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Oxytocin modulates cooperation within and competition between groups:
An integrative review and research agenda☆
Carsten K.W. De Dreu
Department of Psychology, University of Amsterdam, Roetersstraat 15, 1018 WB Amsterdam, The Netherlands
a b s t r a c t a r t i c l ei n f o
Received 16 June 2011
Revised 1 December 2011
Accepted 2 December 2011
Available online 20 December 2011
Theory of mind
The author reviews evidence that hypothalamic release (or infusion) of the neuropeptide oxytocin modulates
the regulation of cooperation and conflict among humans because of three reasons. First, oxytocin enables
social categorization of others into in-group versus out-group. Second, oxytocin dampens amygdala activity
and enables the development of trust. Third, and finally, oxytocin up-regulates neural circuitries (e.g., inferior
frontal gyrus, ventromedial prefrontalcortex, caudatenucleus) involved in empathy and other-concern. Consistent
with an evolutionary perspective on the functionality of cooperation, it is concluded that oxytocin-motivated coop-
eration is mostly parochial—it motivates (i) in-group favoritism, (ii) cooperation towards in-group but not out-
group members, and (iii) defense-motivated non-cooperation towards threatening outsiders. Thus, in addition to
its well-known role in reproduction and pair–bond formation, oxytocin's primary functions include in-group
“tend-and-defend.” This review concludes with avenues for new research on oxytocin's functions in within-group
cooperation and between-group competition.
This article is part of a Special Issue entitled Oxytocin, Vasopressin, and Social Behavior.
© 2011 Elsevier Inc. All rights reserved.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Why and how oxytocin modulates cooperation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Social categorization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Trust and other-concern . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Self-sustaining spiral of (in-group) cooperation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Oxytocin down-regulates fear-signaling and enables trust . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Evidence that oxytocin-modulated trust is parochial . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Oxytocin up-regulates empathic other-concern . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Oxytocin motivates in-group favoritism and parochial cooperation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Oxytocin motivates non-cooperation towards rivaling out-groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conclusions and avenues for future research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Opening the black box . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
From social recognition to social discrimination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
From parochial cooperation to in-group maintenance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
From in-group love to intergroup tension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Coda . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Hormones and Behavior 61 (2012) 419–428
☆ Preparation of this work was supported by the UvA-FMG Research Priority Grant on Affect Regulation, and Grant 432-08-002 of The Netherlands Science Foundation. The
author declares no conflict of interest.
E-mail address: firstname.lastname@example.org.
0018-506X/$ – see front matter © 2011 Elsevier Inc. All rights reserved.
Contents lists available at SciVerse ScienceDirect
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journal homepage: www.elsevier.com/locate/yhbeh
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“Groupswitha greaternumber of courageous,sympathetic and faith-
aid and defend each other … would spread and be victorious over
other tribes” (Darwin, 1873, p. 156).
Humans have an extraordinary capacity to create and promote
social life: They form long-term attachments to close others (Bowlby,
1973; Mikulincer and Shaver, 2007), empathize with others (Batson,
1998), and sacrifice their immediate self-interests to promote the
overarching interests of the groups and communities they belong to
(Dawes, 1980; Penner et al., 2005; Komorita and Parks, 1995; Ostrom,
1998). Humans may even sacrifice their lives, for example as soldiers
or suicide bombers, to promote a common cause, or to protect their
group against enemy forces (Arrow, 2007; Kruglanski et al., 2009;
Stouffer et al., 1949; Tobena, 2009). Together, these and related ob-
servations fit Darwin's insight that, throughout evolution, pro-social
behavior served individual survival and prosperity: Through self-
sacrifice and cooperation humans promote the functioning of their
in-group that provides for levels of security and prosperity well
beyond what individuals could possibly achieve alone (Alexander,
1990; Shinada et al., 2004; Trivers, 1985).
If self-sacrifice and cooperation has survival functionality, the
human brain must have evolved to motivate such pro-social tendencies,
and we should be able to identify neurobiological systems and circuit-
ries that modulate cooperative decision making (Rilling and Sanfey,
2011). Indeed, studies in neuroendocrinology and social neuroscience
provide mounting evidence for an intimate link between pro-sociality
and the evolutionary ancient and highly preserved neuropeptide
oxytocin (Bos et al., 2012; Donaldson and Young, 2008; Madden and
Clutton-Brock, 2011; Ross and Young, 2009). Oxytocin is a nine
amino-acid, cyclic neuropeptide produced in the brain, and has a
well-established role in reproduction and pair–bond formation (e.g.,
Carter et al., 2008). Recent work suggests, however, that its functions
are broader, and that oxytocin plays a critical role also in the formation
and maintenance of social groups more generally, including non-
cooperation towards rivaling out-groups.
Here I review these and related insights, and the evidence from
studies focusing on acute effects of oxytocin in healthy volunteers.1
The first section of this review examines how hypothalamic release
of oxytocin modulates three critical functions underlying parochial
cooperation, namely (i) social categorization of others into in-group
versus out-group; (ii) in-group trust, and (iii) concern for (members
of) the in-group. The second and third sections review the research
evidence for these three propositions and additionally reveal that
oxytocin motivates (iv) non-cooperation towards rivaling out-groups
especially when out-groups threaten one's in-group. The final section
summarizes the main conclusions and identifies avenues for new
research into the link between oxytocin, and the regulation of intra-
and intergroup relations.
Why and how oxytocin modulates cooperation
Harvard economist and Nobel Laureate Thomas Schelling (1960/
1980; also Deutsch, 1973; Luce and Raiffa, 1957) observed that most
group settings contain incentives to compete with others so as to de-
fend and promote immediate self-interest (e.g., personal gains and sta-
tus) as well as incentives to cooperate with others so as to establish
well-functioning long-term relationships that provide greater benefit
to all than mutual competition. Put differently, social systems like fam-
ilies, small groups, communities and work organizations all share two
basic properties: (i) each individual member serves personal interests
best by opting for non-cooperation; and (ii) when all members opt
for non-cooperation, each is worse off than when all had opted for
cooperation (Bornstein, 2003; Dawes, 1980; De Dreu et al., 2008;
Komorita and Parks, 1995).
Given these structural features of most social systems, individuals
would beexpectedtosacrificeimmediate self-interestandtocooperate
only when they have, first of all, trust—the positive expectation that
others will reciprocate one's cooperative effort (Coombs, 1973; Berg
et al., 1995; Pruitt and Kimmel, 1977).2Second, individuals cooperate
to the extent that they have other-concern—the care for others' out-
comes, interests, and well-being (Carnevale and Pruitt, 1992; De Dreu
et al., 2000; De Dreu, 2010; Komorita and Parks, 1995; Weber et al.,
2004; Pruitt and Kimmel, 1977). Other-concern includes a desire for
fairness and to benefit the collective rather than oneself (Bohnet and
Frey, 1999; De Dreu et al., 2000).
Ingeneral, people havestronger trust in and concernfor others they
like and feel close to (Wu et al., 2011), with whom they share com-
mon goals and values (Burnham, 2003), with whom they anticipate fu-
ture interaction, or with whom they share group membership (for
reviews, e.g., Batson, 1998; De Dreu, 2010; De Dreu et al., 2000, 2008;
Komorita and Parks, 1995; Pruitt and Kimmel, 1977). Thus, cooperation
is primarily parochial—people more readily cooperate with members of
their in-group than with members of more or less rivaling out-groups
(Bernhard et al., 2006; Brewer and Kramer, 1986; Choi and Bowles,
2007a, 2007b; Darwin, 1873; Hammond and Axelrod, 2006; Polzer,
1996; Wit and Kerr, 2002; Wildschut et al., 2003).
Because parochialism serves individual and group survival both in
ancestral and contemporary societies (Darwin, 1873), it may have its
root cause in evolved neurobiological circuitries. Fig. 1 shows that
oxytocin is a likely neurohormonal modulator of parochialism because
of three reasons: (i) it facilitates social categorization of others as in-
group versus not in-group, (ii) it enables trust to develop, and (iii) it
up-regulates other-concern. These three critical functions are now
briefly elaborated upon, and in the next sections evaluated in light of
the existing research evidence.
Humans have evolved capacity to quickly distinguish others into
in-group versus out-group (Allport, 1954; Brewer and Kramer,
1986; Kurzban et al., 2001; Mahajan et al., 2011), and such social dis-
crimination may be modulated by oxytocin. Specifically, when a bond
is formed between a mother and her offspring, or between sexual
partners in monogamous species, an olfactory memory is forged in
the olfactory bulb. In mothers, oxytocin released in the brain during
parturition helps to establish the olfactory signatures of the offspring
as memorable (Ferguson et al., 2000; Brennan and Kendrick, 2006;
Tobin et al., 2010). Male rodents engineered to lack (fore-brain) oxyto-
cin receptors no longer discriminated between familiar and unfamiliar
females—compared to normal rodents, these knock-out rodents spent
equal time investigating female rodents with whom they had shared
a cage for several days, but less time investigating novel females who
they met for the first time (Macbeth et al., 2009; also see Ferguson
et al., 2000, 2002).
With regard to social categorization in humans, evidence is exceed-
ingly sparse. However, some first indications derive from a study by
Rimmele et al. (2009) who gave participants intranasal oxytocin or
placebo and showed them a series of pictures of faces. One day later,
1Because of space constraints, studies on the relationship between polymorphism in
oxytocin receptor genes and social behavior (e.g. Apicella et al., 2010; Israel et al.,
2009), and empathy (Roderigues et al., 2009; Tost et al., 2010) are excluded from this
review, as are studies on the relationship between oxytocin and autism spectrum dis-
orders (e.g., Andari et al., 2010).
2Trust as defined here is inversely related to fear of being exploited by others, and
betrayal aversion (Baumgartner et al., 2008; De Dreu, 2010; De Dreu et al., in press).
There are other, broader and multifaceted definitions of trust discussed elsewhere
(e.g., Rousseau et al., 1998).
C.K.W. De Dreu / Hormones and Behavior 61 (2012) 419–428
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nition task, where they had to indicate for a series of pictures whether
they had seen the picture before. Interestingly, results showed that
participants who learned pictures of faces under oxytocin performed
better one day later than those who had learned the pictures under
placebo. Effects were particularly strong on measures of familiarity
and weak on measures of mere recollection — oxytocin makes a face
in memory more familiar.
Familiarity in turn is a key driver of social categorization (Mateo,
2004; Tang-Martinez, 2001), with familiar others being more likely
to be categorized as in-group than unfamiliar others (Castelli and
Zogmeister, 2000; Castelli et al., 2004). However, research is needed
to test the claim that hypothalamic release of infusion of oxytocin
mediates the mere categorization of others into in-group and out-
group (also see Kavaliers and Choleris, 2011).
Trust and other-concern
People develop trust in, and concern for members of their in-group,
and these tendencies are modulated by two distinct neurological
systems. The development of trust, and the reduction of fear and vig-
ilance, is mediated by amygdala activity (Baumgartner et al., 2009;
LeDoux, 2000; Phelps, 2006), and in humans, amygdala activity is
lower when exposed to in-group rather than out-group targets (Beer
et al., 2008; Hein et al., 2010; Van Bavel et al., 2008). Hypothalamic re-
lease of oxytocin dampens neural circuitries involved in fear-signaling
and the regulation of distress (e.g., amygdala; hypothalamic–pituitary–
adrenal axis; LeDoux, 2000; Phelps, 2006). Accordingly, as shown
in Fig. 1, hypothalamic release of oxytocin promotes the development
of trust in in-group others (and may counter-act the tendency to fear
out-group others). Research evidence for this proposition is reviewed
Concern for others is mediated by neural circuitries involved in
empathy and reward processing, including the inferior frontal gyrus
and the inferior parietal lobe necessary for emotion recognition and
contagion, as well as the evolutionary and phylogenetically more
recent ventromedial prefrontal cortex, and the temporoparietal junc-
tion and the medial temporal lobe (Baumgartner et al., 2009; Decety
and Chaminade, 2003; Halko et al., 2009; Keuken et al., 2011;
Shamay-Tsoory, 2011; Schnell et al., 2011).3Empathy refers to the
individual's capacity to experience affective reactions to the observed
or anticipated experiences of another individual, and to take another
person's perspective (Batson, 1998; Frith and Singer, 2008). The neural
circuitries involved in empathy are activated more when individuals
are exposed to in-group rather than out-group members (Harris and
Fiske, 2007; Hein et al., 2010; Van Bavel et al., 2008) and, importantly,
thesecircuitries are up-regulated byhypothalamic release(or infusion)
of oxytocin.4Accordingly, as shown in Fig. 1, hypothalamic release of
oxytocin promotes the development of other-concern for in-group
others (and may counter-act the tendency to withhold other-concern
from out-group others).
Self-sustaining spiral of (in-group) cooperation
The combination of increased trust and other-concern enables
individuals toengageincooperationand tobenefit others ata personal
cost. Importantly, such self-sacrifice may include tendencies to risk
oneself to protect others (e.g., by aggressing against threatening
3In addition to empathy, other-concern may be driven by strategic considerations.
For example, when the individual expects future interaction with the protagonist
and sees the protagonist as relatively powerful, the individual may be motivated to
care of other's outcomes to ensure personal interests in the longer run, or to avoid
the protagonist becomes angry and lashes out. Such strategic considerations for
other-concern are not further discussed here (see e.g., Bazerman et al., 2000; De Dreu
et al., 2000).
4A distinction has been made between affective and cognitive empathy (e.g., Frith
and Singer, 2008), with oxytocin being involved primarily in affective empathy
through its influence on the inferior frontal gyrus and the inferior frontal lobe, and in-
directly in cognitive empathy through its interaction with dopaminergic circuitries
(striatum, ventromedial prefrontal cortex). For a review and discussion, see Shamay-
(Infusion) of Oxytocin
Cooperation by Other
Fig. 1. Hypothalamic release of oxytocin motivates cooperation with in-group others because of lower distress and higher other-concern. Note: Arrows reflect causal pathways.
Up-regulating (down-regulating) effects are denoted with positive (negative) signs.
C.K.W. De Dreu / Hormones and Behavior 61 (2012) 419–428
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outsiders). From Fig. 1 it follows that these tendencies emerge espe-
cially when protagonistsarecategorizedasin-group,andthathypotha-
lamic release (or infusion) of oxytocin amplifies such parochial
here in light of the available evidence.
cooperation (Axelrod, 1984; De Dreu, 2010; Pruitt and Kimmel, 1977),
and there is some evidence that displays of trust and cooperation by
others, especially familiar others like parents and intimate partners,
promotes hypothalamic release of oxytocin (e.g., Ditzen et al., 2007;
Feldman et al., 2010; Gordon et al., 2010; Holt-Lunstad et al., 2008;
Morhenn et al., 2008; Uvnas-Moberg, 1998; Zak et al., 2005). In addi-
tion, intranasal oxytocin augments activity in the caudate nucleus fol-
lowing reciprocated cooperation, suggesting that oxytocin enhances
the reward from reciprocated cooperation and facilitates the learning
that the protagonist can be trusted (Rilling et al., 2012). In all, there is
reason to assume a spiral of oxytocin-mediated and mutually reinfor-
cing cooperation among the individual and his or her protagonist(s)
and that such positive spirals are more likely when protagonists
are categorized as in-group rather than out-group.
Oxytocin down-regulates fear-signaling and enables trust
Produced in the hypothalamus and functioning as both hormone
and neurotransmitter, oxytocin's targets are widespread and include
theamygdala, hippocampus, and regions of the spinal cord that regulate
the parasympathetic branch of the autonomic nervous system (Ludwig
and Leng, 2006; Neumann, 2008; Roderigues et al., 2009). Oxytocin
interacts with the hypothalamic–pituitary–adrenal axis to attenuate
stress responses, and this has a pervasive influence throughout both
the body and the brain (Neumann, 2008; Roderigues and Sapolsky,
2009). Specifically, oxytocin reduces cortisol levels after exposure
to stressors (Heinrichs et al., 2003), inhibits cardiovascular stress re-
sponses (Uvnas-Moberg, 1998), and modulates brain areas and neural
circuitries involved in the processing of fear-related information. For
example, in a neuro-imaging study, Kirsch et al. (2005) gave partici-
pants either an intranasal dose of oxytocin or placebo, and showed
them fearful or neutral stimuli while brain activity was being regis-
tered. Results showed that oxytocin reduced the activation of the
amygdala and attenuated its coupling to brainstem centers responsi-
ble for autonomic and behavioral components of fear (see also
Petrovic et al., 2008).
Fearful, anxiety-provoking stimuli and situations typically moti-
vate an immediate and automatic fight-or-flight response (LeDoux,
2000; Phelps, 2006). However, because of its anxiolytic effects at
both the physiological and neurological level, oxytocin may allow
the individual to consider alternatives to fight-or-flight, including
pro-social approach (Lim and Young, 2006; Heinrichs et al., 2009;
Taylor et al., 2000). Indeed, individuals given oxytocin rather than place-
bo respond less fearfully to angry faces (Evans et al., 2010), and couples
given intranasal oxytocin rather than placebo engage in more construc-
tive discussions of relationship conflicts (Ditzen et al., 2009).
to consider pro-social approach derives from studies infusing exogenous
oxytocin (versus placebo) before participants make monetary donations
to a protagonist in a (variation of) the so-called trust-game (Berg
et al., 1995). The Trust Game involves two participants, each with a
$10 endowment. Player 1 (henceforth Investor) is asked to choose an
amount tobetransferred to Player2 (henceforthTrustee). As explained
to both participants, the amount transferred by Investor is tripled on
the way to Trustee. Trustee is then asked to choose an amount to
back-transfer to Investor. The back-transfer is not tripled, and the final
amounts in the two accounts are paid. In this game, the main reason
for Investors to transfer is trust: the positive expectation that Trustee
will reciprocate and transfer back. To Trustees, the main reason to
back-transfer is other-concern.
A pioneering study by Kosfeld et al. (2005) showed that oxytocin
influences Investor's transfer. Healthy males self-administered through
nasal spray either 24 IU placebo or oxytocin and, after 45 min, engaged
bo group transferred the maximum amount, and the average transfer
was also somewhat higher among participants given oxytocin rather
than placebo. A follow-up study by Baumgartner et al. (2008) consid-
ered Investor transfers both before and after they learned about their
Trustee's back transfer (set at a constant 50% back transfer in 6 trials
of the trust game). Intranasal oxytocin did not influence pre-feedback
transfers by Investors. However, after Investors learned that Trustees
back-transferredin50%of thetrials, Investors givenoxytocincontinued
to make substantial transfers whereas those given placebo significantly
decreased their transfers. Thus, oxytocin inoculated betrayal aversion
among Investors (also see Rilling et al., 2012).5
The effects of oxytocin on trust and betrayal aversion not only per-
tain to monetary, but also social decisions. For example, Keri and Kiss
(2011) observed higher levels of blood plasma oxytocin among partic-
ipants when they released an important secret to their experimenter,
compared to when they exchanged a neutral message. Mikolajczak
et al. (2010a) gave male participants either oxytocin or placebo and
asked them to complete a sexual fantasies questionnaire that inquired
about purposefully very intimate sexual fantasies typically not shared
with strangers. Forty minutes later participants were asked to put
their sexual fantasies questionnaire in an envelope and to hand it
to the experimenter. Participants were free to seal the envelope or
not, and whether the envelope was sealed was taken as reflecting
(dis)trust in the experimenter (i.e., that he might violate his instruc-
tions not to look at the participant's answers). Sixty percent of the par-
ticipants given oxytocin did not seal their envelope, whereas only 3% of
the participants given placebo left their envelope open. This sug-
gests that oxytocin increases trust not only in financial but also in
Evidence that oxytocin-modulated trust is parochial
Research findings thus far are consistent with the proposition in
Fig. 1 that because oxytocin dampens amygdala activity, it enables
trust to develop. An important qualifier in Fig. 1 is that these effects
support for this proposition derives from studies showing stronger effect
of oxytocin on trust and cooperation when protagonists are (displayed
as) benign and trustworthy, or are familiar rather than unfamiliar
to the individual (with familiarity promoting in-group categoriza-
tion; Castelli et al., 2004). Specifically,whentrusteeswereanonymous
strangers, effects of oxytocin on average Investor transfer were sta-
Baumgartner et al., 2008). Furthermore, in both the Mikolajczak et al.
(2010a) and the Keri and Kiss (2011) studies did the experimenter
serve as protagonist and perhaps experimenters are perceived as more
benign than anonymous strangers. Using the trust game, Mikolajczak
et al. (2010b) indeed showed that Investors given oxytocin rather than
placebo transferred more money to Trustees that were described as
a pro-social person (e.g., studying philosophy, practicing first aid).
When Trustees were described in less pro-social terms (e.g., studying
marketing, practicing violent combat sports), oxytocin no longer
influenced Investor behavior. Finally, Declerck et al. (2010) studied
cooperation in an Assurance Game where the only reason not to co-
operate is distrust (Colman, 2003; Bornstein, 2003), and found that
individuals given oxytocin expected more cooperation and cooperated
5An important side-result in both the Kosfeld et al. and the Baumgartner et al. stud-
ies is that when Investors were told that transfer-back decisions would be determined
by a random mechanism, no effects of oxytocin were found. This rules out that trust
and trust betrayal responses merely reflect risk-tolerance and that oxytocin modulates
risk-tolerance rather than trust (see Kosfeld et al., 2005).
C.K.W. De Dreu / Hormones and Behavior 61 (2012) 419–428
Author's personal copy
more but only when they had familiarized themselves with their
protagonist. Absent such positive prior interaction, oxytocin
actually led to less cooperation than placebo.
Taken together, oxytocin reduces fear and anxiety at both the neu-
ral and the behavioral level. It seems that especially with familiar and
relatively benign protagonists this translates in greater trust, and
more cooperation. When protagonists were described in less benign
terms, or when prior interaction was absent and protagonists were
fully anonymous, oxytocin had no or even negative effects on trust
and cooperation. These findings provide indirect support for the
proposition in Fig. 1 that oxytocin modulates trust and cooperation
towards in-group members, and not towards out-groups. More direct
evidence for this proposition will be discussed in the next section,
after we have reviewed work indicating that oxytocin (also) modulates
cooperation and other-concern partly because it up-regulates neural
and behavioral expression of empathy.
Oxytocin up-regulates empathic other-concern
Other-concern more readily develops for others that are in-group,
rather than out-group (Hein et al., 2010; Van der Schalk et al., 2011;
Wu et al., 2011). Furthermore, people feel empathic and develop
high other-concern when in-group others, with whom one seeks to
align and cooperate, are perceived to be sad, distressed, embarrassed,
disappointed, or guilty (Batson, 1998; Van Kleef et al., 2010). Interest-
ingly, there is reason to assume empathy and other-concern is mediated
by oxytocin. In women exposed to infant crying, intranasal oxytocin
modulates activity in the inferior frontal gyrus (Riem et al., 2011),
fathers given oxytocin rather than placebo are more stimulating of
their toddler's exploration and showed less hostility (Naber et al.,
2010), and in males exposed to biological motion, intranasal oxytocin
modulated neural circuitries involved in affective perspective taking
(Keri and Benedek, 2009; Perry et al., 2010; also see Gallese et al.,
2004). Other studies showed that participants given oxytocin rather
than placebo have increased sensitivity to other's fear (Fischer-Shofty
et al., 2010), empathize more for persons depicted in emotionally
charged situations (Hurlemann et al., 2010; but see Singer et al., 2008),
rate images of neutral faces as more trustworthy and attractive
by others (Domes et al., 2007; also see Roderigues et al., 2009).
Consistent with these findings, there is evidence also that oxytocin
modulates behavioral expressions of other-concern and motivates
fairness. Following trust-signaling by their Investors, Trustees' back-
transfers were positively correlated with oxytocin levels in their blood
plasma (Zak et al., 2005; Morhenn et al., 2008). Likewise, individuals
given oxytocin rather than placebo were more willing to interact again
with others who previously included the participant in a ball-tossing
game, but not with others who previously ostracized the individual
(Alvares et al., 2010; also see Andari et al., 2010). Finally, individuals
given oxytocin rather than placebo make more generous offers in
two-party ultimatum bargaining where a proposer is asked to pro-
pose a split of $10 into P for the responder and $10-P for himself
(Zak et al., 2007). The responder can accept the proposal in which
case payments were made, or the responder can reject the proposal,
in which case the proposer and the responder are paid nothing.6
However, whereas proposers in the Zak et al. study were more gener-
ous when given oxytocin, responders' punishment thresholds (i.e., the
level at which they would reject offers) were unaffected by oxytocin.
Also, when asked to split $10 into D for the other person and $10-D
for oneself (the other person had no decision to make), oxytocin had
no effect on the amount offered (Zak et al., 2007).
ral and behavioral expressions of other-concern. It should be noted,
however, that other-concern is a multifaceted construct that includes
a concern for other's outcomes and interests, a desire for fairness, and
a desire for mutual rather than personal benefit (see Footnotes 3 and
4). Targeted research is needed to increase our understanding of the
specific facets of other-concern that are influenced by oxytocin. For
example, in some of the above works (e.g., Zak et al., 2007), oxytocin
seemed to influenced fairness considerations (as measured through
offers in the Ultimatum Game) but not concern for other's outcomes
(as measured through offers in the Dictator Game). Such new research
could also test for alternative interpretations, in that some findings
may reflect increased adherence to pro-social norms, or strategic
maneuvering, rather than “genuine” other-concern.
Oxytocin motivates in-group favoritism and parochial cooperation
itant cooperation emerge especially when protagonists are categorized
as in-group, and not when they are categorized as out-group. This
proposition was tested in De Dreu et al. (2011a). Based on the
well-established finding that humans positively value and evaluate
in-group members (henceforth “in-group favoritism”), and sometimes
negatively value and evaluate out-group members (henceforth “out-
group derogation;” Dovidio and Gaertner, 2010; Yzerbyt and Demoulin,
2010), De Dreu et al. hypothesized that oxytocin motivates in-group
favoritism and, perhaps, out-group derogation. In five experiments,
indigenous Dutch males received oxytocin or placebo in a double-
blind, randomized between-subjects design and, after 40 min, were
exposed to images of in-group targets (Dutch males) or out-group
targets (males from Middle-Eastern descent or, in other experiments,
Germans). Using different methods, De Dreu et al. indeed found that
oxytocin motivated stronger in-group favoritism — in-group targets
elicited stronger positive and more benevolent associations when
participants received oxytocin rather than placebo. Weak and possibly
unreliable effects were found on out-group derogation.
Two aspects of these results are noteworthy. First, and consistent
with Fig. 1, these data suggest that oxytocin up-regulates empathy
for in-group members but does not create more benevolent views
of others generally, or expanded social categories. If that would
have been the case, the data should have shown not only increased
in-group favoritism but also more positive views and evaluations of
out-group members. In none of the experiments reported in De
Dreu et al. (2011a) this was the case (also see Chen et al., 2011; De
Dreu et al., 2011b). Second, these results imply that oxytocin moti-
vates parochial cooperation with in-group members, and not with
individuals belonging to out-groups. Two experiments reported in
De Dreu et al. (2010) tested this implication directly. Males self-
administered through nasal spray 24 IU oxytocin or placebo. Forty
minutes later they were, on the basis of a trivial criterion, categorized
into two three-person groups, and engaged in an Intergroup Prisoner's
Dilemma—Maximizing Differences Game (IPD-MD: Halevy et al.,
2008). Each individual received €10 and was allowed to invest all or
part of it in a within-group pool, and in a between-group pool. Within-
group pool investments were multiplied by 1.5 and equally distributed
among in-group members (i.e., for each Euro invested, each member
received €0.50). Within-group pool investments thus reflect in-group
love—the motivation to benefit in-group members at a cost to oneself.
Between-group pool investments had the same consequence to the
in-group as within-group pool investments, but also reduced the
endowments of the three out-group members by a factor of 1.5 (i.e.,
for each Euro invested, each in-group member gained €0.50, and each
6Proposers motivated by greed should offer the smallest possible portion to the re-
sponder and responders motivated by greed should accept anything greater than zero.
Empirically, few individuals in this type of setting are motivated by greed only: Pro-
posers commonly propose (close to) equal splits (P≈$10-P), and recipients commonly
reject too strong deviations from equal split. Such tendencies toward equal split pro-
posals reflect other-concern (sometimes referred to as inequity aversion, Fehr and
Gächter, 2002): Proposers desire to be fair in and of itself, or anticipate their responder
to respond to unfairness with spite; Responders punish unfairness because of spite and
reputation concerns (e.g., Pillutla and Murnighan, 1996; Van Dijk et al., 2004).
C.K.W. De Dreu / Hormones and Behavior 61 (2012) 419–428
Author's personal copy
out-group member lost €0.50). Between-group pool investments thus
reflect out-group hate—the motivation to hurt the out-group at a cost
to oneself (Halevy et al., 2008).
Fig. 2 summarizes the number of participants predominantly
contributing to in-group love, or out-group hate, as a function of treat-
ment. It shows that across the two experiments reported in De Dreu
et al. (2010), more participants in the oxytocin than placebo condition
opted for an in-group love allocation strategy, whereas more partici-
pants in the placebo than oxytocin condition kept the bulk of their
endowment to themselves (no effects were found on the number
of participants investing in out-group hate). De Dreu et al. reported
that males given oxytocin rather placebo (i) displayed more in-group
love; (ii) expected other in-group members to contribute more to
in-group love; (iii) did not invest more or less in out-group hate; and
(iv) had similar levels of distrust for the out-group.
Taken together, in both humans and non-humans, oxytocin enables
social discrimination (Ferguson et al., 2000; Macbeth et al., 2009),
motivates in-group favoritism (De Dreu et al., 2011a, 2011b), and in-
creases parochial altruism and cooperation. Oxytocin neither increases
nor decreases distrust and spiteful behavior towards rivaling out-groups.
Oxytocin motivates non-cooperation towards rivaling out-groups
help and protect those others through increased competition towards
outsiders (Arrow, 2007). In intergroup competition, one's in-group is
such as launching pre-emptive strikes that reduce or even neutralize
the threat posed by out-groups (Darwin, 1873; Jervis, 1976); and (ii)
offensive aggression aimed at winning the competition and exploit-
ing rivaling out-groups (Boyd and Richerson, 1982). There is indeed
substantial evidence that in intergroup competition, humans pro-
mote their in-group through increased cooperation with in-group
members (Bornstein, 2003; Puurtinen and Mappes, 2009) and in-
creased competition towards the out-group (Wildschut et al., 2003).
The above results on in-group favoritism and parochial coopera-
tion suggested that oxytocin does not motivate out-group derogation
and out-group hate. However, it cannot be excluded that oxytocin
motivates in-group protection through defense-motivated non-
cooperation. Three lines of research support this possibility. First,
there is evidence that lactating Wrister rats selectively bred for high
anxiety-related behaviors (HAB) show more maternal aggression
against a virgin intruder compared to Wrister rats bred for low
anxiety-related behaviors (LAB) (Bosch et al., 2005). In this study,
maternal aggression was positively correlated with oxytocin release
in the paraventricular nucleus among HAB-rats, and negatively
correlated in LAB-rats. Finally, blockade of endogenous oxytocin ac-
tion by infusion of an oxytocin receptor antagonist reduced maternal
aggression among HAB dams, and increased aggression among
LAB dams. Bosch et al. (2005) conclude that maternal aggression
serves to protect offspring, and from this study it thus follows whereas
oxytocin reduces defense-motivated aggression towards outsiders
among low-anxiety individuals, it up-regulates aggression among
Second, De Dreu et al. (2010, Exp. 3) provided a conceptual repli-
cation and extension of the results reported in Bosch et al. (2005).
Participants played a Prisoner's Dilemma between their own in-
group, and a three-person out-group—they choose between coopera-
tion and non-cooperation, with outcomes from their decision accru-
ing to the in-group (the decision maker included). The cardinal
payoffs of the Prisoner's Dilemma were manipulated so that out-
group non-cooperation (rather than cooperation) had either strong
or weak negative effects on in-group outcomes.7In this game, to pre-
vent negative effects of possible out-group non-cooperation, individ-
uals should choose the non-cooperative rather than cooperative
alternative themselves. Results showed that the variation in out-
group threat interacted with whether participants received intrana-
sal oxytocin or placebo. When out-group threat was low, treatment
had no effect on non-cooperation towards the out-group; when
out-group threat was high, however, group members were signifi-
cantly more non-cooperative towards the out-group when they
had been given oxytocin rather than placebo. From these results,
De Dreu et al. concluded that oxytocin motivates humans to be
non-cooperative with rivaling out-groups especially when the out-
group represents a threat to the individual's in-group.
The third and final piece of evidence for the notion that oxytocin
motivates non-cooperation towards out-groups comes from a study
on coalition-formation (De Dreu et al., in press). In this study, partic-
ipants engaged in an intergroup competition and were then asked to
select allies into their team. They were shown faces of potential allies
that were morphed into either high threat (low on trustworthiness
and high on dominance) or low threat (high on trustworthiness and
low on dominance; see e.g. Oosterhof and Todorov, 2008). Males
that were given oxytocin rather than placebo selected more high-
than low-threat allies into their team, and rated these high-threat
targets as more useful. The authors concluded that in intergroup com-
petition, oxytocin motivates humans to select allies that have high
threat potential and appear aggressive rather than friendly, presum-
ably to make their in-group a stronger and more threatening compet-
itor to rivaling out-groups.
Conclusions and avenues for future research
Theresearchreviewed here shows that hypothalamicrelease (or in-
fusion) of oxytocin (i) down-regulates fear and anxiety, and enables
trust to develop especially with protagonists that are familiar and/or
categorized as in-group; (ii) motivates empathic other-concern, in-
group favoritism, and parochial cooperation; and (iii) motivates non-
cooperation towards potentially threatening out-groups. These conclu-
sions subscribe to the emerging insight that oxytocin-induced goodwill
(empathy, trust, cooperation) is far from indiscriminate and highly
targets (Chen et al., 2011; De Dreu et al., 2011a, 2011b; Bartz et al.,
2010). Conclusions also fit the Darwinian insight that human self-
sacrifice and cooperation serve in-group functioning and thereby en-
7The experiment also manipulated the extent to which it was rewarding to exploit
out-group cooperation. Consistent with the finding that oxytocin does not modulate
out-group derogation (De Dreu et al., 2011a, 2011b), or spiteful tendencies to hurt
the out-group (De Dreu et al., 2010 Exp. 1 and 2), oxytocin did not modulate out-
SelfishIn-group Love Out-group Hate
dominant allocation strategy
Fig. 2. In intergroup competition oxytocin increases contributions to benefit the in-
group but not to hurt the out-group. Bars reflect percentage of participants in De Dreu
et al. (2010; Exp. and 2 combined; N=116 males) keeping their endowment (selfish),
donating most to benefit the in-group (in-group love), and donating most to hurt
the out-group (out-group hate).
C.K.W. De Dreu / Hormones and Behavior 61 (2012) 419–428
Author's personal copy
circuitry may have evolved to sustain within-group cooperation, in-
group protection, and if needed, competition towards rivaling out-
groups. In the remainder of this section, I elaborate on these conclu-
sions, and point out gaps in our understanding of the link between
oxytocin on the one hand, and human cooperation on the other.
Opening the black box
The insights and conclusions just summarized emerge when we
take a bird's eye perspective on the emerging research literatures,
yet allow ourselves some leaps of faith too. In fact, there are a number
of concerns with the existing evidence that require new research.
First, the link between oxytocin and brain-level responses in
empathy-related brain circuitries, including the inferior frontal
gyrus and the ventromedial prefrontal cortex needs further testing,
especially when targets are categorized as in-group versus out-
group. Related, the link between oxytocin and expressions of trust,
other-concern, and cooperation toward in-group versus out-group
targets remains elusive. There are important gaps in our understand-
ing of the transfer from hypothalamic release of oxytocin, and expres-
sions of complex social behaviors like cooperation. Research is
needed to fill these voids, and in doing so needs to take into account
the features and characteristics of the targets of cooperation—we
need brain imaging research that takes seriously the notion that
human cooperation and non-cooperation is modulated by the inter-
action between neuropeptides and features of the social environ-
ment within which cooperation takes place (for an example, see
Rilling et al., 2012).
An issue of broader concern is that all studies on oxytocin and
human cooperation forced individuals to decide cooperatively or
not, with non-cooperation being taken as indicative of distrust and
competitive motivation. Such an inference overlooks the fact that in
many social settings individuals prefer leaving the situation rather
than either cooperating with, or competing against their protagonist.
Although oxytocin seems to promote social approach and not social
avoidance (Kemp and Guastella, 2011), it cannot be excluded that
with not-so-benign protagonists, oxytocin associates with increased
withdrawal—when given the choice, oxytocin may actually motivate
people to not playing the trust game, the ultimatum bargaining
game, or the intergroup prisoner's dilemma game. Future research
on oxytocin and human cooperation would benefit from research
paradigms that include a broader repertoire of behavioral options,
withdrawal being one of them (e.g., Miller and Holmes, 1975;
Shalvi et al., 2011).
From social recognition to social discrimination
The research discussed here suggested that effects of oxytocin on
behavioral expressions of trust, empathy, and cooperation may be
stronger when protagonists are categorized as in-group, and may
even reverse when others are seen as out-group. Furthermore,
based on studies into social amnesia in rodents and voles (Ferguson
et al., 2000, 2002; Macbeth et al., 2009), it was hypothesized that oxy-
tocin facilitates classification of others as in-group versus out-group.
Such social discrimination in non-human mammals involves the
olfactory bulb, which is dense with oxytocin receptors (Ferguson
et al., 2000). However, while similar processes may be operating in
humans, humans also discriminate others on the basis of perceptual
and cognitive cues such as physical appearance and attitude similari-
ty, and auditory cues such as native language spoken (for reviews, see
Dovidio and Gaertner, 2010; Yzerbyt and Demoulin, 2010). Research
is needed to test the proposition that oxytocin guides social discrim-
ination in humans, and to uncover whether and how oxytocin modu-
lates the neural circuitries involved in classifying others into in-group
versus out-group on the basis of perceptual and/or auditory cues.
Such research is needed also to clearly distinguish between mere
categorization, and the motivation to treat familiar/in-group others
more positively than unfamiliar/out-group others.
From parochial cooperation to in-group maintenance
The studies reviewed here suggest that oxytocin plays a particu-
larly important role in a broad variety of behaviors aimed at sustain-
ing, maintaining, and improving social life within one's in-group. To
date, research almost exclusively focused on either social cognition
measures of facial recognition and liking, or behavioral measures of
trust and cooperation. However, if indeed oxytocin serves as a single
neurohormonal mechanism underlying these and related forms of
within-group cooperation, it follows that a broader variety of social
behaviors should be affected by oxytocin as well. In free-living meer-
kats, for example, peripheral administration of oxytocin increased a
suite of cooperative behaviors, including digging, guarding, pup-
feeding, and associating with pups (Madden and Clutton-Brock,
2011). In humans, oxytocin motivates the selection of high rather
than low threat allies that are particularly instrumental in protecting
the in-group against outside dangers, including those posed by rival-
ing out-groups (De Dreu et al., in press).
To support group life, humans enforce and comply with group
norms, exchange information, and create and invent ideas and prob-
lem solutions that facilitate group survival and prosperity (Alexander,
1990; De Dreu et al., 2008; Rilling and Sanfey, 2011). An intriguing
question for new research is whether, in humans, oxytocin indeed
modulates a broader variety of group maintenance behaviors, includ-
ing norm compliance and enforcement, information sharing, and per-
haps even group problem solving and creativity. If true, this would
further support and underline the emerging insight that one of the
primary functions of oxytocin includes sustaining and promoting
From in-group love to intergroup tension
Evolutionary theory suggests that intergroup competition fuels
within-group cooperation, and that within-group cooperation serves
survival in intergroup competition and conflict. Throughout this re-
view, it became clear that oxytocin modulates other-concern for be-
nign protagonists, in-group favoritism, and within-group but not
between-group trust and cooperation. Moreover, oxytocin stimulated
non-cooperation towards the out-group when the out-group consti-
tuted an imminent threat to the in-group.
These findings suggest that oxytocin indirectly contributes to
intergroup competition and conflict. First, in-group favoritism alone
or in combination with out-group derogation creates intergroup
bias — in-group members get relatively better treatment and receive
benefits more readily than out-group members (Brewer, 1999;
Dovidio and Gaertner, 2010; Fiske, 2002). Because such unfair treat-
ment triggers negative emotions, violent protest, and aggression
among disfavored and excluded individuals (Hewstone et al., 2002),
by stimulating in-group favoritism oxytocin may trigger a chain re-
action towards intense between-group conflict. Second, through its
effects on within-group cooperation, oxytocin contributes to making
the in-group well-functioning and strong, not only in absolute terms
but also relative to rivaling out-groups. Perceiving such effective and
relatively strong groups may trigger, among rivaling out-groups,
pre-emptive strikes and aggression (De Dreu, 2010; Deutsch, 1973;
Jervis, 1976). Again, oxytocin's effects on within-group cooperation
may, inadvertently, contribute to elevated levels of intergroup ten-
sion, competition, and conflict.
That oxytocin is involved in, and may even indirectly promote
intergroup tension and conflict, is thought-provoking and potentially
relevant to our understanding of the broader social functions of oxy-
tocin. However, the supporting evidence is limited, and replications
and extensions are needed. First, in humans oxytocin motivated
C.K.W. De Dreu / Hormones and Behavior 61 (2012) 419–428
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inter-group competition when out-group threat was high and not
when out-group threat was low (De Dreu et al., 2010). In nursing
rats, oxytocin motivated aggression against intruders when rats
were bred for high anxiety; among those bred for low anxiety, oxyto-
cin actually reduced aggression against intruders (Bosch et al., 2005).
Thus, there may be important individual differences and contextual
factors that temper or even reverse effects of oxytocin on inter-
group competition, and research is needed to uncover and document
these moderators. Second, current evidence is limited to one-shot in-
teractions, in which competing groups had no history or future to-
gether. Theoretically, one would expect escalatory spirals set in
motion by oxytocin-induced in-group favoritism and parochial coop-
eration. Research is needed, however, to test such unfolding of coop-
eration and aggression over time. Furthermore, there is a wealth of
factors associated with intergroup relations known to affect inter-
group competition and cooperation, including power and status dif-
ferences, and history of conflict or cooperation (for reviews see e.g.,
De Dreu, 2010; Dovidio and Gaertner, 2010; Fiske, 2010).
In humans, the evolutionary ancient and highly preserved neuro-
peptide oxytocin modulates a range of cognitive and behavioral func-
tions related to affiliation and pair bonding. Close scrutiny of recent
experiments on social judgment and decision making in humans re-
veals that oxytocin's effects on trust, other-concern, and cooperation
emerge especially with benign protagonists and those seen as belong-
ing to one's in-group. Oxytocin also modulates protection of in-group
members through the selection of high rather than low threat allies
and through competition against threatening outsiders. Thus, in addi-
tion to reproduction and pair–bond formation, the oxytocinergic cir-
cuitry serves to “tend-and-defend” the in-group.
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