Oxytocin modulates selection of allies in
Carsten K. W. De Dreu*, Lindred L. Greer, Michel J. J. Handgraaf,
Shaul Shalvi and Gerben A. Van Kleef
Department of Psychology, University of Amsterdam, Roetersstraat 15, 1018 WB Amsterdam, The Netherlands
Inintergroupcompetition andconflict,humansbenefit from coalitionswith strong partnerswho help them
to protect their in-group and prevail over competing out-groups. Here, we link oxytocin, a neuropeptide
produced in the hypothalamus, to ally selection in intergroup competition. In a double-blind placebo-
controlled experiment, males self-administered oxytocin or placebo, and made selection decisions about
six high-threat and six low-threat targets as potential allies in intergroup competition. Males given oxytocin
rather than placebo viewed high-threat targets as more useful allies and more frequently selected them into
their team than low-threat targets.
Keywords: hormones; coalition formation; person perception; decision-making
In intergroup competition and conflict, both humans and
non-human primates engage allies to defend and promote
group interests [1–4]. Group interests are served by invit-
ing strong partners with high threat potential, who help
defend the in-group and promote its chances of prevailing
in intergroup competition and conflict [5–7]. As such,
coalescing with strong allies who can ward off rivalling
outsiders may be part of a general inclination to defend
and promote one’s own group, which may be driven by
the same neurobiological circuitries as other forms of
in-group cooperation, such as parochial altruism, social
attachment formation and parental care .
Here, we examine this possibility, and focus on oxyto-
cin, a neuropeptide that is produced in the hypothalamus
and functions as hormone and neurotransmitter. Oxytocin
has a well-established role in reproduction and pair-bond
formation [9,10], reduces psychological and physiologi-
cal stress responses, and modulates brain areas and
neural circuitries involved in the processing of fear-related
information [11,12]. For example, when given an intra-
nasal dose of oxytocin (versus placebo), humans had
reduced activation of the amygdala, and attenuated coup-
ling of the amygdala to brainstem centres responsible for
autonomic and behavioural components of fear .
Because fearful, anxiety-provoking stimuli and situ-
ations typically motivate an immediate and automatic
fight-or-flight response [14,15], and because oxytocin
has anxiolytic effects, oxytocin may allow the individual
to consider a broader variety of behavioural strategies
than immediate fight-or-fly responding [16,17]. Indeed,
individuals given oxytocin rather than placebo respond
less fearfully to angry faces , couples given intranasal
oxytocin rather than placebo manage their conflicts more
constructively  and individuals given oxytocin rather
than placebo display reduced betrayal aversion [20,21].
Interestingly, these effects emerge especially when prota-
gonists are familiar or categorized as belonging to one’s
in-group, and not when protagonists are unfamiliar or
belonging to out-groups. Intranasal administration of
oxytocin (versus placebo) increases cooperation only
when participants familiarized themselves with their inter-
action partner , when protagonists were described as
trustworthy , or when protagonists belonged to one’s
in-group . Furthermore, in free-living meerkats, per-
ipheral administration of oxytocin rather than placebo
increased an array of cooperative behaviours, including
digging, associating with pups and, most relevant here,
time spent on guard . Finally, in intergroup compe-
tition, individuals given oxytocin rather than placebo
were less cooperative towards the out-group, especially
when the out-group represented a threat to in-group
outcomes  (see also ).
These emerging insights together suggest that oxytocin
shifts the individual from being focused on self-interest
towards tending for the interests of the in-group and its
members. As noted, one way to protect and promote in-
group interests is to coalesce with strong allies that have
high rather than low threat potential, and are thus par-
ticularly instrumental in allowing the in-group to prevail
in intergroup competition and conflict. Accordingly, we
predicted that individuals given oxytocin rather than pla-
cebo would be more likely to select allies with high rather
than low threat potential.
2. MATERIAL AND METHODS
Seventy-two males received E10 (USD 13) to participate in a
study on medication and decision-making. Exclusion criteria
were medical or psychiatric illness, medication, smoking, and
drug or alcohol abuse. Participants came in groups of six, and
were seated in soundproof cubicles, randomly assigned to the
oxytocin or placebo group (double-blind, placebo-controlled
study design) and tested individually.
Participants self-administered, under experimenter super-
vision,a single intranasaldose of24 IU oxytocin (Syntocinon-
Spray Novartis; three puffs per nostril) or placebo. To avoid
any subjective effects (e.g. olfactory effects), other than
those caused by oxytocin, the placebo contained all the
* Author for correspondence (firstname.lastname@example.org).
Proc. R. Soc. B (2012) 279, 1150–1154
Published online 14 September 2011
Received 11 July 2011
Accepted 25 August 2011
This journal is q 2011 The Royal Society
active ingredients except for the neuropeptide. The placebo
was manufactured by Stichting Apothekers Haarlemse
Ziekenhuizen in coordination with the pharmacy at the
Amsterdam Medical Centre, adhering to the European
Union guidelines on Good Manufacturing Practice and
Good Clinical Practice. The placebo was produced using
the exact recipes and procedures used by Novartis Inc. to pro-
duce the carrier of Syntocinon—the synthetic analogue of
oxytocin. Placebos were delivered in the same bottles as Syn-
tocinon. In short, the only difference between the placebo and
the treatment was the absence versus presence of the active
The experimenter left and participants completed compu-
ter-guided unrelated tests. Because effects of oxytocin plateau
40 min after administration , the computer switched to
theinstructionsfor themaintasksafter30 min.Toprimeinter-
group competition, we followed the procedures and tasks used
in an earlier study (exp. 3 of ). Participants were organized
into two three-person groups and informed that they would
engage in decision-making affecting the financial earnings of
their own and the other group. Specifically, each member of
each group would make a decision that affected their personal
income, that of their fellow in-group members and that of
The decision situation was structured as a prisoner’s
dilemma [27,28]. Each participant had to choose between A
(reflecting cooperation) and B (reflecting non-cooperation),
and was informed that when he and the out-group repre-
sentative chose A (joint cooperation), each individual in each
B by out-group) would result in E0 to each in-group member
(B by participant, A by out-group) would result in E4 to each
in-group member and E0 to each out-group member; and
joint non-cooperation (B by participant, B by out-group)
resulted in E1 to each individual in each group. Thus, a
cooperative choice would benefit the out-group at the expense
of the in-group when the out-group would choose non-
cooperatively, and benefit in-group and out-group equally
when the out-group would (also) make a cooperative choice.
A non-cooperative choice would protect the in-group against
non-cooperation by the out-group, and benefit the in-group
most in case the out-group would choose cooperatively.
Accordingly, to serve one’s in-group best, participants should
Participants made three confidential decisions without
feedback, with one decision being randomly chosen for
pay-out to oneself, one’s fellow in-group members and the
out-group. On average, the instructions and decision-making
took approximately 10 min. Confirming earlier findings 
erative (M ¼ 2.39) towards the out-group than males given
placebo (M ¼ 2.88; directional t(70) ¼ 1.76, p , 0.042;
range 0 ¼ non-cooperative to 3 ¼ fully cooperative).
Following decision-making, participants read that in many
competitions, players can choose who is in their team. Then
they were shown 12 pictures, each on a new screen and ran-
domized per participant. Participants indicated for each
picture whether they would select the target for their team
(0 ¼ no; 1 ¼ yes), as well as how dangerous and how useful
the target was (both 1 ¼ not at all; 5 ¼ very much). We used
six different pictures of faces morphed into low and high
threat by Oosterhof & Todorov  (www.facegen.com;
figure 1). Although people rely on a multitude of cues when
perceiving and interpreting faces , Oosterhof & Todorov
 identified trustworthiness and dominance as the two
orthogonal dimensions that are sufficient to describe face
evaluation. Although face-trustworthiness is more sensitive
to features signalling whether the person should be avoided
or approached, dominance evaluation is more sensitive to fea-
tures signalling physical strength/weakness. Threatening
faces—the focus of the current experiment—should be both
untrustworthy (signalling that the person may have harmful
of causing harm). Although these computer faces are some-
what artificial, the advantage is that other features of the face
(e.g. symmetry) can be kept constant, thus creating optimal
conditions for a clean hypothesis test [29,31]. Indeed, partici-
pants saw high-threat targets as more dangerous (M ¼ 2.75,
s.d. ¼ 0.67) than low-threat targets (M ¼ 2.01, s.d. ¼ 0.62;
F1,70¼ 92.88, p , 0.001). The lack of effects involving treat-
ment (all F , 1) confirms earlier research showing that
oxytocin does not modulate the human ability to perceive
emotional expressions . The total selection task lasted
approximately 7 min. Upon completion of the experiment,
participants were paid and debriefed.
3. ANALYSES AND RESULTS
We averaged responses per measure across the six low-
threat and six high-threat targets, and submitted indices
to 2(treatment: oxytocin/placebo) ? 2(target: high/low
threat) ANOVA with treatment as between-subjects
factor and target as within-subjects factor. For selection,
we only found a treatment ? target interaction (F1,70¼
13.17, p , 0.001; figure 2). Levene’s test showed no het-
erogeneity of variance (F1,70¼ 0.030, p , 0.862). Simple
effects using the overall error term showed that males
given oxytocin preferred high-threat targets more than
males given placebo (F1,70¼ 5.28, p , 0.025), and
paired t-tests showed that they preferred high-threat
over low-threat targets as in-group allies (t(34) ¼ 2.69,
p , 0.011). Males given placebo showed a reversed pat-
tern: they preferred low-threat targets more than males
given oxytocin (F1,70¼ 8.13, p , 0.006), and paired
Figure 1. Examplesof(a)low-and(b)high-threatfacialmorphs
used as targets (adapted from ).
Oxytocin in intergroup conflict
C. K. W. De Dreu et al.
Proc. R. Soc. B (2012)
t-tests showed that they preferred
high-threat targets as in-group allies (t(36) ¼ 22.56,
p , 0.015).
Consistent with our expectation that high-threat allies
were selected because they are seen as more useful, we
found a treatment ? target interaction on usefulness rat-
ings (F1,70¼ 13.11, p , 0.001; figure 3). Levene’s test
for heterogeneity of variance was not significant (F1,70¼
0.056, p , 0.745). Simple main effects showed that indi-
viduals given oxytocin saw high-threat targets as more
useful allies than males given placebo (F1,70¼ 5.27, p ,
0.027); paired t-tests showed that males given oxytocin
rated high-threat targets as more useful allies than low-
threat targets (t(34) ¼ 2.82, p , 0.008). Under placebo,
this pattern was reversed: males given placebo rated
low-threat targets as more useful than those given oxyto-
cin (F1,70¼ 8.08, p , 0.005) and they saw low-threat
allies as more useful than high-threat allies (t(36) ¼
–2.49, p , 0.017). These effects remained unchanged
when we took into account whether targets were selected
or not. This analysis only revealed an additional main
effect for selection decision (F1,70¼ 52.06, p , 0.001),
showing that selected targets were rated as more useful
(M ¼ 3.11) than targets not selected (M ¼ 2.73).
were due to perceived target usefulness, we tested the
effects of treatment on selection decisionswhile controlling
(high-threat selections – low-threat selections) andfor use-
fulness (high-threat usefulness – low-threat usefulness),
and used the bootstrapping method for simple mediation
. This analysis showed that: (i) oxytocin motivated
the selection of high-threat allies (B ¼ 1.02, t ¼ 3.63, p ,
0.005); (ii) oxytocin increased usefulness ratings of high-
threat allies (B ¼ 0.536, t ¼ 3.621, p , 0.001); (iii) useful-
ness ratings positively related to selection of high-threat
targets (B ¼ 0.99, t ¼ 5.128, p , 0.001); and (iv) control-
ling for usefulness rendered the effect of treatment on ally
selection non-significant (B ¼ 0.487, t ¼ 1.83, p , 0.10;
bias and accelerated 95%CI: d ¼0.69/1.06). From these
results, it follows that effects of oxytocin on ally selection
emerge because of the effects of oxytocin on perceived
4. CONCLUSIONS AND DISCUSSION
After being primed with intergroup competition, males
given oxytocin rather than placebo perceived potential
high-threat allies as more useful and more often decided
to select them into their team than potential low-threat
allies. Together, these findings indicate that oxytocin’s
functions include in-group protection [8,24–26]. In
intergroup competition, oxytocin motivates humans to
select allies that have high threat potential and appear
aggressive rather than friendly, presumably to make
their in-group a stronger and more threatening competi-
tor to rival out-groups.
Oxytocin’s anxiolytic effects  have been linked to
reduced betrayal aversion [20,21], and may have been
involved in the presently found tendency to select high-
threat allies. However, oxytocin did not influence ratings
of target dangerousness, ruling out that oxytocin altered
danger perceptions, which in turn might have driven selec-
tion decisions. Furthermore, if reduced fear is the only
explanation, we should have found no differences in
selection decisions and usefulness judgements between
high- and low-threat allies among participants given oxyto-
cin. Clearly, additional processes are involved, and we
conjecture that these relate to the motivation to protect
in-group members against outside dangers, including
those posed by competing out-groups. New research is
needed to identify the neural circuitries involved in oxyto-
cin-modulated in-group protection. In addition, because
in the currentstudyallparticipantswere primed with inter-
group competition prior to the ally selection task, new
research is needed to examine whether current findings
eralize to ally selection in the context of other threats to in-
group functioning and survival, including those posed by
impending non-social catastrophes.
A limitation of the current study is that selection
decisions were costless and facial morphs were approxi-
mations of human faces. While this may have lowered
mundane realism, the advantage is that we have a clean
test of the hypothesis that facially communicated threat
drives our results . One possible concern is, however,
that threat covaries with facial attractiveness . It may
be difficult to see why oxytocin motivates people to prefer
unattractive over attractive allies, but at present it cannot
be excluded that perceived attractiveness explains some of
the variance in selection decisions and usefulness ratings.
New research is needed to conclusively settle this issue.
Another question awaiting future research is whether
the selection of high-threat allies is contingent upon per-
sonal interests at stake. For example, in intergroup
competition and conflict, high-threat allies may help
in-group protection and heighten the probability of win-
ning the conflict, yet high-threat allies may also alter
within-group status hierarchies and claim resources .
Figure 2. Under oxytocin, high (low)-threat targets are more
low-threat high-threat low-threat high-threat
Figure 3. Under oxytocin, high (low)-threat targets are
perceived as more (less) useful as in-group ally, range 1–5;
1152C. K. W. De Dreu et al.Oxytocin in intergroup conflict
Proc. R. Soc. B (2012)
It would be interesting to examine whether (i) high-threat
allies are more likely to be selected to the extent that
within-group status hierarchies are stable rather than
unstable, and (ii) oxytocin modulates such trade-offs
between personal and group interests in ally selection.
Given that oxytocin shifts the focus from personal to
group interest, it may be that oxytocin renders people
more tolerant of dominant newcomers claiming group
resources. But if such claiming behaviour by dominant
newcomers is perceived as a threat to in-group function-
ing and viability, it may well be that oxytocin actually
motivates resistance and defensive aggression.
Oxytocin has been portrayed as ‘trust elixir’ , as the
‘peptide of love’  and as the ‘moral molecule’ .
Whereas such labels suggest that oxytocin in humans
promotes indiscriminate pro-sociality, generosity and hon-
esty, current findings do not fit such an interpretation.
Individuals given oxytocin did not select more allies—
they were not generally more inclusive than participants
tial allies as less dangerous than those given placebo,
and they did not see others as generally more useful. Put
differently, we obtained no evidence that oxytocin indis-
criminately alters social cognition and judgement, or
tendencies towards social inclusion. Instead, we found
that under oxytocin, humans perceive high-threat allies as
more useful than low-threat allies, and therefore more
often select high-threat than low-threat allies as partners.
Current results better fit the emerging insight that the
functions of oxytocin are adaptive , and its effects con-
tingent upon context . We conjecture that oxytocin’s
functions extend beyond reproduction and pair-bond for-
mation to include the motivation to tend and defend
group life, including the willingness to ally with partners
that increase the in-group’s threat potential.
Financial support was granted by The Netherlands Science
Foundation (400-06-098). The authors declare no competing
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Proc. R. Soc. B (2012)