# The impact of humor on infant social learning: Insights from social gaze, heart rate variability, and laughter

Romain di Stasi, Rana Esseily, Fabien Cerrotti, Lauriane Rat-Fischer

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## To cite this version:

Romain di Stasi, Rana Esseily, Fabien Cerrotti, Lauriane Rat-Fischer. The impact of humor on infant social learning: Insights from social gaze, heart rate variability, and laughter. Cognitive Development, 2025, 74, pp.101575. 10.1016/j.cogdev.2025.101575. hal-05036469

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# The impact of humor on infant social learning: Insights from social gaze, heart rate variability, and laughter

Romain di Stasi*, Rana Esseily, Fabien Cerrotti, Lauriane Rat-Fischer

Laboratoire Ethologie Cognition Développement (UR LECD), Université Paris Nanterre, Nanterre 92000, France

## ARTICLE INFO

**Keywords:**

Humor

Learning

Surprise

Positive emotions

Arousal

HRV

Infants

## ABSTRACT

Humor is a universal aspect of human culture, serving both social and cognitive functions. This study investigates humor's influence on infant learning, focusing on physiological, behavioral, and emotional responses. Building on previous research, we examined three questions: (1) Does humor affect infants' behaviors during a social learning task, such as looking, laughing, and smiling? (2) Does humor enhance learning, and is this effect age-dependent? (3) Can arousal, measured by heart rate variability (HRV), explain the relationship between humor and learning? We tested 88 infants aged 14–22 months, exposing them to either a humorous or neutral demonstration of a tool-use task. We assessed infants' performance after demonstration and coded social gazes and emotional reactions. We also analyzed physiological arousal indicators, namely heart rate variability (HRV), through a connected wristband. Our analyses revealed that infants exposed to humorous demonstrations showed enhanced learning compared to those in the neutral condition, independently of whether they laughed or not. This suggests that laughing does not mediate the effect of humor on learning. Additionally, infants in the humorous condition looked more at the experimenter, which could indicate increased engagement or attentional processes. Finally, our HRV analyses revealed that infants who succeeded in the task exhibited higher HRV than those who did not. This study is the first to explore the mechanisms underlying humor's effect on infant learning, emphasizing its complexity and the need for a multidimensional approach integrating cognitive, behavioral, and physiological factors.

## 1. Introduction

Humor is a kind of play present in every human culture (Mireault & Reddy, 2016). It can be defined as feeling or expressing something surprising and subjectively funny, accompanied by a positive emotional response expressed through specific behaviors such as laughing, chuckling, or smiling (Booth-Butterfield & Booth-Butterfield, 1991; Martin, 2007). According to Gervais and Wilson (2005), the core nature of humor lies in its non-serious social incongruity. While the idea that incongruity is at the core of humor is widely accepted in literature, its definition varies among authors. Some suggest that incongruity does not necessarily involve surprise and may imply a form of ambiguity (e.g., Shultz, 1976). In the context of our study, however, we consider incongruity to be a disruption of expectations (Mireault & Reddy, 2020) that necessarily involves an element of surprise (Suls et al., 1972; see also Hurley et al., 2011, pp. 45–53 for a detailed discussion). Furthermore, not all incongruity is inherently humorous. For example, being hit by a

<footnote>* Correspondence to: Laboratoire Ethologie Cognition Développement (UR LECD) Université Paris Nanterre, 200 avenue de la République, France.
E-mail addresses: romain.di-stasi@outlook.com, r.distasi@parisnanterre.fr (R. di Stasi), resseily@parisnanterre.fr, ranaesseily@hotmail.com (R. Esseily), cerrotti.fabien@gmail.com (F. Cerrotti), l.ratfischer@parisnanterre.fr, lratfischer@gmail.com (L. Rat-Fischer).</footnote>
car while walking on the sidewalk is incongruous in that it disrupts expectations but is not humorous (Martin, 2007). Likewise, pouring a bucket of water over one's head may be incongruous and surprising, yet not necessarily funny (Mireault & Reddy, 2016). For congruity to be perceived as humorous, it must either be resolved like a problem-solving task (Suls et al., 1972), perceived as an attempt to amuse an audience (Hoicka et al., 2008; Hoicka & Gattis, 2012), or at least understood within a playful context (Rothbart, 1973). In this article, we consider the foundation of humor to lie in the recognition of this playful context, which becomes more complex as development progresses. Thus, humor is based on a disruption of expectations that is done in a non-serious way, i.e., to make the audience laugh and be amused. In adults, humor has been shown to serve both positive and negative functions such as reducing stress or enhancing social relationships, but also denigrating or isolating someone socially (Banas et al., 2011; Martin, 2007). During development, humor serves two functions according to Mireault & Reddy: the first, referred to as "laughing for love", is to promote social interactions by participating in infants' emotional development, and the second, "laughing to learn", is to strengthen learning and cognitive development (p13, Mireault & Reddy, 2016). While these two functions are clearly intertwined in development, in this paper, we tackle specifically the role of humor on learning in infants. Before examining humor's impact, we must clarify our definition of "learning". Learning is defined as any change in an individual's behavior resulting from experience, including their own actions (De Houwer et al., 2013). Our focus is specifically on social learning, which involves learning through observation or interaction with others (Kendal et al., 2018). In this study, infants observed an experimenter demonstrating how to use a rake-like tool to retrieve an out-of-reach toy, focusing on learning related to tool use.

Researchers became interested in the effect of humor on learning in the late seventies with a focus on classroom learning in adults and children (McGhee, 1979; Sroufe & Wunsch, 1972). The use of humor in classrooms has been shown, for example, to improve students' performance by 10% at their final exam (Ziv, 1988), or to facilitate the learning of a second language (Tong & Tsung, 2020). Several factors have been proposed to explain how a humorous context enhances learning, in particular by increasing attention and interest toward the pedagogical material associated with humor (Bryant & Zillmann, 1989) or by enhancing motivation for learning (Wanzer et al., 2010). In children, studies found that educational television programs containing humor were preferred by second graders - compared to classical programs (Wakshlag et al., 1981), and similar programs also stimulated memorization in kindergarten children (Zillmann et al., 1980). Although humor usually enhances learning, it needs to be appropriately used to have such an effect (Banas et al., 2011; Ziv, 1988). Otherwise, it can have either no effect on learning (Houser et al., 2007; Ozdoğru & McMorris, 2013) or even a detrimental effect (Bolkan et al., 2018). For example, Hoicka et al. (2017) showed that children aged 3–5 years trusted less an agent with humorous intentions compared to an agent with sincere intentions when learning new words. These findings underscore the importance of understanding the link between humor and learning, as well as the factors and mechanisms behind it.

Research on humor and learning in infants has received little attention, likely because humor is often associated with advanced cognitive abilities such as theory of mind (ToM; Mireault, Poutre, et al., 2012; Mireault & Reddy, 2016; Soy Telli, 2023). Many authors studying humor in adults argue that incongruity must be recognized and resolved for an event to be perceived as humorous (Shultz, 1976; Suls et al., 1972). However, this argument primarily applies to verbal humor in adults and may not be suitable for understanding non-verbal humor in infancy. Indeed, early studies (Shultz, 1976) hypothesized that children under six would be unable to resolve incongruities and, therefore, unable to perceive humor. However, more recent research challenges this view by showing that infants can engage with humorous stimuli long before developing advanced cognitive abilities (Hoicka et al., 2008, 2017; Mireault & Reddy, 2016; Soy Telli, 2023). While humor perception in infancy does not require full incongruity resolution, it does seem to involve recognizing violations of expected behaviors and responding in a socially interactive manner. Rothbart (1973) proposed that infants do not need to fully comprehend humor's complexity to laugh; instead, they only need to interpret it as a form of play. This perspective aligns with findings that infants as young as three months of age respond to humorous stimuli (Mireault et al., 2012; Wolff, 1963, cited by Mireault & Reddy, 2016), and some parents report humorous reactions as early as one month (Addyman & Addyman, 2013; Hoicka et al., 2022). This raises an important theoretical question: should infant humor be considered an early precursor ("proto-humor") to more advanced humor, a distinct phenomenon, or simply a form of humor adapted to early cognitive and social abilities? Some authors propose the proto-humor distinction to separate early playful reactions from humor that requires incongruity resolution and higher-order cognition (Hurley et al., 2011). However, this distinction remains debated, and recent findings suggest that early humor may share fundamental mechanisms with later humor, albeit in a simpler form. In our study, we adopt the perspective that humor in infancy shares core characteristics with humor in older children and adults but operates within the constraints of early cognitive development. Specifically, we consider humor in infants as a socially embedded process that relies on detecting incongruities but does not require full resolution. Humor, play, and surprise are closely intertwined in infancy, as playful interactions often involve unexpected behaviors that violate conventional patterns. These moments capture attention, engage social partners, and create learning opportunities. Therefore, rather than strictly distinguishing between humor and proto-humor, we use the term "humor" to encompass early playful interactions that elicit laughter and social engagement. This inclusive approach aligns with recent empirical work highlighting humor's role in early social bonding, attention regulation, and learning. Next, we explore the link between humor and learning in infants.

To our knowledge, only three studies have investigated the influence of humor on learning in infants (Esseily et al., 2015; Hoicka et al., 2008; Hoicka & Butcher, 2016), suggesting that even in early infancy, humor may play a role in enhancing cognitive and social learning. In Hoicka et al. (2008)'s study, the authors found that when parents of 19- to 26-month-old infants read humorous books, their utterances were more abstract as compared to when reading neutral books. According to the authors, reading humorous material may make the learning situation more playful and enjoyable, and could thus positively enhance learning. Additionally, a high level of abstraction in parents' utterances is associated with a higher level of abstraction in infants' language development (Kleeck et al., 2018). Related to this finding, Hoicka and Butcher (2016) found that parents use specific cues when making jokes to their infants compared to other false beliefs such as pretense for example. They use disbelief language (e.g., the ducks say "moo") which can help
infants recognize the humorous intent. By using specific language cues when making humor, parents' guide infants in understanding that joking breaks conventional norms. This guidance not only aids in humor comprehension but also broader cognitive and social development, as it teaches infants about the varied ways language and actions can be used to convey different meanings. These two studies suggest that humor provides infants with learning opportunities; however, in both studies, the link between humor and learning remains indirect. Esseily et al. (2015) were the first to directly test the effect of humor on learning in infants. In this study, 18-month-old infants had to learn by observation to retrieve an out-of-reach toy using a rake-like tool. In a humorous demonstration condition, the experimenter created an incongruous event by throwing the toy over its shoulder after each retrieval while smiling and looking at the infant. Among the infants who laughed at the humorous demonstration, 94% reproduced the target action, compared to 19% of infants in the same group who did not laugh, and 25% of infants exposed to a neutral demonstration (i.e., manipulating the toy instead of throwing it). In this paper, we build upon this first study to try disentangling between the factors and mechanisms that underlie the positive effect of humor on learning. This study focuses on three key factors: laughter, which is known to promote affiliation between individuals sharing a joke (Dezecache & Dunbar, 2012; Dunbar, 2022; Manninen et al., 2017); positive emotions, which have significant cognitive and physiological implications for learning (see Clark & Noudoost, 2014; Legrand et al., 2021; McGaugh, 2000; Nielson et al., 2005 for the link between emotion, physiology, and learning; or Fredrickson & Branigan, 2005; Poirel et al., 2012 for connections between emotion, cognition, and learning); and surprise, which is also known to affect learning (Stahl & Feigenson, 2015, 2017). In the following sections, we provide a concise review of the literature on these factors and their relationship to learning, followed by an exploration of the cognitive and physiological mechanisms that may underlie these associations.

Laughter is a typical reaction associated with a humorous context (Martin, 2007; Mireault & Reddy, 2016; Provine, 2017). Some authors suggest that laughing with others involves specific physiological processes such as endorphins production, which in turn promotes social cohesion and affiliation (Dezecache & Dunbar, 2012; Dunbar, 2022). Thus, in the study by Esseily et al. (2015), infants who laughed during the humorous demonstration may have become more affiliated with the experimenter, which could have in turn enhanced their imitation. The positive relationship between affiliation and learning has already been demonstrated in several studies in infancy, and in particular in 16-month-old infants, using mimicry and a similar social learning task (Somogyi & Esseily, 2014).

The association between humor and learning may also be explained by a more general effect of positive emotions, known to enhance learning and memory in adults and in children. For example, in adults, Um et al. (2012) showed that the use of stimuli in multimedia instructional materials, designed to trigger positive emotions, allowed better learning than when using neutral materials. In 14–16-year-old children, Olmos-Raya et al. (2018) showed that the induction of positive emotions in a virtual educational game also induced a better retention of information. The effect of positive emotions on learning can be explained by several cognitive and physiological mechanisms. Authors have shown for example that positive emotions can broaden the scope of attention (Fredrickson & Branigan, 2005; Poirel et al., 2012). This may affect the learning process by opening the possibilities of integrating different elements in the environment, making the cognitive processes more flexible (Fredrickson & Branigan, 2005). Concerning the physiological mechanisms, dopamine has been identified as an important hormone involved in the positive association between emotion and learning. The induction of positive emotions leads to an increase in the brain dopamine level, allowing for greater cognitive flexibility and creativity (Ashby et al., 1999), two important elements for learning.

The fact that humor improves learning could be related to the initial emotion it elicits-surprise. As mentioned earlier, any humorous situation is based on incongruity (Gervais & Wilson, 2005; Mireault & Reddy, 2016), which is a disruption of expectations (Mireault & Reddy, 2020), and it induces surprise (Esseily et al., 2015; Gervais & Wilson, 2005; Suls et al., 1972). Stahl and Feigenson have shown, both in school-aged children (Stahl & Feigenson, 2017) and in infants (Stahl & Feigenson, 2015), that surprise has a significant effect on learning. In their 2015 study, they tested 110 11-months olds across four experiments in which they introduced violations of expectations regarding object solidity, spatiotemporal continuity, and physical support. For example, in a solidity violation, an object appeared to pass through a solid wall, contradicting the intuitive belief that solid objects cannot pass through each other. The results showed that infants preferred to explore objects that defied their expectations, adapting their behavior according to the type of violation observed, such as striking an object or repeatedly dropping it after witnessing a solidity violation. The authors concluded that violation of expectations maximizes infants’ exploratory behavior and, consequently, their learning. Since humor is based on a disruption of expectations, it is very likely that surprise explains, at least in part, why humor improves learning in infants.

Emotions such as positive emotions and/or surprise can also activate the autonomic nervous system (ANS). The ANS is divided into two subsystems: the sympathetic (SNS) and parasympathetic nervous systems (PNS, Mauss & Robinson, 2009). The first system is linked to an activation while the latter is linked to a relaxation state (Mauss & Robinson, 2009). These two systems have long been considered antagonistic, with activation of one inducing deactivation of the other. However, the relationship between these systems is much more complex, and they can also be co-activated (Berntson et al., 1993). It is commonly accepted that an increase in SNS activity, a decrease in PNS activity, or both simultaneously, indicates the intensity of emotions (both positive and negative), also known as arousal (Collet et al., 1997; Kreibig, 2010; Marín-Morales et al., 2021; Mauss & Robinson, 2009). Arousal has been suggested to play an important role in the learning process (Clark & Noudoost, 2014; Legrand et al., 2021; McGaugh, 2000; Nielson et al., 2005). Some researchers have shown that high-arousal compared to low-arousal emotions enhance learning (Chung et al., 2015; Nielson et al., 2005). Arousal is linked to the production of cortisol, epinephrine, and norepinephrine (for both positive and negative emotions, Kuhlmann & Wolf, 2006; McGaugh, 2000; Nielson et al., 2005; Pátkai, 1971), which are known to improve memory retention (Kuhlmann & Wolf, 2006; McGaugh, 2000) and attention (Clark & Noudoost, 2014). Given that humor is a complex construct, it is likely to involve different physiological mechanisms and states of arousal. Humor can provoke an increase in arousal due to its incongruous nature and disruption of expectation, leading to surprise (Csink et al., 2021), but also to the associated laughter and mirth (Averill, 1969; Foster et al., 2002; Marci et al., 2004; McGhee, 1983). Humor can also be associated with a decrease in arousal because it reduces stress (Banas et al., 2011; Borrelli et al., 2021; Martin, 2007; Oliveira & Arriaga, 2022) via the activation of the
parasympathetic nervous system (PNS). It is therefore reasonable to think, as Fry (1994) suggested, that humor initially increases arousal through the surprise effect produced by the incongruity of the event, but ultimately decreases arousal by reducing stress and promoting relaxation (see also Bennett & Lengacher, 2008 for a review). However, assessing this dynamical process is challenging when relying solely on physiological markers of the autonomic nervous system. Therefore, in this study, we will investigate the overall pattern (i.e., a general increase or decrease) rather than the specific fluctuations of arousal.

In the current study, we assessed the influence of humor on learning in infants using a procedure similar to Esseily et al. (2015), integrating both physiological and behavioral measures. We tested 14- to 22-month-old infants with the rake-imitation task, a task typically mastered between 18 and 24 months of age (e.g., Fagard et al., 2016; Rat-Fischer et al., 2012b). The general aim of this study was to investigate the effect of humor on learning and to understand its underlying mechanisms. This investigation was structured around three research questions: First, we asked whether adding humor to the demonstration has a consistent effect on infants' behaviors during a social learning task (e.g., looking, smiling, laughing) across all ages. Measuring infants' gaze toward the experimenter can help assess their level of engagement or attention, which may be influenced by surprise or, more broadly, by the humorous context. This behavior is often referred to as "social looking" in the literature, and research suggests that infants tend to look more at an experimenter when they encounter unexpected events (Dunn & Bremner, 2017; Mireault et al., 2014; Scherer et al., 2004; Walden et al., 2007). While social looking is frequently used in the context of the violation of expectation (VoE) paradigm (Dunn & Bremner, 2017; Mireault et al., 2014), it is important to note that, on its own, this measure does not clearly distinguish whether infants are looking more because they are surprised or simply because they are more socially engaged with the experimenter for various reasons. For instance, infants who smile are more inclined to gaze toward the experimenter while doing so (Cohn & Tronick, 1987; Venezia et al., 2004). Similarly, since laughter is fundamentally social (Martin, 2007; Provine, 2017), infants are likely to look at the experimenter when they laugh. Conversely, regarding laughter, the reverse is also possible. Mireault et al. (2014) suggest that the frequency of social looking may depend on how infants interpret the situation; for instance, infants who do not find an event humorous may look more frequently at the experimenter to seek cues, rather than as an expression of shared amusement. For all these reasons, we consider social looking a marker of attentional or social engagement as in Mireault et al. (2014)'s study, possibly due to surprise or other social motivations. Additionally, measuring their smiles and laughter indicates whether they perceive the humor as funny. The question of when humor perception emerges during development is complex, with no consensus in the literature. As seen earlier, some authors suggest that infants can perceive simple forms of humor between 3 and 5 months of age (Mireault et al., 2012 Wolff, 1963, cited by Mireault & Reddy, 2016) or even as early as 1 month (Addyman & Addyman, 2013; Hoicka et al., 2022). Others argue that humor is more complex, and that it is only around 18 months that infants understand and differentiate between humor and other mental states, such as wrongness and mistakes (Hoicka et al., 2008; McGhee, 1979; Mireault & Reddy, 2016). According to McGhee (1979), this capacity to tolerate the unexpected without thinking it is wrong develops at the same time as symbolic play, where play involves some form of representation or acting-as-if, a capacity that typically develops around 18 months. By testing infants from 14 months onwards, we aimed to assess at what age our humorous demonstration was perceived as such. The second research question was whether adding humor increased learning in infants and whether this effect was age dependent. If infants younger than 18 months responded positively to the humorous demonstration, we hypothesized that humor would also enhance learning in these younger age groups, consistent with studies showing that additional information in a demonstration can improve observational learning before 18 months (Esseily et al., 2013; Fagard et al., 2016; Somogyi & Esseily, 2014). As previously mentioned, Esseily et al. (2015) showed that 18-month-old infants who laughed during a humorous demonstration using a rake to retrieve an out-of-reach toy were more successful than those who did not laugh or were exposed to a neutral demonstration. In line with this, we expected 14- to 22-month-old infants exposed to the humorous demonstration, especially those who laughed, would perform better than those who did not laugh and/or those exposed to a neutral demonstration. The third and last main question of our study was whether arousal could explain why humor enhances learning in infants.

We had two hypotheses regarding the relationship between arousal and learning in a humorous context. Humor may increase arousal in infants through surprise (Hoicka, 2014; Hurley et al., 2011; Martin, 2007; Mireault & Reddy, 2016; Suls et al., 1972), mirth and/or laughter (Averill, 1969; Foster et al., 2002; Marci et al., 2004; McGhee, 1983). Since literature suggests a positive link between arousal and learning in adults (Chung et al., 2015; Clark & Noudoost, 2014; Legrand et al., 2021; McGaugh, 2000; Nielson et al., 2005), we hypothesized that this increase in arousal could explain infants' enhanced learning. Alternatively, humor may reduce stress (Banas et al., 2011; Borrelli et al., 2021; Martin, 2007; Oliveira & Arriaga, 2022). Stress is known to have a detrimental effect on learning in adults (e.g., Frazier et al., 2019; Schwabe & Wolf, 2010), and probably in infants too. Several authors suggest that one of the reasons humor improves learning in adults and school-aged children is linked to its ability to reduce stress, which can improve learning performance (Banas et al., 2011; Martin, 2007). Therefore, we also hypothesized that humor may positively influence learning in infants by reducing stress, possibly generated by the novelty of the experimental setup, thus reducing arousal. We assessed the influence of arousal in infants during the humorous demonstration by measuring their heart rate variability (HRV), a known marker of arousal (Marín-Morales et al., 2021; Mohammadpoor Faskhodi et al., 2023; Rajendra Acharya et al., 2006). It is a non-invasive and reliable biomarker for studying vagus nerve activity: high HRV indicates high vagal activity, while low HRV indicates low vagal activity (Heiss et al., 2021; Laborde et al., 2017; Marín-Morales et al., 2021; Shaffer & Ginsberg, 2017). While an increase in the activity of the vagus nerve may indicate increased arousal in specific cases of aversive emotions (Marín-Morales et al., 2021), it is directly correlated with a decrease in stress (Field & Diego, 2008; Mohammadpoor Faskhodi et al., 2023) and thus a decrease in arousal. Additionally, HRV reflects attentional processes, as the vagus nerve is activated during prolonged attentional tasks (Field & Diego, 2008), resulting in increased HRV (Heiss et al., 2021; Laborde et al., 2017; Marín-Morales et al., 2021; Shaffer & Ginsberg, 2017). According to Porges and Furman (2011), the development of the myelinated vagal system supports infants' ability to regulate physiological state through social engagement. This system, which continues to mature throughout the first year postpartum, and its
myelination continues into adolescence (Becker et al., 1993; Geuna et al., 2002; Sachis et al., 1982). As the vagus nerve becomes more myelinated, infants gain a greater capacity for sustained attention and social engagement (Porges & Furman, 2011). The hypothesis that vagus nerve activity is linked to attentional engagement aligns with the idea that humor may help infants feel more relaxed and less stressed, thus enhancing learning. Here, we will investigate the impact of the humorous demonstration on arousal considering a negative link between arousal and HRV: an increase in HRV corresponds to a decrease in infants' arousal, and a decrease in HRV corresponds to an increase in arousal. We will examine whether adding humor to the demonstration and/or laughing affects infants' HRV and whether changes in HRV positively affect their learning.

## 2. Methods

### 2.1. Participants

The data collection and recruitment of participants took place between February 2021 and September 2022. The study was conducted both at the BabyLab (N = 17) of the University of Paris Nanterre (Nanterre, France) or at the participants' homes (N = 102), depending on the parents' preferences. On of the 119 infants tested, 31 were excluded *a posteriori* for the following reasons: eleven infants as they succeeded the pre-test and therefore could not be tested for social learning; twelve infants for refusal to participate or fussiness during the experiment; and eight infants because of external bias (e.g., high environmental noise due to construction work, verbal help from parents during the trials). The proportion of infants excluded at BabyLab or the parent's home is approximately the same: 29 % (5 out of 17) of infants tested at the BabyLab, and 25 % (29 out of 102) of infants tested at home. Thus, a total of 88 infants were included in the study (mean age = 18 months; min = 14 months, max = 23 months; 47 females). Of these, 41 were exposed to the neutral demonstration (control condition), and 47 were exposed to the humorous demonstration. The slight imbalance between the two groups is due to the need to ensure that at least 21 infants who laughed were included in our sample. Given that the proportion of infants who laughed in the humorous condition was lower than those who did not laugh (34 %), we recruited a slightly larger number of participants in this group. Infants were recruited both via social media and from a list of local families who expressed interest in participating in studies on infant development. Prior to the experiment, parents were informed about the procedure and stayed blind to the experimental hypotheses. They provided written informed consent prior to their infant's participation. Specific demographic information on average parental income, education or age of the parents was not available for this study. This study received ethics clearance from the Research Ethics Committee of the University Paris Nanterre (approval number: 2021-01-01).

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Illustration of the experimental procedure. L1 corresponds to the position of the experimenter during the warm-up and laterality phases, and during the pre-test and test of the experimental phase (see procedure section). L2 corresponds to the experimenter's position during the demonstration. B corresponds to the infant's position, sitting on the lap of one of their parents P.</figcaption></figure>
## 2.2. Materials

The experiment was performed on a table surrounded by three chairs (see Fig. 1). The infant sat on their parents' lap (chair P). The experimenter sat either in front of the infant (chair L1) or perpendicular to the infant (chair L2) depending on the phase of the experiment (see Fig. 1 and the procedure for further details). A first set of toys was used at the beginning of the experiment (see warm-up phase in the procedure section) to familiarize the infant with the testing situation. This set consisted of a doll, two forks, two spoons, a pitcher, a glass, and a fake plastic cell phone. The set used for the main experiment consisted of a cardboard T-shape tool (17 × 20 cm, referred to as “rake” hereafter), similar to the one used in Esseily et al. (2015) and various plastic toys representing animals of different attractive colors and about similar size (about 2 cm high, 6 long, 4 cm wide, see illustration in Supplementary Appendix S1). To perform fine behavioral analyses, we recorded the experiments using two SONY digital video cameras. The first camera was placed on the right side of L1 (Fig. 1), the second on the left side of L2, over the experimenter’s shoulder. This configuration allowed us to always have a camera facing the infants when they looked either toward L1 or L2, to be able to score their facial expressions (laughing and smiling) during the experiment. We used the E4 Empatica® wristband to continuously measure infants’ physiological data in a non-invasive way during the whole experiment. The wristband uses photoplethysmography (PPG) to measure the Heart Rate Variability (HRV) in an online manner. To do this, the PPG sends red and green light rays that cross the skin and reach the blood vessels, the refraction produced by the oxygen molecules in the blood is then refracted and detected by sensors. Through this intermediary, the PPG measures the Blood Volume Pulse (BVP), corresponding to the quantity of blood expelled into the blood vessels at a frequency of 64 Hz (i.e., 64 times per second). Physiological data was collected by synchronizing the wristband with the E4 RealTime application for smartphones. The data was recorded in real time and sent directly to a cloud secured by Empatica®. Using an algorithm designed by Empatica®, the heart rate was calculated, which then allowed us to assess heart rate variability (HRV). The wristband was placed on the infant’s left ankle to record physiological data without disturbing the infant’s natural hand movements and to reduce motion noise in BVP recording (Sevil et al., 2020). Initially, the wristband was designed for adults. More recently, it has been validated for use in infants, with the ankle (Bian et al., 2018) or the calf as suitable sites (Bainbridge et al., 2021). Out of the 88 infants included in the experiment, 78 accepted the wristband.

Due to the experiments being conducted mostly during the Covid-19 pandemic, the experimenter wore a transparent mask throughout the entire experiment. These transparent masks, designed by LSF PI TOUS charity®, were initially created to facilitate communication with and between deaf or hard-of-hearing individuals using sign language during the pandemic. They provide excellent particle filtration (i.e., 90% for 3-micron particles), and their transparency allowed the experimenter to use social and emotional cues when interacting with the infants. These masks ensured maximum protection against contamination, in accordance with the French national standards, including specifications from the French Standardization Association (AFNOR), and certifications provided by the French Ministry of Armed Forces (DGA) and the French Institute for Textiles and Clothing (IFTH).

## 2.3. Procedure

The whole session lasted for about an hour, including the reading and signature of the informed consent letters, and the experimental session which lasted for about 15–18 minutes. To be able to retrospectively synchronize the images from the cameras with the physiological data, the experimenter first switched on the two cameras, then switched on the wristband while emitting a sound. The experimental session contained three consecutive phases: a warm-up phase and a laterality phase which were the same for all infants, and an experimental phase that differed according to the type of demonstration to which they were exposed (neutral or humorous).

The goal of the warm-up phase was twofold: first to familiarize the infant with the experimental situation, and second to have a baseline for assessing the quality of the physiological measures (see “Behavioral and physiological data” section). At the beginning of this phase, the experimenter placed a set of toys on the table in front of the infants and within its reach. While the infant could start interacting with the toys, he placed the wristband on the infants’ ankle and started the recording. This phase lasted for about three minutes during which the infant could play with the toys while the experimenter briefly explained the procedure to the parent.

In the laterality phase, the experimenter assessed infants’ hand preference using 10 chosen items from Sacco et al. (2006) (see Supplementary S1). The information from the laterality test was used to present the rake used in the following phase next to the infant’s preferred hand. If the infant did not show a hand preference, the rake was presented to the infant’s right hand.

The experimental phase included a pre-test, a demonstration, and a test, following (Esseily et al., 2015) procedure. First, in the pretest, infants’ capacity to use the rake as a tool to retrieve an out-of-reach toy prior to demonstration was assessed. To do so, the experimenter placed the rake near the infant’s preferred hand (as assessed in the laterality phase) and placed a toy out of its reach. If the infant succeeded at retrieving the toy using the rake at the pretest, they completed the procedure but were excluded from the final analysis. Second, a demonstration was performed by the experimenter, consisting in eight consecutive demonstrations of the raking action. Beforehand the experimenter changed sides to sit perpendicular to the infant (L2, Fig. 1). The demonstration was repeated eight times, either in a humorous or a neutral manner. In the humorous condition, the experimenter repeatedly retrieved the toy and threw it over their shoulder while smiling at the infant and saying “hop” in a high-pitched, playful voice. In the neutral condition, the experimenter kept the toy in his hands after retrieval and said, “That’s nice, isn’t it?” while looking and smiling at the infant. The neutral demonstration could not be completely neutral to avoid making the infant uncomfortable with the experimenter wearing a transparent mask. In both conditions, the tone of the voice corresponded to infant-directed speech (IDS, e.g., Monnot, 1999). However, during the humorous demonstration, the demonstrator used exaggerated features of IDS, including a higher mean level of fundamental frequency, greater amplitude, and higher pitch variation, and produced a more exaggerated smile. These choices were motivated by literature on parent-infant humorous interactions (Hoicka & Butcher, 2016; Hoicka & Gattis, 2012).
Finally, the test was like the pre-test. The experimenter returned to the L1 position (Fig. 1) and placed the toy out of the infant's reach, and the tool next to the infant's preferred or right hand. Infants had between 3 and 5 attempts to retrieve the toy, with a standard of 3 trials. Additional trials were added under the following circumstances: (i) if the infant successfully used the rake in one of the three trials but clumsily (corresponding to a score of 3, see the section "Behavioral and physiological data" for further details about scoring), two additional trials were conducted to be sure that he did not succeed by chance; (ii) if the three consecutive trials lasted for less than one minute (for example, if the infant rapidly discarded the tool without using it, thus ending the trial), one or two additional trials were added depending on the total time spent in the test; and (iii) if the infant was not motivated by a given toy (i.e., did not try to reach for the toy), one additional trial was added for each unengaged attempt, up to a total of two additional trials.

## 2.4. Behavioral and physiological data

The behavioral coding was performed using The Observer XT 15 (Noldus Team, 2021). To measure infants' performance after demonstration, we used the same tool-use score as in (Esseily et al., 2015). Infants' performance in the pre-test and test were scored from 1 to 4. A score of 1 was assigned when the infant was interested in either the toy or the rake but did not connect the two; 2 when the infant touched the toy with the rake without retrieving it; 3 when the infant successfully retrieved the toy with several attempts; and 4 when the infant retrieved the toy directly with a perfect imitation of the modeled raking gesture (see Supplementary S2). Both in the pretest and in the test, we used each infant's highest score over all trials during a 30 second period for the pretest and a one-minute period for the test. Even if the test exceeded this time, we did not consider scores beyond these periods for the sake of standardization.

In addition, we coded two behavioral variables during demonstration: frequency and duration of gazes toward the experimenter. As mentioned in the introduction, we considered infants' gaze toward the experimenter as an indicator of their attentional engagement during the demonstration, hypothesizing that this behavior might be related to surprise or, more broadly, to the humorous demonstration. Historically, these gaze behaviors have been considered indicators of social referencing (Mireault et al., 2014), defined as the process by which an individual seeks emotional information from others to make sense of an external event that would otherwise remain ambiguous or surprising (Kim et al., 2010; Klinnert et al., 1983; Mireault et al., 2014). In our context, however, infants' behavior cannot be classified as social referencing since, by definition, social referencing involves emotional contagion, which was not present in the experimenter's interactions. For this reason, we refer to these behaviors as "social looking", which has previously been used in contexts such as the violation of expectation (VoE) (e.g., Walden et al., 2007) or humor (e.g., Mireault et al., 2014). These behaviors generally indicate infants' attentional engagement and may reflect surprise, known to redirect attention toward the surprising event (Baillargeon et al., 1985; Barto et al., 2013), or other attentional processes associated with humor. Indeed, humor can increase attention through factors beyond surprise. As we have seen, humor induces an increase in arousal (Averill, 1969; Foster et al., 2002; Marci et al., 2004; McGhee, 1983), and arousal itself can enhance attention (Wass, 2021; Wass et al., 2016). Finally, humor involves positive emotions, such as joy, which can strengthen infants' attention by broadening their perceptual scope and by facilitating visual engagement with their environment (Fredrickson & Branigan, 2005; Poirel et al., 2012). Thus, we expect to observe more frequent gazes toward the experimenter in the humorous condition compared to the neutral condition. For all these reasons, we expect that infants exposed to the humorous demonstration will produce more gazes toward the experimenter than those exposed to the neutral one. Finally, we categorized the infant's highest emotional reaction during the demonstration. If the infant laughed at least once, we considered their highest emotional reaction to be "laugh"; if they smiled at least once but did not laugh, we categorized their highest reaction as "smile". If neither behavior occurred, the infant was classified as "neutral". Unlike previous studies on humor, such as Mireault et al. (2014), which grouped smiles and laughter, we distinguished between these two behaviors for two key reasons. First, this distinction allowed for direct comparison with Esseily et al. (2015), who found that only laughter—but not smiling—predicted better learning outcomes. Second, while smiling is often associated with positive affect, it does not always indicate humor perception. In our study, smiling occurred in both the humorous and neutral conditions, reinforcing that it can reflect general social engagement rather than humor per se. This aligns with research suggesting that smiles can result from various emotional states, including interest, mild amusement, or even ambiguous affective responses (Messinger et al., 1999). However, the absence of a significant increase in smiling does not negate the role of humor in infant learning. Smiling and laughter are not interchangeable humor indicators, as laughter is a stronger and more specific response to humorous stimuli (e.g., Mazzocconi & Ginzburg, 2023; Mireault & Reddy, 2016). Additionally, infants in the humorous condition laughed significantly more than those in the neutral condition, suggesting that they did perceive the event as humorous. Thus, while smiling alone may not reliably indicate humor perception, laughter, social engagement, and learning outcomes suggest that humor played a role in infants' responses. Moreover, it is important to acknowledge that the neutral condition could not be entirely devoid of social engagement. A completely neutral interaction might have made infants uncomfortable, as the experimenter was unfamiliar. The experimenter maintained a warm but non-humorous demeanor to maintain a natural and engaging social context. Some infants were expected to smile at the neutral demonstration in this setting, but laughter was unlikely. This further reinforces the distinction between the humorous and neutral conditions, as laughter was significantly more frequent in the humorous condition.

The physiological data extracted from the E4 wristband corresponded to the heart rate (HR). We synchronized this variable with the two videotaped recordings using The Observer XT 15 (Noldus Team, 2021). We then extracted both the behavioral and physiological data into a single file for each participant at a sample rate of 4 Hz. Using Python version 3.10.5 (van Rossum & Drake, 2006) and the packages pandas (McKinney, 2010) and numpy (Harris et al., 2020), we analyzed and processed the HR data to calculate the Heart Rate Variability (HRV), a reliable biomarker for studying arousal (Heiss et al., 2021; Laborde et al., 2017; Shaffer & Ginsberg, 2017). Research on heart rate variability (HRV) in infants remains relatively rare. However, the HRV analysis methods used in our study are consistent with approaches applied across different age groups, including adults and older children (e.g., Shaffer & Ginsberg, 2017).
HRV can be measured in different ways, including time domain measurements (e.g., RMSSD, SDNN) and frequency domain mea-
surements (e.g., LF power, HF power, LF/HF ratio). The choice of measurement depends on data collection duration and research
objectives, but these measurements can be highly correlated (see Shaffer & Ginsberg, 2017 for details). For our study, we used Root
Mean Squared Successive Differences (RMSSD) of the duration between two heartbeats (see Supplementary S3 for the formula), which
is suitable for short data collection periods such as 60, 30, or 10-second epochs (Heiss et al., 2021; Laborde et al., 2017; Shaffer &
Ginsberg, 2017). We analyzed the HRV during the entire demonstration for each infant. To ensure that the differences in HRV were not
due to inter-individual variations independent of our experimental condition, we checked the quality of our data by subtracting the
HRV of the first 20 seconds of the demonstration from the last 20 seconds of the warm-up phase, defined as our baseline. The analysis
of HRV during the whole demonstration and the baseline difference yielded similar statistical patterns. Specifically, when considering
the tool-use score as the dependent variable (DV), our logistic regression showed $\chi^2(1) = 57.861, p < 0.001$ for the whole demon-
stration, and $\chi^2(1) = 60.606, p < 0.001$ for the difference with the baseline. Similarly, when considering success/error as a binomial
DV, we obtained $\chi^2(1) = 4.052, p < 0.05$ for the whole demonstration, and $\chi^2(1) = 5.619, p < 0.05$ for the baseline difference. For
clarity and to ensure the robustness of our results, we report only the HRV analysis for the entire demonstration in this article.
However, the full output of the models comparing HRV and baseline, which serves as a quality check of our data, is available in the OSF
repository under the "Output Models" section at the following link: https://osf.io/bdr3y/?view_
only=c068361f55bc4fb29f1781fad1b41cc5.

2.5. Inter-rater scores and evaluation of the heart rate data quality

All behaviors were scored by two principal raters (rater 1 and rater 2). Each of them analyzed 50 % of the videos and were trained by a third rater (rater 3). Rater 3 analyzed 25 % of the videos from each of the two principal raters to assess inter-rater reliability. Since the behavioral category "infant's highest emotional reaction during the demonstration" was used as a key independent variable in several models, rater 3 double-coded 100 % of the videos for this variable. We conducted an inter-rater reliability analysis using the Intraclass Correlation Coefficient (Koo & Li, 2016) and percentages of agreement (see formula (1) in Supplementary S3.1). The IntraClass Correlation Coefficient was calculated using the function "ICC" from the package psych (Revelle, 2024) in R (version 4.0.4, R Core Team, 2021) on the frequency and duration of gazes at the experimenter. Intraclass Correlation Coefficients (ICC3) between raters 1 and 2 (combined) and rater 3 were 0.80 for frequency and 0.92 for duration, which are considered satisfactory (Koo & Li, 2016). The percentages of agreement between raters 1 and 2 (combined) and rater 3 for these measures were 86.11 % and 81.21 %, respectively, which are also satisfactory. For the "infant's highest emotional reaction" dependent variable, our percentage agreement was 92.86 %, and for the highest tool-use score after the demonstration, it was 90.23 %. Comparisons between raters 1 and 3 and between raters 2 and 3 are available in the OSF platform section "Reliability inter-Observer". It is important to note that although the hypotheses were not explicitly communicated to coders 1 and 2, they could be readily inferred from reading the paper of Esseily et al. (2015), which all coders had read. Additionally, coder 3 was not blind to the hypotheses. Therefore, we cannot exclude a potential coder bias related to hypothesis awareness in assessing differences between the experimental and control conditions.

To ensure the quality of the Blood Volume Pulse (BVP), which is used to assess heart rate (HR) and its variation (i.e., Heart Rate Variability, HRV), we visually inspected the signal of each participant and excluded those with too many artifacts (i.e., signals that did not correspond to a normal BVP signal). As a result, 10 out of the 78 participants were excluded from the analyses due to excessive noise (more than 30 % of the signal did not correspond to a characteristic BVP signal). Additionally, we used two methods to assess data quality by examining the percentage of outliers for the heart rate, as reported by Forcolin et al. (2018). The "percentage change" method defines an extreme value as any heart rate that deviates by 30 % from the mean, while the "standard deviation" method defines an extreme value as any data that deviates by 4 or 5 standard deviations, depending on the study. In our case, we set the limit to 4, as in Forcolin et al. (2018). The percentage change method showed 0.00 % extreme values after preprocessing with the Empatica® algorithm, while the standard deviation method showed 0.74 % extreme values, which is lower than the percentage found in pediatric analyses on infants, such as in Kemper et al. (2007), indicating good raw data quality. Since HRV calculation can be sensitive to extreme values, we removed those identified with the standard deviation method (the less conservative of the two used). The processed HR values corresponded to normal values for infants of this age, approximately 85–150 beats per minute (Fleming et al., 2011).

2.6. Statistical analyses

The statistical analyses were performed on R (R Core Team, 2021). To determine our minimum sample size, we conducted a
G-Power analysis using the "pwr.anova.test" function within the pwr package (Champely et al., 2020). Drawing upon the findings of
Esseily et al. (2015), we assumed that laughter would exert a strong effect on learning, denoted as "f = 0.4". The G-Power analysis
yielded a minimum required sample size of 63 participants, which translates to 21 participants per group (i.e., neutral, smile, and
laugh). Our final sample comprised 88 participants, and among them, 21 infants laughed during the demonstration (5 during the
neutral demonstration and 16 during the humorous demonstration). To check whether the gender of the individuals was balanced
within the age groups (in months) and between the two conditions (neutral and humor), we performed a chi-square test of sex in-
dependence within age groups and between the two conditions using the "chisq.test" function of the package stats (R Core Team,
2021). Given that both tests were not significant ($\chi^2_{(16, N = 88)} = 13.348, p = 0.6471$ for age and $\chi^2_{(2, N = 88)} = 2.6704, p = 0.2631$ for
condition), and that no sex-related effects were reported in previous studies using the same tool-use task during the second year of life
(e.g., Rat-Fischer et al., 2012b, 2012a, 2024), we did not include sex as an independent variable in our models.

For each of our statistical models, several interactions were tested (described in the following sections). The non-significant
interactions were systematically removed from the model. For each variable in the models, an effect size was calculated using Cohen's d when comparing means, and the estimate (B) and Standard Error (SE) when performing correlations. Cohen's d were calculated using the function "cohens_d" of the package *rstatix* (Kassambara, 2023) and are systematically reported alongside test statistics in the results section where applicable. All graphs were produced with the ggplot2 package (Wickham, 2016). Finally, for each model, the collinearity between the factors and the homogeneity of variance were checked using the "check_model" function of the Performance package (Lüdecke et al., 2021). In this way it was possible to check the robustness of each model even in case of imbalance of the factors.

### 2.6.1. Q1 - Does adding humor to the demonstration have an effect on infants' behavioral and physiological responses?

To answer this question, we ran five models. The first two models were logistic models using the "glm" function from the *stats* package (R Core Team, 2021). The aim was to assess whether the humorous demonstration had an effect on infants' positive emotional reactions (i.e., smiling or laughing). Both models included the same independent variables: the age of the infant in days, the type of demonstration (neutral or humorous), and their interaction. The first model (Model Q1-Smile) used smiling as a binomial dependent variable (infants who smiled at least once during the demonstration scored 1, otherwise 0). The second model (Model Q1-Laugh) used laughter as the dependent variable, similarly binomial: 1 when infants laughed at least once during the demonstration, and 0 when they did not. The other two models were linear models using the "lm" function in the *stats* package (R Core Team, 2021). These models were designed to assess whether infants made longer and more frequent gazes at the experimenter when exposed to the humorous demonstration compared to the neutral condition, and when they laughed during the demonstration compared to when they did not. Both models included the same independent variables: the type of demonstration, whether the infant laughed or not, and the interaction between these two variables. In the first model (Model Q1-Gazes_D, where D stands for the duration), the dependent variable was the duration of infants' gaze at the experimenter during the demonstration. In the second model (Model Q1-Gazes_F, where F stands for the frequency), the dependent variable was the frequency of this same behavior. We considered only the presence or absence of laughter and smiles during the demonstration for several reasons. First, laughter was relatively rare and brief, with limited inter-individual variability, making a statistical duration analysis less pertinent. Regarding laughter frequency, we observed limited variability among individuals: those who laughed did so between one and three times, rarely more, which reduced the relevance of detailed frequency analysis. We did not have specific hypotheses regarding their duration or frequency of smiles and similarly opted for a presence/absence analysis. This choice was also due to the fact that infants' smiles could be triggered by factors unrelated to the humorous demonstration; many infants, for instance, smiled during the neutral demonstration. Differences in the frequency and duration of smiles thus seemed more likely to reflect temperament variations rather than responses to humor. Concerning gazes toward the experimenter, all infants except one looked at the experimenter, making an analysis based on presence or absence less pertinent. For this behavior, we measured the duration and frequency of gazes toward the experimenter. In the VoE literature, gaze duration is often used as it allows for a parallel with fixation time behaviors (Baillargeon et al., 1985 see also (Margoni et al., 2024 for a review). However, gaze duration provides information only on the length of infants' attentional engagement during the task without indicating the number of disengagements (Mireault et al., 2014; Peltola et al., 2008). In our study, the humorous demonstration involved a social dimension distinct from a traditional VoE paradigm. Thus, gaze frequency additionally allowed us to evaluate the interaction between the infant and the experimenter. This distinction between prolonged durations (indicating sustained engagement) and high frequencies of gazes (potentially indicating frequent returns of attention to the interaction) enabled us to examine the fine dynamics of visual behaviors in our humorous context. Finally, although the duration of the demonstrations is relatively similar across infants (p = 0.205), we checked the quality of the model by normalizing all behaviors over the duration of the demonstration for each individual. The results were similar and are available in supplementary material on the OSF platform. In both models, we did not include the infants' age since we did not have a specific hypothesis about the links between age and social referencing within this age range.

Finally, we assessed the influence of both the type of demonstration and of infant's laughter on their Heart Rate Variability using a linear model (Model Q1-HRVvsDemo) with the "lm" function from the *stats* package (R Core Team, 2021). We considered the infant's Heart Rate Variability (HRV) during the entire demonstration as the dependent variable. The independent variables included the type of demonstration to which infants were exposed (neutral or humorous) and whether the infant laughed during it. We analyzed the interactions between these dependent variables. In this model, we did not include the infants' age since we did not have a specific hypothesis about the links between HRV and humor and/or laughter within this age range. As mentioned in the "Behavioral and Physiological Data" section, we checked the quality of this model to ensure that the observed differences in HRV were not due to inter-individual differences independent of our experimental condition. For this, we replaced the HRV during the whole demonstration with the difference between the HRV in the first 20 seconds of the demonstration and the last 20 seconds of the warm-up phase, which we defined as our baseline. As mentioned earlier, the statistical tests confirmed that the results were very similar. Specifically, logistic regression analyses for the tool-use score and success/error as a binomial dependent variable yielded comparable $\chi^2$ values for the demonstration and the baseline difference (see the end of the "Behavioral and Physiological Data" section for further details). Since it is redundant with the prior analysis and is not a classic way to analyze HRV, we report the results of this "quality check" only on the OSF platform in the section "Output Model", the analyses are available in the "HRV analyses" section.

### 2.6.2. Q2 - Does adding humor increase learning in infants, and does it depend on age?

To answer this question, we ran a logistic model (Model Q2-Tool_Use_Score) using the "clm" function from the *ordinal* package (Christensen, 2023). The aim was to assess if the humorous demonstration and/or infant's laughter positively affected their learning. The dependent variable in this model was infants' highest tool-use score after demonstration. The independent variables included the age of the infant in days, the type of demonstration, and whether the infant laughed or not. We analyzed the interactions between age
and type of demonstration, as well as the interaction between the type of demonstration and whether the infant laughed or not during the demonstration. To determine if we obtained the same results when using tool-use score or when evaluating performance in a binary pass/fail manner, we conducted additional analyses. We performed the same logistic model (Model Q2-Tool_Use Success) with the dependent variable being whether the infant succeeded at the tool use task in a binomial manner (1 if the infant succeeded, 0 if they did not). For this, we used the “glm” function from the *stats* package (R Core Team, 2021). Finally, to test the hypothesis of an 18-month threshold in the relationship between humor and learning in infants, and in line with McGhee, (1979) theory, which posits that humor comprehension develops alongside pretend play, we reanalyzed the set of models described in this section, dividing our sample into two groups: infants younger than 18 months (Model Q2-Tool_Use_Score_Under18; Model Q2-Tool_Use Success_Under18) and those older than 18 months (Model Q2-Tool_Use_Score_Over18; Model Q2-Tool_Use Success_Over18).

### 2.6.3. Q3 - Can arousal explain why humor enhances learning in infants?

To answer this question, we ran a linear model (Model Q3-HRVvsSuccess) using the “lm” function from the *stats* package (R Core Team, 2021), with infants’ Heart Rate Variability (HRV) during the entire demonstration as the dependent variable. A permutation test was applied to this model using the “PermTest” function from the *Pgirmess* package (Giraudoux et al., 2023). Using a continuous measure helped avoid the loss of statistical power associated with a binary success variable. Moreover, it allowed us to account for the mediating effect of the demonstration condition on HRV. For completeness, we also conducted an alternative analysis (see supplementary materials in the section “Outputs Models” on the OSF plateform) where success was treated as the dependent variable and HRV as the independent variable, as suggested. The results remained consistent, with the permutation test yielding *p* < 0.05, while the logistic regression resulted in $\chi^2(1) = 4.0516, p < 0.05$. In this model, we included the following independent variables: whether the infants succeeded at the tool-use task after the demonstration (measured as a binary factor), the type of demonstration (neutral or humorous), and their interaction. As in the previous model, we assessed the robustness of the dependent variable by replacing HRV during the entire demonstration with the difference between HRV in the first 20 seconds of the demonstration and HRV during the baseline (the last 20 seconds of the warm-up phase). The results were similar and are available on the OSF platform in the “Output Model” section.

### 2.6.4. Extraction of the statistics of the models

To extract the p-values of the models we used different methods depending on the type of the model. For the logistic regressions using the “glm” function, we used the function *Anova* of the package *car* (Fox & Weisberg, 2019). For the logistic model using “clm” we used the function “clm.Anova” of the package *RVAideMemoire* (Herve, 2011). Finally for the linear regression, we used the “PermTest” function of the R package *pgirmess* (Giraudoux et al., 2023).

## 2.7. Transparency and openness

This study was not preregistered. The data and R and Python scripts used for processing and analysis are available online in the OSF repository. The link is at the end of the “Behavioral and Physiological Data” section.

# 3. Results

## 3.1. Q1: does adding humor to the demonstration have an effect on infants' behavioral and physiological responses?

Out of 47 infants exposed to the humorous demonstration, 16 laughed (34 %) and 20 smiled (43 %), whereas out of 41 infants exposed to the neutral demonstration, 5 laughed (12 %) and 11 smiled (27 %). We did not expect infants to laugh during the neutral demonstration. However, all 5 infants who laughed during the neutral demonstration were tested at home, suggesting that infants may be more relaxed at home than those at BabyLab (see discussion section). Additionally, we observed that infants laughed at almost any age in the humorous condition. See Table 1 for more details on the number and percentage of infants in each condition as a function of

<table><caption>**Table 1**
Number and proportion of infants who laughed at each age (in month) as a function of the type of demonstration (i.e., neutral or humorous).</caption><thead><tr><th colspan="11">Type of demonstration</th></tr><tr><th colspan="4">Neutral</th><th colspan="4">Humor</th></tr><tr><th>Age in Month</th><th>Non-Laugh</th><th>Laughing</th><th>% of laughter</th><th>Age in Month</th><th>Non-Laugh</th><th>Laughing</th><th>% of laughter</th></tr></thead><tbody><tr><td>14 (n = 4)</td><td>3</td><td>1</td><td>25.00</td><td>14 (n = 4)</td><td>4</td><td>0</td><td>0.00</td></tr><tr><td>15 (n = 4)</td><td>4</td><td>0</td><td>0.00</td><td>15 (n = 6)</td><td>5</td><td>2</td><td>28.57</td></tr><tr><td>16 (n = 5)</td><td>5</td><td>0</td><td>0.00</td><td>16 (n = 6)</td><td>5</td><td>1</td><td>16.67</td></tr><tr><td>17 (n = 4)</td><td>4</td><td>0</td><td>0.00</td><td>17 (n = 7)</td><td>3</td><td>4</td><td>57.14</td></tr><tr><td>18 (n = 8)</td><td>6</td><td>2</td><td>25.00</td><td>18 (n = 6)</td><td>4</td><td>2</td><td>33.33</td></tr><tr><td>19 (n = 5)</td><td>4</td><td>1</td><td>20.00</td><td>19 (n = 5)</td><td>2</td><td>3</td><td>60.00</td></tr><tr><td>20 (n = 6)</td><td>5</td><td>1</td><td>16.67</td><td>20 (n = 5)</td><td>3</td><td>2</td><td>40.00</td></tr><tr><td>21 (n = 4)</td><td>4</td><td>0</td><td>0.00</td><td>21 (n = 5)</td><td>3</td><td>2</td><td>40.00</td></tr><tr><td>22 (n = 1)</td><td>1</td><td>0</td><td>0.00</td><td>22 (n = 2)</td><td>2</td><td>0</td><td>0.00</td></tr></tbody></table>
age.

Effect of humor on smiling: the older the infant, the more they smiled at the experimenter (Model Q1-Smile: `glm - χ² (1) = 4.202, p = 0.040`). However, contrary to our expectations, infants exposed to the humorous demonstration did not smile significantly more than those exposed to the neutral demonstration (`glm - χ² (1) = 2.611, p = 0.106`). The interaction between age and the type of demonstration was not significant and was thus removed from the model.

Effect of humor on laughing: the number of infants who laughed at the humorous demonstration was significantly higher than those exposed to the neutral one (Model Q1-Laugh: `glm - χ² (1) = 6.076, p = 0.014`). However, older infants were not more inclined to laugh at the demonstration than younger ones (`glm - χ² (1) = 0.267, p = 0.606`), and the interaction between age in days and the type of demonstration was not significant.

Effect of humor on social gazes: Infants exposed to the humorous demonstration gazed at the experimenter significantly more often (Model Q1 - Gazes_F: `d = -0.869, p < 0.001`, based on 1000 permutations; Fig. 2A) and for a longer duration (Model Q1 - Gazes_D: `d = -0.507, p = 0.014`, based on 1000 permutations; Fig. 2B). However, laughing infants did not look at the experimenter more than those who did not laugh (Model Q1-Gazes_F: `d = -0.401, p = 0.645`, based on 1000 permutations), nor was the duration of their gaze towards the experimenter higher compared to non-laughing infants (Model Q1-Gazes_D: `d = -0.288, p = 0.591`, based on 1000 permutations). In both models, the interactions between the type of demonstration and whether the infant laughed or not were initially included in the full model but were not significant. Consequently, they were removed in the simplified model, and the reported statistical values come from this final model without the interaction term.

Effect of humor on Heart Rate Variability (HRV): infants exposed to the humorous demonstration did not have a significantly different HRV from those exposed to the neutral demonstration (Model Q1-HRVvsDemo: `d = -0.062, p = 0.801`, based on 1000 permutations), nor did infants who laughed have a significantly different HRV (`d = 0.107, p = 0.671`, based on 1000 permutations) from those who did not. Finally, the interaction between the type of demonstration and whether the infant laughed or not was not significant.

## 3.2. Q2- Does adding humor increase learning in infants, and does it depend on age?

Infants exposed to a humorous demonstration had significantly higher tool-use scores after the demonstration than infants exposed to a neutral demonstration (Model Q2-Tool_Use_Score: `d = -0.447, clm - χ² (1) = 4.790, p < 0.05`, Fig. 3. A). Older infants also had significantly higher tool-use scores after the demonstration than younger ones (`clm - χ² (1) = 10.290, p = 0.001`, Fig. 3. B). However,

<figure><img src="image_4.png" /></figure>

**Fig. 2.** Frequency and duration of gazes toward the experimenter according to the type of demonstration. Graphs A and B respectively represent the frequency and duration of gazes toward the experimenter during the neutral and the humorous demonstrations. In all graphs, points indicate individual averages, shaded areas represent the probability density function, boxes show the mean and standard errors (SE), and error bars represent the standard deviation (SD). Asterisks correspond to the degree of significance: \* p < 0.05, \*\* p < 0.01.
<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Effect of the type of demonstration and age on infants' tool use score after demonstration. (A) Highest tool-use score as a function of the type of demonstration (light grey = neutral; red (or dark grey for the black and white printed version) = humorous). In this box, points indicate individual averages, areas represent the probability density function, boxes show the mean, and standard errors (SE) and error bars represent the standard deviation (SD). (B) Highest tool-use score after demonstration as a function of age (in days) and type of demonstration. In these graphs, the lines were drawn using a logistic function. Each point represents infants' highest tool-use score after demonstration. The light grey points represent infants exposed to the neutral demonstration, and the red points (or dark grey) those exposed to the humorous one. Asterisks correspond to the degree of significance: \* p < 0.05, \*\* p < 0.01.</figcaption></figure>

infants who laughed did not have higher tool-use scores after the demonstration than those who did not (d = -0.0208, clm - χ² (1) = 0.066, p = 0.797). Finally, neither the interaction between the type of demonstration and the infant's age in days, nor the interaction between the type of demonstration and whether the infant laughed was significant. In fact, even if the logistic regression lines cross at 450 days (14.8 months) and 650 days (21.4 months), these interactions were not statistically significant.

The model on the binary version of the performance (success/failure) yielded the same results. Infants exposed to the humorous demonstration succeeded more often at the tool-use task compared to those exposed to the neutral one (Model Q2-Tool_Use Success: d = -0.432, glm - χ² (1) = 4.742, p < 0.05). Similarly to the first model, older infants were more likely to succeed at the task (glm - χ² (1) = 9.163, p < 0.01), and laughing infants did not perform better than those who did not (d = -0.077, glm - χ² (1) = 0.148, p = 0.700). In both models, the interaction between the type of demonstration and age in days (glm - χ² (1) = 0.293, p = 0.588), as well as the interaction between the type of demonstration and whether the infant laughed (glm - χ² (1) = 0.006, p = 0.936), were not significant thus removed from the model.

About our analyses of the relationship between humor and learning beyond the 18-month threshold, our results indicate that, whether in terms of learning scores or success considered as a binary outcome (0 for unsuccessful, 1 for successful), infants exposed to the humorous demonstration were more successful in using the rake to retrieve the out-of-reach toy than those exposed to the neutral demonstration (Model Q2-Tool_Use_Score_Over18: clm - d = -0.609, χ² (1) = 4.2876, p < 0.05; Model Q2-Tool_Use Success_Over18: d= -0.562, glm - χ² (1) = 4.7654, p < 0.05). Conversely, when considering only infants under 18 months, no significant difference was observed between conditions (Model Q2-Tool_Use_Score_Under18: clm - d = -0.425, χ² (1) = 1.2795, p = 0.2580; Model Q2-Success_Under18: glm - d = -0.431, χ² (1) = 1.19164, p = 0.2750). Across all models, laughter did not significantly influence our dependent variables.

### 3.3. Q3- Can arousal explain why humor enhances learning in infants?

Infants who succeeded at the tool-use task after demonstration had a significantly higher HRV than those who did not succeed (Model Q3-HRVvsSuccess: d = -0.499, p < 0.05, based on 1000 permutations, Fig. 4). There was no effect of the type of demonstration on the infant's HRV as already observed in the model Q1-HRVvsDemo (d = -0.062, p = 0.953, based on 1000 permutations). Finally, the interaction between the infant's success and the type of demonstration was not significant (p = 0.990, based on 1000 permutations) and was thus removed from the model.
<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Influence of the HRV during the entire demonstration on infant's performance at the tool-use task after demonstration. Points indicate individual averages, shaded areas represent the probability density function, boxes show the mean and standard errors (SE), and error bars represent the standard deviation (SD). Asterisks correspond to the degree of significance: \* *p* < 0.05.</figcaption></figure>

# 4. Discussion

The general aim of this study was to investigate the effect of humor on social learning during the second year of life and to understand its underlying mechanisms. We first asked whether adding humor to the demonstration has a consistent effect on infants' behavior (i.e., laughing, smiling, looking). We first confirmed that the humorous demonstration proposed by Esseily et al. (2015) made infants laugh, and second, we showed that this effect extends throughout the entire second year of life. We also showed that infants exposed to the humorous demonstration look at the experimenter more often and for a longer duration than those exposed to the neutral demonstration. Second, we tested whether adding humor increased learning in infants and whether this is age dependent. We found that the humorous demonstration positively enhanced infants' social learning throughout the second year of life. Third, we investigated whether arousal could explain why humor enhances learning in infants. Our analyses of the physiological processes showed that infants with the highest learning performance had the highest heart rate variabilities (HRV) during the demonstration, independently of demonstration type. We discuss these findings below and propose several mechanisms likely to explain our results, such as enhanced attention, positive emotions, arousal, and stress reduction.

## 4.1. Effect of humor on infants’ emotional reactions

Our results show that infants laughed at all ages tested, at least starting at 15 months. This observation is consistent with previous findings on the emergence of humor during development (Addyman & Addyman, 2013; Hoicka et al., 2022; Mireault et al., 2012; Mireault, Sparrow, et al., 2012; Mireault & Reddy, 2016; Soy Telli, 2023), suggesting that reacting positively to humor does not necessarily require complex cognitive capacities such as advanced theory of mind (Mireault et al., 2012; Mireault & Reddy, 2016) or a high level of representation or acting-as-if, which develops during the emergence of pretend play behavior at 18 months as suggested by McGhee (1979).

The lack of an observed effect of the experimental condition on infants' smiles is surprising, as we expected that infants in the humorous condition would be more likely to smile than those in the neutral condition. However, it is worth noting that even though the neutral demonstration was not intended to be funny for the infants, it was not entirely neutral. If the experimenter had adopted a completely neutral posture, the infant -who is not familiar with the experimenter- might have perceived the situation as uncomfortable. Furthermore, although laughter is not exclusively associated with humorous contexts (see Mazzocconi & Ginzburg, 2023, or Mireault & Reddy, 2016, who suggest that laughter can occur in various contexts for adults), smiling is an even more general behavior that can be triggered by a wide variety of factors independent of humor. It is important to remember that infants' smiles can occur in various contexts and convey a wider range of emotions compared to laughter. For example, smiles can appear during social play (Fogel
et al., 2000) or in generally positive interactions (Messinger et al., 1999) and can be associated with positive emotions not directly related to humor (Fogel et al., 2000; Messinger et al., 1999). Additionally, infants sometimes smile to express negative emotions (Dawson et al., 1997, cited in Messinger et al., 1999). In our context, infants may have found the social learning environment enjoyable, which could explain the absence of differences in smiles between the two experimental conditions. Unlike smiles, laughter may require that infants perceive the emotion as more intense and amusing, which explains why we do not observe the same results for smiles and laughter. While laughter can also occur in contexts other than humor, it is likely more closely related to the humorous condition in this case than smiles are. Finally, interestingly, we observed an effect of age on infants' smiles: older infants smiled more frequently during the demonstration, independently of whether it was humorous or neutral. This observation could be explained by the fact that older infants were more comfortable in the study setting, having more experience with new objects and strangers than younger infants (Venezia et al., 2004).

## 4.2. Effect of humor on infants' learning

Infants exposed to a humorous demonstration performed better than those exposed to a neutral one. This finding aligns with previous literature on adults (e.g., Tong & Tsung, 2020; Wanzer et al., 2010; Ziv, 1988 see also Banas et al., 2011 for review) and children (Wakshlag et al., 1981; Zillmann et al., 1980 see also Bryant & Zillmann, 1989; Martin, 2007 for reviews). Further analyses determined whether this effect differed before and after 18 months, as McGhee, (1979) hypothesis suggested that humor understanding develops along with pretend play. Although we found no significant interaction between age and the effect of humor on learning, our results showed that infants over 18 months benefited more from humorous demonstration than younger infants. Conversely, for infants under 18 months of age, no significant difference in learning performance was observed between conditions. These results suggest that age may play a role in modulating the influence of humor on learning. Another possibility is that the rake task used may be too difficult for infants younger than 18 months (e.g., Rat-Fischer et al., 2012b), which may hinder the positive effect of humor on learning. Future research should investigate these possibilities with larger samples and a model specifically designed to detect age-related effects. Several factors could explain the effect of humor, or more specifically, the effect of the humorous demonstration, as mentioned in the introduction. A primary candidate was novelty or surprise, which are known to improve learning in infants (Houillon et al., 2013; Kidd et al., 2012, 2014; Mather & Plunkett, 2012; Murayama et al., 2019; Oudeyer et al., 2016; Stahl & Feigenson, 2015, 2017; Twomey & Westermann, 2018). During our humorous demonstration, the experimenter repeatedly threw the toy over his shoulder after each retrieval, an action likely to be perceived as incongruous, novel, and surprising by most infants. We found that infants exposed to the humorous demonstration looked longer and more frequently at the experimenter compared to those in the neutral demonstration. Such social gazes are observed when infants encounter ambiguous or unexpected events (Dunn & Bremner, 2017; Mireault et al., 2014; Scherer et al., 2004; Walden et al., 2007), particularly in surprising contexts (Dunn & Bremner, 2017; Scherer et al., 2004; Walden et al., 2007). This incongruous/surprising event most likely captured the participants' attention (Mireault & Reddy, 2020). Even if the throwing action was not perceived as surprising by some infants, it remained novel since it is not something an adult usually does. According to Loizou (2005), "incongruity is a sophisticated kind of novelty" (p.43). Although surprise and novelty are closely related, their cognitive processes differ (Barto et al., 2013). However, several studies suggest that both concepts improve learning in infants in various contexts (Houillon et al., 2013; Kidd et al., 2012, 2014; Mather & Plunkett, 2012; Murayama et al., 2019; Stahl & Feigenson, 2015, 2017; Twomey & Westermann, 2018). This could explain why infants performed better when exposed to the humorous demonstration.

The second candidate factor involved in humor is laughter. This is one of the most obvious and universal manifestations of humor, as it is a loud and stereotyped behavior that is easily recognizable (Gervais & Wilson, 2005; Martin, 2007; Provine, 2017). Contrary to the study by Esseily et al. (2015), we did not find that infants who laughed during the demonstration learned better than those who did not. We propose several possible explanations for this lack of effect. First, in our study, we tested infants aged 14–22 months, whereas in the study by Esseily et al. (2015), only 18-month-old infants were tested. During their second year of life, infants significantly develop their ability to learn through observation, especially in tool use (Esseily et al., 2010; Fagard et al., 2016; Rat-Fischer et al., 2012a, 2012b, 2024). Thus, as they grow older, they become more capable of succeeding in complex tasks, such as using a rake, without laughter being necessary to facilitate their learning. This could explain why, although only 30 % of the 20–22-month-old infants laughed, 60 % of them succeeded when exposed to the humorous demonstration. Conversely, laughter in younger infants may not be sufficient to improve their learning, given the complexity of the task. For example, Rat-Fischer et al. (2012b) showed that tool-use tasks are rarely successfully completed after a neutral demonstration before the age of 18 months. Another explanation for the lack of effect of laughter could lie in the fact that most infants were tested at home (76 out of 88), while only 12 were tested at the BabyLab as in Esseily et al. (2015). Since infants are likely more comfortable in their home environment, it is possible that this setting was more conducive to learning, even for those who did not laugh. It is noteworthy that the five infants who laughed during the neutral demonstration were all tested at home, which indeed suggests greater comfort at home than at the BabyLab. However, this hypothesis deserves further exploration, given the limited number of infants tested at the BabyLab in this study. Finally, our results show that infants exposed to the humorous demonstration learned better than those exposed to the neutral demonstration, regardless of their age. Although infants under 18 months rarely achieved a success score (i.e., a score of 3 or 4), their performance was consistently better with the humorous demonstration, more frequently achieving a score of 2 (contact between the rake and the toy) than a score of 1 (no contact). This could be explained by an effect of humor, which may have motivated them more to try to reproduce the action. This link between humor and motivation is consistent with studies on adults, showing that humor can enhance motivation, even without immediate improvements in learning (Houser et al., 2007; Özdogru & McMorris, 2013). Thus, the daily repetition of the humorous demonstration could foster latent learning, allowing them to succeed earlier than in a non-humorous context. Although we did not
measure motivation or conduct longitudinal analyses, this hypothesis could be explored in future research.

## 4.3. Physiological processes involved in enhanced learning

### 4.3.1. Does humor affect arousal levels?

Our initial hypotheses stated that humor would either increase arousal (Averill, 1969; Foster et al., 2002; Marci et al., 2004; McGhee, 1983) or decrease it (Borrelli et al., 2021; Martin, 2007; McGhee, 1983; Oliveira & Arriaga, 2022), and that this change would in turn correlate with their learning performance. Our analyses failed to show such a link between HRV and the type of demonstration.

One explanation for this could be that the neutral demonstration, while intended to be neutral, was still playful and may have aroused curiosity in the infants, making it difficult to consider it completely neutral. Curiosity has been shown to drive attention, leading to an increased focus on intriguing stimuli (Oudeyer et al., 2016; Poli et al., 2020; Twomey & Westermann, 2018), which could have influenced HRV (Field & Diego, 2008; Richards & Casey, 1991; Siennicka et al., 2019). Moreover, our methodological approach may not have been able to capture the fluctuations in infants' emotions and arousal throughout the demonstrations, as we only measured the general impact of the type of demonstration on arousal. This complexity in emotional responses during social learning could have obscured any potential HRV effects. Positive emotional reactions, such as laughter and smiles, are known to be linked to HRV changes (Duarte & Pinto-Gouveia, 2017; Fogel et al., 2000; Geisler et al., 2010; Oliveira & Arriaga, 2022), which could have masked the potential effect of the type of demonstration on HRV, especially since some infants laughed and smiled during the neutral demonstration. Additionally, other emotions, such as frustration, could have also played a role. Even though fussy infants were excluded from our study, some infants may have been frustrated by waiting for the demonstrator to complete the demonstration before they could access the toy. Frustration, which is arousing (Braungart-Rieker & Stifter, 1996; Calkins et al., 2002), typically leads to a decrease in HRV (Mohammadpoor Faskhodi et al., 2023). Since the neutral demonstration was less playful than the humorous one, some infants may have become more impatient, resulting in a decrease in their HRV. An important point to consider is Porges' polyvagal theory (Porges, 2022 and 2011 cited in Heiss et al., 2021) and the work of Porges and Furman (2011). According to this theory, the level of vagal nerve myelination varies based on an individual's experiences, both during the prenatal period and postnatally. For instance, Porges (2011, as cited in Heiss et al., 2021) suggests that the quality of attachment between an infant and their parents could play a crucial role in the myelination of the vagus nerve. The more securely attached the infant is to their parents, the greater the degree of vagal myelination, which enhances the infant's capacity to regulate emotions and associated arousal. This regulation also implies inter-individual differences in arousal regulation among infants: some infants in the humor condition may have shown vagal activation, while others, who are less able to regulate their emotions, might not have exhibited this same activation. Finally, it is essential to consider a methodological factor. If this portable wristband is particularly interesting for assessing emotions outside the artificial conditions of the laboratory, it may lack the precision needed to detect subtle differences in infants. Several studies have evaluated the quality of HRV data obtained using the E4 wristband, but they do not systematically conclude that these data are always reliable. For instance, Stuyck et al. (2022) demonstrated that HRV data calculated using RMSSD exhibited an acceptable bias under minimal movement conditions. However, the authors cautioned about the degradation of data quality when participants engaged in significant movement. Similarly, Sinichi et al. (2024) observed that data quality declined sharply in ambulatory conditions, sometimes falling below the level of consumer-grade smartwatches. Furthermore, Hu et al. (2024) reported that even in static conditions, only 26.14 % of BVP signal segments were free of artifacts, with this figure dropping to 8.76 % in ambulatory conditions. An important point to consider is the precision of the E4 Empatica wristband, which has a sampling rate of 64 Hz. While this rate is standard among portable devices using BVP to assess HRV, it may not provide sufficient precision to capture subtle variations in this variable. Kwon et al. (2018) demonstrated that HRV assessments conducted with a sampling rate below 100 Hz often lack accuracy. This limitation could explain why, although infants with the best learning performance exhibited higher HRV indices, no differences were detected between the neutral and humorous conditions. These findings suggest that further research using devices with higher sampling rates is warranted to understand better the relationship between HRV, humor, and learning in infants. Another factor to consider is the potential cognitive fatigue experienced by the infants during the demonstration phase. According to Lee et al. (2022), mental fatigue can alter HRV indices, introducing noise into the data and potentially masking differences between conditions. However, we estimate that the duration of our protocol (15–18 minutes) was reasonable and is comparable to that of similar studies (Esseily et al., 2015; Rat-Fischer et al., 2012b, 2012a; Somogyi & Esseily, 2014). First, although the experimental conditions did not involve significant movements—with infants seated on their parent's laps, minimizing the risk of artifacts—we visually inspected the BVP signals, which appeared to be of good quality. Our quality analyses further confirmed the reliability of the data, with only 0.74 % outliers in the highest estimation. Another critical point is that, as previously explained, higher HRV can reflect enhanced attentional processes (Richards & Casey, 1991; Siennicka et al., 2019). In our study, although HRV analyses did not show significant differences in attentional engagement between the humorous and neutral conditions, infants with higher HRV indices consistently demonstrated better learning performance. This interpretation is further supported by behavioral observations: infants in the humorous condition looked at the experimenter more frequently and for longer durations, behaviors associated with increased attention. Together, these findings suggest a convergence between physiological and behavioral indicators of attention and reinforce our confidence in the HRV results obtained.

### 4.3.2. Effect of arousal on learning

Our analyses showed that the infants who succeeded in the tool-use task after the demonstration had higher HRV than those who failed, confirming the role of arousal in learning. This difference in HRV can be explained in two ways: either these infants experienced a greater reduction in stress during the demonstration, or they showed increased attention, or both. Given that we observed no signs of
stress in the infants and excluded overly fussy infants from our study, it is difficult to assess the impact of stress reduction. The majority of the infants who succeeded were those exposed to the humorous demonstration. These babies also looked at the experimenter more frequently and for longer durations. Since fixing their gaze on the experimenter in this context may indicate increased attention (Çetinçelik et al., 2021), we hypothesize that the increase in HRV reflects heightened attention (Richards & Casey, 1991; Siennicka et al., 2019) rather than stress reduction. However, while our results support the attention hypothesis, they do not rule out the stress reduction hypothesis: the most attentive infants might also have been the calmest during the demonstration. It is indeed difficult for an infant to be attentive when overly aroused. Some studies in adults suggest that stress can impair attention (Schwabe & Wolf, 2010).

In summary, although we did not find a significant difference in HRV depending on the type of demonstration, three results converge: (1) the infants who learned better were those exposed to the humorous demonstration, (2) these infants looked at the experimenter more frequently and for longer durations than those exposed to the neutral demonstration, suggesting they were more attentive (Çetinçelik et al., 2021), and (3) the infants who succeeded had higher HRV, which, in light of previous results, could be linked to increased attention (Richards & Casey, 1991; Siennicka et al., 2019). However, given that HRV is influenced by many factors, this could explain why we did not observe a significant difference based on the type of demonstration. Since these results point in the same direction, we hypothesize that the infants exposed to the humorous demonstration performed better because they were more attentive during the demonstration.

## 4.4. Broadening the scope: How humor might enhance various forms of learning in early childhood

It is important to note that our study is situated within the context of social learning involving a tool-use task in which infants were required to use a rake to retrieve an out-of-reach toy after observing the experimenter demonstrate its use. We selected this tool use task because it clearly measures whether infants acquire a novel skill through observation. As Rat-Fischer et al. (2024) highlight, such tasks are particularly suited to examining social learning in contexts where direct imitation of action allows researchers to determine whether genuine learning has occurred. In comparison, other problem-solving tasks, such as those involving object exploration or manipulation without tools, make it more challenging to evaluate learning precisely, as they do not always clearly distinguish between understanding and random exploration. However, it would be valuable to explore other types of tasks to broaden our understanding of learning mechanisms. For instance, Patzwald et al. (2018)'s findings indicate that infants do not simply replicate actions mechanically but use emotional cues to decide which goals to imitate. These results suggest that tasks focused on goal imitation rather than on the instrumental learning of new actions could provide complementary insights into the role of emotional cues in social learning. Looking forward, a promising avenue for future research would be to investigate whether emotional cues, such as those conveyed through humor or other forms of affective expression, facilitate not only the learning of complex actions but also the imitation of action goals. Such an approach could significantly enhance our understanding of how infants integrate emotional signals to guide their behavior and clarify the extent to which humor influences various learning processes. This potential for future research underscores the importance of our work and the impact it could have on the field of developmental psychology and social learning.

Finally, another important question arises as to whether we have truly evaluated the effect of humor on learning in our context or rather an effect related to incongruity and surprise. While our findings could suggest an influence of incongruity, several factors strongly support an interpretation in favor of humor. First, previous research (Esseily et al., 2015) has already demonstrated an effect of laughter on learning in 18-month-old infants. Second, in the current study, the absence of a laughter effect could be due to multiple factors: our broader age range, variations in pretend play engagement (see our separate analysis distinguishing infants under and over 18 months), and the fact that some infants laughed even in the neutral condition, potentially blurring the laughter-related effects. While we observed that infants exposed to the humorous condition laughed more than those in the neutral condition and also performed better, no significant effect was found on smiling. Moreover, in contrast to the findings of Esseily et al. (2015), we did not observe an effect of laughter on infants' performance in our task, which prevents us from determining whether our results are attributable to humor or incongruity. We acknowledge that further research is needed to disentangle the effect of surprise and amusement, particularly at the key age of 18 months, where previous studies have identified significant laughter-related learning effects. This would allow us to determine whether infants genuinely perceive humor in our task, meaning they can resolve the incongruity by understanding why the action of tossing the object is unexpected and funny (see Suls's incongruity resolution theory, 1972). Otherwise, their reaction may fall more under proto-humor, where incongruity evokes an amused reaction without full cognitive resolution. This distinction is important because, according to Suls et al. (1972), humor involves understanding the rules being violated. In contrast, we propose that proto-humor can occur without complete cognitive resolution of the incongruity.

## 4.5. Study's limitations

First, as mentioned in the previous section, determining whether the positive influence of the humorous demonstration on infants' learning is directly attributable to humor itself or to the incongruous nature of the demonstration—independent of its humorous aspect—remains a challenge. Another limitation concerns using the E4 Empatica bracelet, which has a limited sampling rate, as discussed in the section “Physiological Processes Involved in Enhanced Learning”. Although our analyses suggest good data quality, this equipment may have contributed to the lack of significant differences between the neutral and humorous conditions. Additionally, despite our precautions to prevent coders 1 and 2 from being exposed to our hypotheses, as described in the “Inter-rater scores and evaluation of heart rate data quality” section, these hypotheses could still have been inferred from Esseily et al. (2015). Therefore, we cannot entirely rule out the possibility of bias. Another methodological limitation concerns the neutrality of the neutral condition. Eliminating social engagement was not feasible, as an unfamiliar experimenter behaving strictly neutrally could have caused
discomfort in infants. Such discomfort may have artificially amplified the difference between conditions, leading to a false positive effect, where the humorous demonstration appears to enhance learning simply because it contrasts with a socially uncomfortable neutral demonstration. To prevent this, we maintained minimal engagement to ensure a valid comparison. A further consideration is how infants process humor and whether different levels of humor understanding influence learning. While laughter was more frequent in the humorous condition, we cannot determine whether all infants perceived the event in the same way. Some may have simply detected an unexpected and playful event, while others may have processed the incongruity in a more advanced way. Since our study defines humor broadly as playful interactions that elicit laughter and social engagement, further research is needed to explore how individual differences in humor perception affect learning outcomes. Further research should explore whether distinguishing between humor and proto-humor influences learning outcomes (see discussion in the previous paragraph). Finally, while we observe a positive effect of humor on learning, it is essential to assess whether this effect is generalizable to other forms of learning and across cultures. Although humor is present in all human societies, its perception varies culturally (Fry, 1994). As seen in adults, certain forms of humor may not be perceived similarly by infants from different cultural backgrounds, even at very young ages (see Jiang et al., 2019; Kuiper et al., 2010; Yue et al., 2016; Zhang, 2005 for cross-cultural studies on humor perception in adults). These perceptual differences could also influence infants' learning. These limitations highlight the complexity of studying humor perception in infancy and underscore the need for future research to refine our understanding of how different aspects of humor—such as incongruity, playfulness, and laughter—contribute to early learning across diverse contexts.

## 5. Conclusion

This study reveals three key findings. First, demonstrating a task to an infant with humorous intent, whether they find it funny or not, enhances their social learning. Second, adding humor to the demonstration can surprise infants, as humor is inherently incongruous. This surprise might contribute to the observed improvement in learning. One of the findings supporting this hypothesis is the observation of social gaze behaviors: infants gaze at the experimenter more frequently and for longer durations when exposed to a humorous demonstration than a neutral one. While social gazes may indicate surprise, as previously discussed, they may also stem from other social motivations. Humor often involves positive interactions with the experimenter, likely to increase the infant's expectation of a positive response. Whether these glances are due to surprise or other social motivations, these increased looks likely allowed the infants to be more attentive to the task, providing a greater learning opportunity. The attention hypothesis is partially supported by the observation that the most successful infants also exhibited the highest vagal activity (i.e., the highest HRV), suggesting that increased attention is correlated with better learning outcomes. Finally, we could not establish a direct link between vagal activity and the type of demonstration. This could be due to the nature of the neutral demonstration, which, although not humorous, may have been intriguing enough to capture the attention of some infants. By combining HRV and gazes toward the experimenter, we better understood one of the mechanisms by which humor enhances learning in infants: the effect of a humorous context in capturing their attention. Emotions are complex and encompass cognitive, behavioral, and physiological dimensions (Baciadonna et al., 2018). An integrative approach is essential to study them, exploring each dimension fully. Future research could further refine the understanding of how humor, vagal activity, and arousal interact in a learning context, building on the insights provided by this study.

### CRediT authorship contribution statement

di Stasi Romain: Writing – original draft, Software, Project administration, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Esseily Rana: Writing – review & editing, Supervision, Methodology, Funding acquisition, Conceptualization. Cerrotti Fabien: Software, Methodology, Formal analysis. Rat-Fischer Lauriane: Writing – review & editing, Validation, Supervision, Project administration, Methodology, Investigation, Funding acquisition, Conceptualization.

### Declaration of Generative AI and AI-assisted technologies in the writing process

During the preparation of this work, the author(s) used ChatGPT4o to improve the readability and clarity of the manuscript. After using this tool, the author(s) carefully reviewed and edited the content to ensure its accuracy and take full responsibility for the content of the published article.

### Funding

[Funding information will be provided at the appropriate stage, in accordance with the journal's confidentiality policy].

### Funding sources

This work was funded by a grant from the Agence National de Recherche to L. Rat-Fischer (Ref. ANR-20-CE28-0020), a grant from Université Paris Lumière (UPL) to R. Esseily, and funding from the Institut Universitaire de France (IUF) to R. Esseily.

### Declaration of Competing Interest

The authors declare that they have no conflicts of interest.
## Acknowledgements

We thank Ildiko Kiraly and Laurence Conty for their valuable suggestions concerning the analysis of physiological data. We also thank the students who participated in the recruitment of participants and the testing: Claire de Fremond, Mariem Diané. We thank the student who participated in the analyses: Pierre-Eric Beucler and Alexandra Fakhri. This work was funded by a grant from the Agence National de Recherche to L. Rat-Fischer (Ref. ANR-20-CE28-0020), a grant from Université Paris Lumière (UPL) to R. Esseily, and funding from the Institut Universitaire de France (IUF) to R. Esseily.

## Appendix A. Supporting information

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.cogdev.2025.101575.

## Data Availability

The raw data and scripts used for this article are available on the OSF platform, with the link provided in the manuscript.

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