Ryan T. Johnson and S. Marc Breedlove1
Neuroscience Program, Michigan State University, East Lansing, MI 48824-1101

In his autobiography, Mark Twain
recalls his boyhood love of abducting
live bats from a local cave and installing them in his coat pocket. Arriving home, he would trick his poor
mother, whom he described as full of trustand conﬁdence: “When I said, ‘There’s
something in my coat pocket for you,’ shewould put her hand in. But she always took
it out again, herself; I didn’t have to tellher.” Evolutionary psychologists might
posit that women have been selected fornurturing qualities, which might include
a warm, trusting nature such as afﬂicted
Twain’s mother. Selection probably would
not involve the evolution of separate genes
found only in females or in males but
rather genes that are present in both sexes
but are more likely to be expressed in one
sex than the other. Thus, there has beena good deal of interest in the idea that
hormones that are more prevalent in one
sex could modulate genes to engender sex
differences in behavior. For trust and cooperativity, one particular hormone—thepeptide oxytocin—has been implicated inseveral studies (1–3). This peptide is speciﬁcally released during childbirth, lactation, and orgasm, so it is in greater
circulation in women than in men in atleast some contexts and does seem topromote more trusting behavior (2). In
PNAS, Bos et al. (4) pinpoint anotherhormone that may modulate interpersonal
trust, but in the opposite direction: testosterone (T). They report that providing
this steroid hormone to women reduces
their interpersonal trust, speciﬁcally how
trustworthy they rated strangers based on
facial photographs.
What is especially intriguing is that theT treatment, 0.5 mg infused as liquid under the tongue, had this effect of reducing
trust only in some of the women. Whenthe 24 subjects were divided into the dozen
who were most trusting when given placebo and the dozen who were least trusting, it was only the more trusting women
who showed any response to androgens.
Women who were already skeptical intheir judgment of trustworthy faces did notchange their judgment under the inﬂuenceof T. Rather, it was the 12 women who
gave the highest ratings of trust underplacebo who became signiﬁcantly more
skeptical after T treatment.
The study is well designed, as the authors
use a double-blind, placebo-controlled
procedure, so neither the subject nor theexperimenter working with her knew
whether she had gotten T or a placebo. This
www.pnas.org/cgi/doi/10.1073/pnas.1005376107

Fig. 1. Potential model for hormonal effects on
interpersonal trust. The amygdala (center) is active
during fearful responses or detecting threat infaces, and many neurons there possess androgen
receptors, enabling them to respond to T. Bos
et al. (4) suggest that T may reduce interpersonal
trust by acting on vasopressinergic neurons in theamygdala to increase communication to brainstem systems that activate fearful responses, while
reducing communication to orbitofrontal cortex.
Oxytocin boosts interpersonal trust, perhaps byexerting opposing effects on these same systems.

is an important consideration because
other groups have found that women who
think they are receiving T, whether they
actually do or not, behave more unfairly ina bargaining task, whereas among women
who are unaware of whether they receive Tor placebo, those receiving T actually behave more fairly (5). Also, Bos et al. (4) usea within-subjects design, meaning that eachwoman rated the trustworthiness of unfamiliar faces twice, ﬁrst after receiving one
type of infusion (either T or placebo) andagain 3 d later after receiving the other typeof infusion. On the one hand, this repeatedmeasures design offers an excellent control for many other sources of variance, including heritable factors, socioeconomic
status, and prior experience among others,
so the experiment should be very sensitive toany effects of T on trust. However, precisely
because even very subtle effects of T can bedetected, these ﬁndings do not suggest we
could make accurate predictions about any
particular woman’s tendency to trust basedon her circulating levels of T. The ﬁeld haslearned this important lesson many times
before. The oldest version was the ﬁnding
that, although eliminating T entirely
through castration abolishes male copulatory behavior in many species, among gonadally intact males, there is no correlation
between circulating T and sexual vigor.
Therefore, despite the ability of exogenous

T to reduce trust in the naive women inthis study, we would not expect Lucrezia
Borgia to have had higher circulating levels
of T than Mother Teresa. Indeed, the authors found no correlation between baseline
measures of salivary T and their subjects’
tendency to rate faces as trustworthy.
These infusions of sublingual T in
women raise circulating levels of the hormone 10-fold within 15 min, but levels
should return to baseline within 2 h.
Because the exogenous hormone wasavailable for only a short time, perhaps its
effect on trust was also short-lived, butfor now, the time course of exogenous
T effects on trust is unknown. Because
endogenously produced T levels normallyvary across time, these ﬁndings also raise
the question of whether ﬂuctuating androgen secretion may normally modulate
a person’s judgment of whether to trust
people. There are circadian rhythms in
T secretion, in both men and women, so isthere also a circadian rhythm in how theyjudge trustworthiness in faces? There isalso variation in circulating T in womenacross the menstrual cycle, with a modest
peak in circulating T just a few days before
ovulation (6), the very period during
which copulation is most likely to result inpregnancy. What’s more, androgens suchas T have been reported to boost women’s
libido in several studies (7–9), including
one study using the same sublingual doseof T, which increased sexual arousal (10).
If androgens normally boost female libido,
a peak in T before ovulation makes senseto evolutionary psychologists who might
expect women to be most interested in sex
when they are most fertile. What thepresent ﬁndings suggest is that womenmight also reach their peak in skepticism
about the trustworthiness of other people,
presumably including potential mates, atabout this same point in the ovulatory cycle. Heightened skepticism about a potential mate’s trustworthiness also makes
evolutionary sense in scenarios where afather’s ongoing support is crucial forsurvival of the infant. In those circumstances, misjudging a man’s commitment
to help raise joint offspring may result incatastrophe. Despite the many studies
Author contributions: R.T.J. and S.M.B. wrote the paper.
The authors declare no conﬂict of interest.
See companion article on page 9991 in issue 22 of volume
107.
1

To whom correspondence should be addressed. E-mail:
breedsm@msu.edu.

PNAS | June 22, 2010 | vol. 107 | no. 25 | 11149–11150

COMMENTARY

Human trust: Testosterone raises suspicion

examining ﬂuctuations in cognitive functions in women across the menstrual cycle,
including changes in preferences for selfresembling faces (11) and for femininity infaces (12), it is not clear whether anyone
has examined how women’s ratings oftrustworthiness of unfamiliar faces varyacross the menstrual cycle. Do those ratings decline just before ovulation? Doesan androgen-fueled bump in skepticism
before ovulation make women more cautious when evaluating a handsome
stranger (13)?
Another question is where T may be
acting in the brain to affect trust. Bos et al.
(4) point out that the amygdala has been
implicated in many studies of social judgment, including making judgments aboutother people’s faces, and it is also a hot-spot for neurons expressing the androgen
receptors that T acts upon to regulate
gene expression (14, 15). Thus, it is possible that T may alter social judgments byacting directly on the amygdala, perhaps,
the authors suggest, by regulating thestrength of signaling between the amygdala and other brain regions implicated insocial evaluation, such as the orbitofrontal
cortex (Fig. 1). It will be interesting tosee where oxytocin and T might interact
as they exert their opposite effects on
ratings of trustworthiness in others.
For that matter, we are in the midst of
a boom of studies manipulating various
hormones to look for effects on socialjudgment, so we may expect other hormones to join the fray, which would expand
the interactions beyond just oxytocin and T.

Of course, social context and past experience also normally affect how naive weare when judging others. However, somepeople never seem to learn. Twain reportsthat his mother fell for that “surprise” in his
coat pocket more than once: “It was remarkable, the way she couldn’t learn to likeprivate bats. The more experience she had,the more she could not change her views.”
Apparently, Twain’s mother was exactly
the type of woman who might havebeneﬁtted from a judicious dose of T whenweighing her son’s words.

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11150 | www.pnas.org/cgi/doi/10.1073/pnas.1005376107

ACKNOWLEDGMENTS. Support for this work wasprovided by National Institute of Mental Health
Grant F31MH084655 (to R.T.J.), National Institutesof Health Grants R01NS028421 and R21MH082273,
and National Science Foundation Grant IOS0956831(to S.M.B.).

Johnson and Breedlove