Research Report

Do 18-Month-Olds Really Attribute Mental
States to Others? A Critical Test

Psychological Science22(7) 878­–880© The Author(s) 2011
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DOI: 10.1177/0956797611411584
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Atsushi Senju1,Victoria Southgate1, Charlotte Snape2,
Mark Leonard2, and Gergely Csibra3
1

Centre for Brain and Cognitive Development, Birkbeck, University of London; 2Department of Psychological Sciences,Birkbeck, University of London; and 3Department of Cognitive Science, Central European University

Abstract
In the research reported here, we investigated whether 18-month-olds would use their own past experience of visual accessto attribute perception and consequent beliefs to other people. Infants in this study wore either opaque blindfolds (opaquecondition) or trick blindfolds that looked opaque but were actually transparent (trick condition). Then both groups of infants
observed an actor wearing one of the same blindfolds that they themselves had experienced, while a puppet removed an
object from its location. Anticipatory eye movements revealed that infants who had experienced opaque blindfolds expected
the actor to behave in accordance with a false belief about the object’s location, but that infants who had experienced trick
blindfolds did not exhibit that expectation. Our results suggest that 18-month-olds used self-experience with the blindfolds
to assess the actor’s visual access and to update her belief state accordingly. These data constitute compelling evidence that
18-month-olds infer perceptual access and appreciate its causal role in altering the epistemic states of other people.
Keywords
theory of mind, infants, eye tracking, social cognition
Received 10/20/10; Revision accepted 3/22/11

Recent studies have indicated that human infants are able tospontaneously understand that other people’s actions aredriven by their beliefs about the real world, even when thosebeliefs are false (Buttelmann, Carpenter, & Tomasello, 2009;
Kovács, Téglás, & Endress, 2010; Scott & Baillargeon,2009; Song, Onishi, Baillargeon, & Fisher, 2008; Southgate,Chevallier, & Csibra, 2010; Southgate, Senju, & Csibra, 2007;
Surian, Caldi, & Sperber, 2007). These findings have been met
with skepticism because children at this age usually fail traditional explicit tests of false-belief understanding; some psychologists have suggested that the infants’ success in theserecent studies reflects a reliance on behavioral cues rather thanmental-state attribution (Perner & Ruffman, 2005). Such alternative explanations cannot be completely ruled out because allthese studies used behavioral cues to indicate the presence oftrue or false beliefs.
One way to attempt to resolve this controversy is to createa paradigm in which different infants would attribute differentmental states to exactly the same person in the same event. A
manipulation that could lead to such an ideal experiment was
proposed by Heyes (1998) and implemented by Meltzoff andBrooks (2008) to test 18-month-olds’ attribution of visual
access to other people. In Meltzoff and Brooks’s study, infants
were provided with either an opaque blindfold or a trick

blindfold (which appeared opaque but was in fact transparent).
Those who had worn the trick blindfold subsequently followeda blindfolded adult’s gaze, but those who had worn the opaque
blindfold did not. These results suggest that infants’ attributionof perceptual access to other people is modulated by their own
past experience of visual access.
In the study we report here, we investigated whether
18-month-olds would use their own past experience of visual
access to attribute perception and consequent beliefs to anotherperson. Infants first wore either opaque blindfolds or trick
blindfolds (which appeared to be identical to the opaque blindfolds) and subsequently watched a video sequence in which anactor wore what appeared to be the same kind of blindfold
while an object was displaced in front of her. By measuringvisual anticipation, we assessed whether the infants’ own
experience with the blindfolds influenced their predictions of
where the actor would search for the displaced object. Because
the infants could not observe either themselves or other people
wearing the blindfolds, they had no opportunity to acquire
Corresponding Author:
Atsushi Senju, Centre for Brain and Cognitive Development, Birkbeck,
University of London, Malet Street, London WC1E 7HX, United Kingdom
E-mail: a.senju@bbk.ac.uk

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Mental-State Attribution
possible associations between blindfold wearing and observable behaviors; thus, any differences in infants’ expectations
following the different blindfold experiences would necessarily be the result of attributing an unobservable mental state tothe actor.

MethodThirty-six infants (23 male, 13 female; ages 17.5 to 18.5
months, M = 18.0 months) were randomly assigned to either
an opaque (n = 18) or a trick (n = 18) condition. Twelve additional infants were tested but were excluded from the analyses,
because they did not show anticipatory saccades by the end of
the familiarization trials in the test phase (n = 4; 2 in theopaque and 2 in the trick condition), looked away fromthe stimuli at the critical moment in the test trial (n = 4; 2 in theopaque and 2 in the trick condition), did not look at one of thetwo windows (the location to which the anticipatory saccadeshould have been made) on the test trial (n = 3; 1 in the opaqueand 2 in the trick condition), or did not complete the familiarization phase (1 infant in the trick condition).
During familiarization, each infant was introduced to one
of two pairs of blindfolds, either a pair of opaque blindfolds ora pair of trick blindfolds. Each pair consisted of a broad cloth(29 cm × 29 cm) and a narrow cloth (51 cm × 6 cm); all blindfolds were made of black cloth with pink trimming around theedge to make them easily identifiable. The opaque and trick
blindfolds looked identical, but the absence of a thick middle
layer in the trick blindfolds made it possible for the wearer tosee through. Infants were presented with various pictures andtoys while a blindfold was interposed between their eyes andthe objects. On each such trial, the experimenter asked,“Where’s the [object label]?” Across trials, each infant experienced both blindfolds in the pair appropriate to that infant’scondition. This familiarization lasted for 5 min and provided
the infant with experience about the optical properties of theopaque or the trick (see-through) blindfolds, depending on thecondition to which the infant was assigned.
During the test phase, infants watched a video similar to theone used in our previous study (Southgate et al., 2007). The
video, which was presented on a Tobii 1750 eye tracker (TobiiTechnology, Danderyd, Sweden), consisted of five trials. Inthe video, a female actor sat behind a panel with two windows.
In front of the panel there were two boxes on a table. The first
two trials began with a small toy in view on one of the boxes.
Next, simultaneous illumination of the windows and soundingof a chime indicated that the actor was going to reach throughone of the windows. After a 1,750-ms delay, the actor reachedthrough a window (the left window in the first trial and theright window in the second trial) toward the toy. In the thirdand fourth trials, the sequence began when a puppet appeared,opened the lid of one of the boxes, picked up a toy (the same
one used in Trials 1 and 2) from its initial location (i.e., on thecenter of the table) and placed it into the box, closed the lid,
and left the scene. Then the windows were illuminated, andsimultaneously the chime was sounded. After a 1,750-ms

delay, the actor reached through the window corresponding tothe location of the toy. To be included in the analyses, infants
had to show anticipatory saccades (i.e., saccades to the window corresponding to the location of the toy before the actor
reached through the window) by the fourth trial. In the final(fifth) trial, which was the test trial, the puppet picked up thetoy from its initial location (which was the same as in Trials 3and 4) and placed it into the left-hand box, after which theactor put on a blindfold ostensibly previously experienced by
the infant (the narrow cloth). The puppet then removed the toyfrom the box and left the scene with it. Once the puppet had
disappeared, the actor removed the blindfold, the windows
were illuminated, and the chime was sounded. (See Video S1
in the Supplemental Material available online, which shows
the whole video sequence used in the test phase.)

Results
On the basis of the eye-tracking data for the test trial, we coded
the direction of each infant’s first saccade following the illumination of the windows. We also calculated the infant’s differential looking score (DLS) by subtracting total looking timeto the right window from total looking time to the left window
and dividing this value by the total looking time to both windows (Senju, Southgate, White, & Frith, 2009). We found that
14 of the 18 infants in the opaque condition made first saccades toward the left window (p = .031, g = .278, binomialtest), whereas only 6 of the 18 infants in the trick condition didso (p = .238, g = .167, binomial test). Thus, the location of firstgaze was influenced by infants’ experience with the blindfold(p = .018, Φ = .447, Fisher’s exact test). The DLS also differedsignificantly between the groups, t(33) = 2.668, p = .012, d =0.871, being above zero in the opaque condition, DLS = .39,t(17) = 2.32, p = .03, d = 0.549, but not in the trick condition,
DLS = −.31, t(17) = −1.53, p = .15, d = 0.360.

Discussion
The finding that infants in the two conditions had different
expectations about the actor’s action after displacement of theobject suggests that the infants used self-experience with theblindfold to assess the actor’s visual access, update her belief
state accordingly, and use this attribution for predicting heraction. Infants in the opaque condition expected the actor tosearch in the left box, which suggests that they attributed a false
belief to the actor. Infants in the trick condition, by contrast, did
not show such a bias, which suggests that they did not attributea belief about a specific location to the actor: Because the object
had been removed from the scene, the actor (if she had seen thepuppet’s actions) should have had no strong basis on which tochoose one box over the other. Our findings are inconsistent
with the proposal that infants rely solely on behavioral cues
available in the stimuli to make predictions about another person’s actions (e.g., Perner & Ruffman, 2005), because the same
video sequence was presented in both conditions. Our results
strongly suggest that 18-month-olds pay attention to other880		
people’s visual access to events and appreciate the causal role ofthat access in altering belief states.
Meltzoff and Brooks (2008) offered several alternative
explanations for how infants can use first-person experiencewith a blindfold to understand its effects on another person.
However, even the leanest interpretation of such extrapolationfrom the first to the third person entails the understandingof the unobservable mental state of seeing (Heyes, 1998;
Meltzoff & Brooks, 2008; Penn & Povinelli, 2007). Thus, atthe very least, infants in the present study encoded thatthe actor either could or could not see the displacement of theobject when she wore the blindfold, and the infants were able
to use this information to correctly anticipate the actor’s future
behavior. We are unable to determine from this study whether
or not the link between visual access and consequent behavior
that 18-month-olds are able to make is mediated by the attribution of an additional mental-state concept, such as belief.
Nevertheless, our findings provide further evidence that thecapacity for mental-state attribution in infants should not bedismissed as simply the precursor of an adult theory of mind,
relying on simpler, nonmentalistic processing. Our findings,
together with those of other studies of early belief attribution,
point to the existence of a genuine ability to attribute mental
states beginning at least at the middle of the second year of life
(Kovács et al., 2010).
Acknowledgments
We would like to thank all the participants and parents who supported our study. We acknowledge Geoff Bird, Rechele Brooks,
Helena Ribeiro, and Yura Senju for their help.

Declaration of Conflicting Interests
The authors declared that they had no conflicts of interest withrespect to their authorship or the publication of this article.

Funding
This study was supported by a research fellowship from the United
Kingdom Economic and Social Research Council (RES-063-270207) and a grant from the United Kingdom Medical ResearchCouncil (G0701484).

Senju et al.
Supplemental Material
Additional supporting information may be found at http://pss.sagepub.com/content/by/supplemental-data

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