Testosterone decreases trust in socially naïve humans
Peter A. Bosa, David Terburga, and Jack van Honka,b,1
a
Department of Psychology, Utrecht University, 3584 CS, Utrecht, The Netherlands; and bDepartment of Psychiatry and Mental Health, University of CapeTown, Groote Schuur Hospital (J-2), Observatory 7925, South Africa

Trust plays an important role in the formation and maintenance ofhuman social relationships. But trusting others is associated witha cost, given the prevalence of cheaters and deceivers in humansociety. Recent research has shown that the peptide hormone
oxytocin increases trust in humans. However, oxytocin also makes
individuals susceptible to betrayal, because under inﬂuence ofoxytocin, subjects perseverate in giving trust to others they know
are untrustworthy. Testosterone, a steroid hormone associatedwith competition and dominance, is often viewed as an inhibitor ofsociality, and may have antagonistic properties with oxytocin. The
following experiment tests this possibility in a placebo-controlled,
within-subjects design involving the administration of testosterone
to 24 female subjects. We show that compared with the placebo,
testosterone signiﬁcantly decreases interpersonal trust, and, as
further analyses established, this effect is determined by those who
give trust easily. We suggest that testosterone adaptively increasessocial vigilance in these trusting individuals to better prepare themfor competition over status and valued resources. In conclusion, ourdata provide unique insights into the hormonal regulation of human sociality by showing that testosterone downregulates interpersonal trust in an adaptive manner.
hormones| judgment | oxytocin | competition | vigilanceThe hormonal regulation of human social relationships has
recently been approached by several disciplines, including
psychology, economics, and neuroscience (1–5). Of the many important ﬁndings, the discovery that oxytocin increases interpersonal trust (3), as well as the perseveration of trust toward theuntrustworthy (2), has been of considerable interest given current
debates about the evolution of prosocial behavior in humans andother animals (1). Here we investigate whether testosterone,a hormone associated with success in competition for resourcesand dominance (6), and an alleged inhibitor of sociality (4), may
counteract the role of oxytocin in interpersonal trust. More speciﬁcally, we investigate whether, and in what way, testosterone
administration in humans decreases interpersonal trust with
unfamiliar others.
Humans are highly social and cooperative animals, and theirinterpersonal relationships importantly rely upon trust. Without
trust, suspicion spreads through human social interaction, allowing
fear to threaten relationships by instilling vigilance for treacheryand betrayal. Compared with other animals, humans are much
more likely to trust and cooperate with genetically unrelated andunfamiliar others, and these differences might constitute social
adaptations that underlie their evolutionary success (3). Trust has,
however, a downside: naïve, trusting humans run a much greaterrisk of being misguided and deceived by others. In the same waythat we have evolved capacities to help others, we have also evolved
capacities to deceive and cheat. Thus, those who are willing tobelieve what others say, or fail to probe the motivations underlying their actions, may fall prey to considerable economic andsocial costs.
Although humans are essentially social animals (7), competition
for resources also underlies the evolution of our species. It is thus
critical to understand both the evolutionary and momentto-moment dynamic between competition and trust, as bothhave played a critical role in the construction and destruction ofsociety (8).

www.pnas.org/cgi/doi/10.1073/pnas.0911700107

Recent research in humans using an economic exchange task
has shown that administration of oxytocin, a peptide hormone
known for its role in attachment and bonding (9), increases interpersonal trust in an economic game, as evidenced by higher
monetary allocations to unfamiliar others (3). Other studies havealso shown, however, that oxytocin induces perseverative trust:
following oxytocin administration, subjects continue to allocate
substantial amounts of funds to untrustworthy others, despite
being told that their opponents had repeatedly violated theirtrust (2). These ﬁndings highlight the Janus face of trust: highlevels of interpersonal trust are beneﬁcial in social interactions,but may place individuals at great personal risk (8).
Testosterone, a steroid hormone with potentially toxic consequences for human sociality (4), might counteract the maladaptive aspects of trust. Testosterone has been associated withsocial dominance and success in competition (6), and may restraininterpersonal trust to ensure social scrutiny for status and economic concerns. Indeed, testosterone levels in humans correlate
positively with ﬁnancial gain on the stock market and, as such,appear predictive of economic shrewdness (10). These ﬁndings,
however, are only correlational and thus do not clarify testosterone’s relation with interpersonal trust. To explore the possible
causal role of testosterone in trusting behavior and, in particular,
to test whether testosterone decreases interpersonal trust in
humans, we investigated the effect of a single administrationof testosterone to healthy volunteers in a trust experiment. In
a double-blind, counterbalanced design, we sublingually administered either 0.5 mg of testosterone or a placebo to 24 adultfemales on two separate days (72 h interval between treatments).
Only women participated, because the parameters (quantity andtime course) for inducing neurophysiological effects after a single
sublingual administration of 0.5 mg of testosterone have been
established in women (11), but are unknown in men (for details,
see Methods and Materials).
We used facial trustworthiness evaluations as a measure ofinterpersonal trust to control for the inherent rewarding properties of economic exchange tasks. The association of testosterone
with reward and risk-taking is very strong, and could potentiallyinterfere with the measure for trust in an economic exchangetask (3, 4). Importantly, trustworthiness judgments of nonfamiliar
faces is not only a highly validated procedure (12, 13) unconfounded by reward, but these judgments are also highly correlatedwith investments in an economic-trust task (14). A recent study
showed higher trustworthiness ratings to unfamiliar others after
oxytocin administration compared with placebo, demonstrating
the validity of using a comparable paradigm for measuring trustworthiness (15). For these reasons, the trustworthiness task is our
method of choice for measuring the effect of testosterone administration on subjects’ interpersonal trust levels.

Author contributions: P.A.B., D.T., and J.v.H. designed research; P.A.B. performed research; P.A.B. analyzed data; and P.A.B., D.T., and J.v.H. wrote the paper.
The authors declare no conﬂict of interest.
This article is a PNAS Direct Submission.
1

To whom correspondence should be addressed. E-mail: j.vanhonk@uu.nl.

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Edited by Bruce S. McEwen, The Rockefeller University, New York, NY, and approved April 23, 2010 (received for review October 9, 2009)15

20
r2 = 0.43

10
0
-10
-20
-30
-40
-20

-10

*

10

5

10

20

30

40

50

Fig. 2. Plot of the baseline trust ratings, correlated against the effect of
testosterone on trust judgments. The points on the left side of the graph,
representing subjects who displayed low interpersonal trust in baseline
measures, are clustered around zero for an effect of testosterone, indicatingthat their behavior was not affected by hormone treatment. In contrast, inthe subjects displaying high interpersonal baseline trust, represented by the
points on the right side of the graph, testosterone signiﬁcantly decreased
interpersonal trust.

In sum, the effects of testosterone on interpersonal trust are
not mediated by either mood or subjective preconceptions (17);they are pure effects of the hormone on behavior.
Furthermore, as can be seen in Materials and Methods, testosterone levels that were measured from saliva before the experiment did not predict the trustworthiness scores, and the individual
variance in these baseline testosterone levels between conditions
did also not explain the effect of testosterone administration oninterpersonal trust.
Discussion
Our ﬁndings license the conclusion that testosterone decreases
interpersonal trust, and in an apparently adaptive manner. The
hormone acted selectively on our high-trusting subjects, defensibly to down-regulate their trust to a level more advantageousin the competition for resources. Our data coincide with correlational evidence showing that higher testosterone levels predict

25
P
T

0

baseline trust

average trustworthiness rating

average trustworthiness rating

20

effect of testosterone on tustworthiness ratings

Results
In agreement with our hypothesis, we show a signiﬁcant overall
reduction in trustworthiness ratings after testosterone comparedwith placebo [F(1, 23) = 4.56, P = 0.044; Fig. 1]. This signiﬁcantreduction has an effect size of Cohen’s d = 0.36. To address
the possibility of individual differences, we applied a linear regression to examine whether subjects’ individual basic trust levels(observed from their ratings in the placebo condition) predictedtestosterone-induced changes in trustworthiness. This analysis
yielded a statistically signiﬁcant correlation (r = −0.66, P = 0.001;
Fig. 2), with individuals’ baseline trust levels explaining 43% of
the variance in the effect of testosterone on interpersonal trust.
To better qualify this effect, we applied a median split on the 24
basic-trust levels to create groups of 12 high- and 12 low-trusting
subjects. Analyses (Fig. 3) showed no effects in low-trusting subjects [F(1, 11) = 0.79, NS], but high-trusting subjects presented
a substantial reduction in interpersonal trust following testosterone administration [F(1, 11) = 10.89, P = 0.007]. The effect sizesof testosterone’s effect in the low- and high-trust group are, respectively, d = 0.08 and d = 0.92. Note that the absence of aneffect in the low-trust group is not caused by a ﬂoor effect. That is,the dependent measure could range from −100 to 100, which did
not restrict the ratings of the faces in the low-trust group, because
the average scores ranged from −13.5 to 8.5 and were normally
distributed. In sum, testosterone administration reduced interpersonal trust, but only in subjects who were generally trusting,and therefore more at risk for deceit.
To control for potential secondary mood-generated effects of
testosterone on interpersonal trust, we administered the shortened version of the proﬁle-of-mood-states (POMS) (16) before
the trustworthiness task for both the placebo and testosterone
conditions; the POMS includes the subscales tension–anxiety,depression, anger, fatigue, and vigor. Paired t tests for the subscales showed nonsigniﬁcant effects (all P’s > 0.24, two-tailed).
Furthermore, subjects were asked after the experiment to indicate
or guess the day they received testosterone. Subjects’ scores wereat chance (binomial = 0.84, two-tailed), and there was no statistically signiﬁcant relationship between the subjective guess of the
day of testosterone administration and trustworthiness ratings;
an ANOVA that used testosterone-induced change in trust as
a between-subject factor and correct-versus-incorrect guesses asa between-subject factor was not signiﬁcant [F(1, 22) = 0.18, NS].

P
T

**

20
15
10
5
0
-5
low trusting0
total group

Fig. 1. Testosterone induced a signiﬁcant decrease in interpersonal trust in
the total group (n = 24). A repeated-measures ANOVA (testosterone placebo) showed [F(1, 23) = 4.56, *P = 0.044]. White bars represent placebo (P),black bars represent testosterone (T), and error bars represent SEM.

9992 | www.pnas.org/cgi/doi/10.1073/pnas.0911700107

high trustingFig. 3. Separate repeated-measures ANOVAs for the low- and high-trusting
subject groups showed that low-trusting participants were completely unaffected by testosterone administration [F(1, 11) = 0.79, NS], whereas hightrusting participants showed a sizeable reduction in the evaluation of facial
trustworthiness [F(1, 11) = 10.89, **P = 0.007]. White bars represent placebo(P), black bars represent testosterone (T), and error bars represent SEM.

Bos et al.

Bos et al.

In conclusion, we show that testosterone plays a causal role in
reducing interpersonal trust among unfamiliar individuals. The
way in which testosterone decreases trust is consistent with itsrole in economic decision-making and competitive interactions.
The attribution of trust toward unfamiliar others was especiallydecreased in subjects who run the greatest risk of being misled byothers, that is, those who grant trust easily. Consequently, testosterone increased social vigilance in trusting humans, presumably to better prepare them for the hard-edged competition
over status and valued resources. These ﬁndings provide insight
into the hormonal regulation of human sociality by showing thatthe hormone testosterone down-regulates interpersonal trust in
an adaptive manner.
Materials and Methods
Subjects. The Ethics Committee of the University Medical Centre Utrecht
approved the protocol of our experiment wherein 24 healthy young women(mean age 20.2) participated. All women received testosterone and placebo,in randomized order, with a 72-h latency between sessions. Subjects had no(history of) psychiatric disorders or neurological or endocrine abnormalities.
They did not smoke, and used no medication other than contraceptives. We
controlled for inﬂuences of hormonal change due to menstrual cycle byonly including women who used single-phase contraceptives, and testingthem during the 3-week period they were on these contraceptives and notduring menstruation (see also ref. 34). In this 3-week contraceptive period,
menstrual-cycle inﬂuences are virtually absent. Moreover, any effects of thecontraceptives would be equal during the placebo or testosterone condition.
Substance Administration. The drug samples consisted of 0.5 mg of testosterone, 5 mg of (the carrier) cyclodextrine, 5 mg of ethanol, and 5 mL ofwater. Testosterone was omitted from the placebo samples, and both testosterone and placebo were administered sublingually. Previous experimental research established the time course of changes in blood levels of
testosterone and physiological responsiveness in typical young women after
a single sublingual administration of 0.5 mg of testosterone (11). A 10-foldincrease in total testosterone was observed 15 min after intake with testosterone levels returning to baseline within 1.5 h (11). It was also shownthat this single administration of testosterone signiﬁcantly elevated vaginal
pulse amplitude in healthy young women, which peaks around 4 h. Thus,
physiological effects after single sublingual administrations of 0.5 mg testosterone peak 2.5 h after the testosterone level in the blood has returned
to baseline. Note, that vaginal pulse amplitude, a centrally driven response
evoked by erotic material, is the only physiological measure known to possess a nonhabitual nature, thus allowing multiple measures throughout theday (11, 35). There is no method available to assess the time course of effects
of testosterone in human males, whereas in females, the present timecourse method may have unique applicability in the treatment of sexual
dysfunction (35, 36). Crucially, the reliability and generalizability of behavioral effects after a 4-h delay has been successfully established in more than
20 studies, addressing sexual, social, and emotional behaviors in young
typical women (e.g., refs. 5, 17, 35, and 37–41). Therefore, in the present
protocol, a 4-h delay between testosterone administration and measurement of mood and the trustworthiness ratings was again used.
Physiological Levels and Potential Neuroendocrine Mechanisms. The 10-foldincrease in testosterone levels that our method induces (11) seems rather
high in the light of increases seen in treatment studies. However, it is important to note that there are important differences between the chronic
treatments, which do not consider a time course of effects, and our singleadministration approach. Our single sublingual administration of 0.5 mg
testosterone produces an increase in absolute levels of testosterone in mostcases higher than that seen with chronic treatment, but within and during
a very short period. Crucially, it is conjectured by van der Made et al. (35)that this increase will not produce a proportional increase in the free fraction of testosterone; the amount of testosterone reaching the brain will bemuch less. A sex hormone binding globulin (SHBG) saturation thresholdmechanism has been postulated: The increase of testosterone into the bodywill ﬁrst bind to SHBG (and to albumin, to a smaller extend) before being
able to produce an increase in the free fraction (35). The increase of testosterone produced by the sublingual 0.5 mg administration method does
not compare with the 10-fold increase in total testosterone in the blood, butwould be large enough to pass this putative SHBG threshold, resulting ina short increase in the free testosterone fraction. This short increase, how-

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ﬁnancial gain on the stock market (10), but seem somewhat at
odds with recent ﬁndings of more fair bargaining behavior on theultimatum game after testosterone administration (17). However,the down-regulation of trust after testosterone administration atpresent was restricted to the high trusting, thus most socially naïve
half of our subject group, and may for that reason be adaptive inthe competition for status and resources. The ultimatum game,however, is an economic paradigm that measures fairness andnot trust (3); in the ultimatum game, fair offers are logicallymore often accepted. With fair offers, the proposer takes control
over the game and both players make money. In sum, more fair
offers by the proposer in the ultimatum game after testosterone
administration are also adaptive for achieving status and resources (17). Hence, the context (i.e., trusting behaviors against fairness behaviors) in the above cases obviously deﬁned the—at ﬁrst
sight—differential effects of the hormone, which ultimately havecommon ground. In many mammalian species, testosterone’s rolein social behaviors is simply conﬁned to motivating aggression incompetition for status and resources. In humans, however, thehormone seems to motivate for rational decision-making, social
scrutiny, and cleverness (17), the apparent tools for success ina modern society (4, 6, 18; but see ref. 19). Viewed from thisperspective, testosterone’s relation to risk-taking behaviors inhumans (20) might also be reevaluated, as success on the stock
market cannot be established by unrestrained risk-taking, but
requires a ﬁne-tuned grasp of the balance between ﬁnancial threat
and reward (10).
At present, there is little understanding of the neurobiologicalmechanisms by which testosterone acts on interpersonal trust.
Nonetheless, animal data have shown that the amygdala is an
important target of this hormone in the brain (21). Human neuroimaging studies support this ﬁnding by demonstrating the involvement of the human amygdala in the detection of facial threat(22), in social evaluations of faces (23), and speciﬁcally, in evaluations of trustworthiness from faces (24). Furthermore, social
evaluations of faces are impaired in patients with bilateral lesions
to the amygdala, and these patients appear more trusting in theirinteractions with strangers (13). However, the amygdala does notstand alone in the social evaluation of faces; in particular, theorbitofrontal cortex (OFC), which shows strong connectivity to
the amygdala, also plays an important role in these social processes. Moreover, the amygdala and OFC are thought to act inconcert in the regulation of many social behaviors (25, 26), andthe communication of these structures is affected by testosterone.
In humans, administration of testosterone induces rapid reductions in the functional connectivity between amygdala and OFC in
response to facial threat (27) and, conversely, seems to activate
the amygdala–brainstem defense circuit (5). Interestingly, animal
research shows that testosterone may induce amygdala–brainstem
functional connectivity by acting on the social peptide vasopressin(21, 28). Vasopressin, whose expression is regulated by testosterone (29), increases outputs of the amygdala to the brainstem byacting on distinct neuronal populations within the amygdala (28).
Oxytocin, the hormone that increases interpersonal trust (3),acts in a manner opposite to vasopressin, decreasing the outputs
to the brainstem (28, 30), but also increasing the involvement offrontal cortical regions, such as the OFC (31). Thus, testosterone
and oxytocin seem to act as hormonal antagonists at the level ofthe amygdala, providing an adaptive balance in behavioral
responses to social cues. In sum, we suggest that testosterone in
the present study may have induced a prefrontal limbic shift insocial-emotional processing by regulating peptide expression in
the amygdala (21, 28). This shift toward evolutionary older brain
regions puts the brain in a defensive or vigilant mode (28, 31–33),
and consequently may have down-regulated interpersonal trust. A
socially vigilant stance is vital for gaining and maintaining dominance or leadership, and for success in competition for resources(10, 17, 18).

ever, is responsible for cognitive, affective, and behavioral effects observed
a few hours later, which have been reported in numerous studies in human
females, as noted previously.
Generalizability of Effects to Males. The parameters (quantity and time course)
for inducing neurophysiological effects after a single sublingual administration of 0.5 mg of testosterone are thus known in women, but not in men.
Nonetheless, based on ﬁndings from our correlational research on testosterone and human social behavior in which we used males and females, we
expect the effects of testosterone administration to be similar for males andfemales (41, 42). Moreover, we have repeatedly shown that testosterone
administration in females results in more male-typical social behavior (37,43). Finally, others have shown that testosterone administration in females(17) seems to increase status-seeking behavior, and this ﬁnding agrees withcorrelations between endogenous testosterone levels and status-related
behaviors shown in men (for a review, see ref. 18) and women (44–47). This
research adds to the growing evidence that testosterone plays an importantrole in female social behavior (48–51). In sum, the relation between testosterone and social behavior apparently has much communality in humanmales and females.
Behavioral Experiment. The stimuli in the trustworthiness task consisted of 150
grayscale frontal pictures of unfamiliar faces with neutral emotional
expressions, of which 100 were adapted from Adolphs et al. (13) and 50 weretaken from the Psychological Image Collection at Stirling (PICS; http://pics.
psych.stir.ac.uk/). For our within-subjects design, we created two sets of 75
stimuli that were matched based on trustworthiness ratings in a previous
study with 36 healthy adult subjects (52).
On each test day all stimuli of one set were presented once, in random
order, both sets being counterbalanced with administration order. Pictures
were presented in the middle of a 17-inch LCD display subtending a visual
angle of ≈8 ° on a gray background. Directly below the stimulus a visual analog scale was presented ranging from (left to right) “very untrustworthy” to“neutral” to “very trustworthy.” For each stimulus, subjects were presented
with the question “How trustworthy do you think this person is?” and asked
to answer by clicking on the scale with a mouse cursor. After the response toeach trial, a button appeared with the word next; the subject’s response to
the scale could be adjusted until this button was clicked, and then it disappeared. For each presentation trial, the scale was reset to the neutral
position. The stimuli were presented using software written in E-Prime (Psychology Software Tools, Inc.). Subjects performed trustworthiness ratings
once on each set, counterbalanced with order of administration.
For data analysis, the scale positions were coded from −100 (very untrustworthy) to 0 (neutral) to +100 (very trustworthy) in steps of 1. These

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scores were averaged for each subject and both test sessions to obtain individual measures of trustfulness in testosterone and placebo conditions.
Testosterone Saliva Measurement. Salivary sampling was chosen to obtainbaseline testosterone levels. Salivary testosterone has proven to be a reliablenoninvasive biomarker in the social (42, 44, 46, 49, 51) and clinical sciences(36, 53), and has also been successfully applied in economic research (10, 54).
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[1,2,6,7-3H]-Testosterone (TRK402; Amersham Nederland BV) was used asa tracer following chromatographic veriﬁcation of its purity. The lower limit
of detection was 10 pmol/L, and interassay variation was 16.1, 11.5, and5.1% at 21, 100, and 230 pmol/L, respectively (n = 4, 5, 5). Samples of two
subjects were contaminated, showing out-of-normal range levels, andtherefore not included in further analysis.
Our analyses showed that testosterone levels measured from saliva before
administration did not differ between the testosterone and placebo administration condition in the complete group [F(1, 21) = 2.19, NS] or in thehigh-trusting subject group, which was accountable for our effects [F(1, 11) =1.27, NS]. Furthermore, differences in baseline testosterone levels between
subjects’ placebo and testosterone condition (entered as a covariate in theoriginal analyses) did not explain any variance in the effects of testosterone
administration on trust in the complete group [F(1, 20) = 0.42, NS] or in thehigh-trusting group [F(1, 10) = 1.24, NS]. Finally, low-trusting subjects compared with high-trusting subjects did not show higher baseline testosterone levels in their placebo [F(1, 20) = 0.72, NS] or testosterone condition[F(1, 20) = 0.87, NS] in the experiment. Thus, our ﬁndings on testosterone
administration cannot be attributed to variation in baseline testosterone
levels in subjects between conditions, or to differences in baseline testosterone between conditions in general. Finally, testosterone baseline levels
can also not account for our exclusive effect in the high-trust group.
ACKNOWLEDGMENTS. We thank Ralph Adolphs for the use of his stimulus
material and Marc Hauser for constructive feedback on the manuscript. We
also thank the reviewers for their comments, which helped us greatly to
improve the manuscript. Funding for this study was provided by a Utrecht
University High Potential Grant and the Hope for Depression Research
Foundation.

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