Conflict between groups is a recurring theme in human history. We tend to form social bonds with others who share the same characteristics as ourselves, whether that is nationality, ethnicity, or supporting the same football team. Individuals that belong to the same group as us comprise our ‘in-group’. All other individuals make up our ‘out-groups’. Competition and conflict with out-groups – from benign sporting rivalry to warfare – has a key role in shaping human cultures and societies.

Such conflict often requires individuals to act in ways that harm their own self-interests. It also requires them to coordinate their actions with other members of their in-group. How does our biology drive this behavior? When small groups prepare for conflict with other groups, they often perform social bonding routines and rituals. These trigger the brain to release a hormone called oxytocin into the bloodstream. Known as the ‘love hormone’, oxytocin helps promote pair bonding as well as social bonding with in-group members. Studies in both humans and monkeys show that boosting oxytocin levels artificially via a nasal spray makes individuals more trusting and cooperative.

But Zhang et al. now show that the ‘love hormone’ also helps individuals launch more coordinated ‘attacks’ on out-groups. In a study involving a multi-round economic contest game between groups of ‘attackers’ and ‘defenders’, oxytocin did not make attackers less aggressive. Instead it enabled them to better coordinate their attacks. Each contest game involved three attackers individually contributing money to a group pool to outbid the other group and win more money, and three defenders making similar contributions to their own group pool to defend it against the rivals’ attacks and protect themselves from losing all their money. Attackers who used an oxytocin nasal spray were better at tracking their rivals' defensive strategies than attackers whose nasal spray contained a placebo. Under the influence of oxytocin, the attackers timed their strikes to occur when their rivals were vulnerable. Over time, the oxytocin users became better at coordinating their behavior with other members of their in-group. This resulted in more earnings.

Success – and even survival – in intergroup conflicts depends on how willing individuals are to make contributions that incur a personal cost. They also depend on how well individuals coordinate their contributions. Social strategies, such as leading by example, and neurobiological mechanisms such as oxytocin can both help achieve the coordination needed to exploit out-group rivals.

Conflict between groups of people can profoundly change their demographic, economic and cultural outlook. Across human history, intergroup conflict functioned as a critical change agent and selection pressure that may have shaped the biological preparedness for in-group oriented cooperation and self-sacrifice on the one hand, and out-group hostility and aggression on the other hand (Bar-Tal et al., 2007; Boyd and Richerson, 1982; Choi and Bowles, 2007; De Dreu et al., 2010; Macfarlan et al., 2014). Out-group hostility and aggression serves to subordinate and exploit rivaling out-groups (henceforth out-group attack), and/or to defend the in-group against such hostility from neighboring groups (henceforth in-group defense) (De Dreu et al., 2016a; Halevy et al., 2008; Rusch, 2014a; Rusch, 2014b; Lopez, 2017; Wrangham, 2018). Recent work has showed that individuals are more strongly motivated to contribute to defend one’s own group than to attack out-groups and, importantly, that out-group attacks frequently fail because individual contributions to out-group attacks are poorly coordinated (De Dreu et al., 2016a). Indeed, attacking rivals who are expected to have strong defenses more likely results in failure and waste, than attacking groups expected to be weak and defenseless (De Dreu et al., 2016b; Grossman and Kim, 2002; Goeree et al., 2003). Thus, to succeed in intergroup competition and conflict, group members not only need to contribute to their group’s competitive strength, but they also need to coordinate within their group the intensity and timing of attacks.

How group members coordinate behavior into effective joint actions during intergroup competition and conflict remains poorly understood (De Dreu et al., 2016a) and we lack insight into the underlying neurobiological mechanism. Here, we target the neuro-hormone oxytocin as a neurobiological mechanism underlying within-group coordination of out-group attack in dynamic intergroup contests. Studies in ethnography and anthropology have shown that small groups preparing for intergroup conflict build cohesion and commitment through social bonding routines and rituals. Groups selectively invite friends to join a raid (Glowacki et al., 2016), bond like family (Macfarlan et al., 2014), and engage in cultural rituals that simulate self-sacrifice, cooperation and coordination (Xygalatas et al., 2013). Neuroendocrine studies (Burkett et al., 2016; Carter, 2014; Ma et al., 2016b; Samuni et al., 2017) have linked these and related practices to the release of oxytocin, a nine-amino acid neuropeptide. In turn, intranasal administration of oxytocin has been shown to modulate neural responses in brain regions involved in threat detection and reward processing (Ma et al., 2016b; Paloyelis et al., 2016; Wang et al., 2017; Liu et al., 2019). Moreover, in both non-human and human primates, elevated levels of oxytocin via intranasal administration have been linked to a range of cognitive and behavioral effects including within-group conformity, trust, affiliation, and cooperation (Arueti et al., 2013; Aydogan et al., 2017; De Dreu et al., 2010; Madden and Clutton-Brock, 2011; Rilling et al., 2012; Samuni et al., 2017; Stallen et al., 2012; Yan et al., 2018).

Whereas these work together point to oxytocin as a possible neurobiological mechanism underlying group coordination, three issues remain unclear. First, we lack empirical evidence for the possibility that oxytocin promotes group-level coordination of collective action in general, and during intergroup competition and conflict in particular. Second, we poorly understand how group members coordinate collective action during intergroup competition and conflict, and whether oxytocin can directly influence coordination and/or the strategy group members use to coordinate. Third, we do not know whether oxytocin differentially modulates tacit coordination within groups when collective contributions are focused on attacking one’s rival, versus defending the in-group against possible attacks by one’s rival.

To address these issues, we examined the role of oxytocin in 80 interactive, multi-round contests between three-person attacker and three-person defender groups. Group members on each side made individual contributions to their group pool (aimed at attacking the other side, or at defending against such possible attacks). We provided individuals with real-time feedback on group investments and success after each contest round, which allowed group members to learn and adapt to their rival’s past investments, and use the history of play as a focal point to coordinate their future attacks and defenses. Indeed, when lacking explicit coordination mechanisms such as a leader or decision-making protocols (De Dreu et al., 2016a; Gavrilets and Fortunato, 2014; Hermalin, 1998), groups use social norms and focal points to tacitly coordinate collective action (Halevy and Chou, 2014; Schelling, 1960). In multi-round intergroup contests, the rival’s history of play can serve as a focal point for groups to coordinate their contributions to attack and/or defend (De Dreu et al., 2016b). Thus, when attacking out-groups, group members may coordinate their contributions on their rival’s historical level of defense, and attack when historical defense is low and the target appears vulnerable; and not attack when historical defense is high and the target appears strong and difficult to beat.

We expected effects of oxytocin to be stronger during out-group attack than during in-group defense. In-group defense is first and foremost an adaptation to the attackers’ (past) aggression and is typically well-coordinated—individuals fight towards the same goal of self-preservation and group survival. Although there is some evidence that oxytocin may increase protective aggression (for a review see De Dreu and Kret, 2016), administering oxytocin may contribute little to the relatively high baseline levels of contribution and coordination during in-group defense. In contrast, out-group attacks are typically less strong, more variable, and less well-coordinated (De Dreu et al., 2016a), leaving more room for oxytocin administration to influence behavior. We anticipated two possibilities for the effect of oxytocin. On the one hand, oxytocin has been linked to prosociality (Madden and Clutton-Brock, 2011; Rilling et al., 2012; Liu et al., 2019), empathy (Hurlemann et al., 2010; Bartz et al., 2010) and consolation (Burkett et al., 2016). This line of research would lead us to anticipate that oxytocin may enable individuals in attacker groups to coordinate on a peaceful no-attack strategy that is independent of the rivals’ history of defense. On the other hand, oxytocin has been shown to enhance in-group conformity (Aydogan et al., 2017; Stallen et al., 2012; De Dreu and Kret, 2016), increase behavioral coordination and neural synchronization within pairs of individuals performing a joint task (Arueti et al., 2013; Mu et al., 2016), enhance facial mimicry, motor imitation, and neural responses linked to action-intention mirroring (Korb et al., 2016; De Coster et al., 2014; Kret and De Dreu, 2017; Levy et al., 2016), and improve memory and learning from feedback (Striepens et al., 2012; Guastella et al., 2008; Ma et al., 2016a). These functionalities alone and in combination may facilitate coordination on social norms and shared focal points that emerge during group interaction, suggesting that oxytocin may facilitate the use of rival’s history, enable individuals in attacker groups to coordinate better on the level and timing of their attacks, and to efficiently appropriate resources from their out-group. Evidence for this possibility would be consistent with earlier findings that oxytocin shifts the focus from self-interest to in-group interests (De Dreu and Kret, 2016), limits trust and cooperation to in-group members (De Dreu et al., 2010; Ma et al., 2015; Ten Velden et al., 2017), and promotes aggression toward threatening rivals (Madden and Clutton-Brock, 2011; Samuni et al., 2017; Striepens et al., 2012; Ne'eman et al., 2016).

We examined these possibilities using a dynamic, fully incentivized Intergroup Attacker-Defender Contest game (De Dreu et al., 2016a). The IADC is an all-pay contest (Abbink et al., 2010; Dechenaux et al., 2015) involving six individuals randomly assigned to a three-person attacker and a three-person defender group. In the IADC game, 3 attackers made individual contributions to the group pool to subordinate the other group and increase gains through victory (but always keep the remaining resources), and 3 defenders contributed to their group pool to defend rival’s attack and protect against loss and defeat (defenders ‘survive’ from left with 0 only if defense succeeds). We created 80 IADC sessions (with N = 480 healthy males); in 40 (40) sessions, participants received intranasal oxytocin (matching placebo) (Figure 1). Each IADC session involved two blocks of 15 investment rounds with real-time feedback in between rounds. For each investment round (Table 1), each individual received an initial endowment of 20 MUs (Monetary Units). Each individual decided the amount (I_i, 0 ≤ I_i ≤ 20) to the group’s pool G (0 ≤ G ≤ 60, G_Attacker = I_Attacker-1 + I_Attacker-2 + I_Attacker-3, G_Defender = I_Defender-1 + I_Defender-2 + I_Defender-3). Contributions were wasted, but when G_Attacker ≤ G_Defender, attackers failed and defenders survived and all six individuals kept their remaining endowment (leftovers, 20 – I_i). However, when G_Attacker > G_Defender, defenders failed and left with 0. The attacker group won and took away defender group’s remaining MU (spoils from winning, 60 – G_Defender), which were divided equally among three attacker group members (each attacker member received: (60 – G_Defender)/3) and added to their remaining endowments (20 – I_Attacker-i). Thus, contributions in the attacker group (G_Attacker) and in the defender group (G_Defender) reflect the contribution level to out-group attack and to in-group defense, respectively. In one 15-round block, individuals made their decisions individually and simultaneously without communication (i.e. Simultaneous decision-making block). In the other 15-round block (order counterbalanced), individuals within groups made their decisions in sequence: one randomly drawn member made his decision first, followed by the second randomly selected member who was informed about the first member’s contribution, and then the third and final member made his decision (i.e. Sequential decision-making block, De Dreu et al., 2016a; Gavrilets and Fortunato, 2014; Figure 2).

Figure 1

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Figure 2

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The sequential decision-making protocol acts as a ‘coordination device’ that facilitates behavioral coordination within groups (De Dreu et al., 2016a; Gavrilets and Fortunato, 2014; Hermalin, 1998). This treatment thus provides a benchmark to compare groups in the simultaneous decision-protocol who lack an explicit coordination mechanism and have to find other means—such as a shared social norm or the rival’s past defense — to coordinate individual contributions into effective joint action (De Dreu et al., 2016a). We operationalized within-group coordination as behavioral alignment of contribution to the group, indicated by the variance in contributions to group fighting, with lower group-level variance reflecting better coordination. We expected that effects of oxytocin on group coordination would emerge especially under simultaneous rather than sequential decision-making.

Before examining within-group coordination, we examined treatment effects on contributions to group pool. Individual contributions were averaged across the three members within each three-person group, and submitted to a 2 (Treatment: Oxytocin vs. Placebo) $\times$ 2 (Role: Attack vs. Defense) $\times$ 2 (Procedure: Simultaneous vs. Sequential) $\times$ 15 (Rounds) mixed-model Analysis of Variance (ANOVA) with Treatment between-sessions. Individuals contributed more under sequential than simultaneous decision-making (M ± SE = 6.89 ± 0.20 vs. 6.47 ± 0.24; F(1, 78) = 5.109, p = 0.027, η² = 0.061), and somewhat less when given oxytocin rather than placebo (M ± SE = 6.28 ± 0.27 vs. 7.08 ± 0.30; F(1, 78) = 3.719, p = 0.057, η² = 0.046; marginal significance). Figure 3A showed higher contributions to in-group defense than to out-group attack, especially in earlier rounds (Role, F(1, 78) = 287.903, p < 0.001; η² = 0.787; Role $\times$ Round, F(14, 65) = 4.529, p < 0.001, η² = 0.494).

Figure 3

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Treatment effects also emerged when we examined the number of non-contributors. There were more non-contributors in attacker compared to defender groups (M ± SE = 20.23 ± 0.90 vs. 4.25 ± 0.44; F(1, 78) = 408.489, p < 0.001; η² = 0.840), and more non-contributors in groups given oxytocin than placebo (M ± SE = 13.46 ± 0.90 vs. 11.01 ± 0.75; F(1,78) = 4.345, p = 0.040; η² = 0.053). Crucially, oxytocin increased the number of non-contributors in attacker groups but not in defender groups (Role $\times$ Treatment, F(1, 78) = 5.043, p = 0.028, η² = 0.061, Figure 3B). This Role $\times$ Treatment effect is especially true when decisions were made simultaneously (F(1, 78) = 5.712, p = 0.019, η² = 0.068) but less so when decisions were made sequentially (F(1, 78) = 2.143, p = 0.147; η² = 0.027). We then examined participants’ decision time when deciding to (not) contribute, we showed that individuals in attacker groups made their decisions to not contribute faster than to contribute (F(1, 78) =137.679, p < 0.001, η² = 0.641). Oxytocin increased the speed with which individuals in attacker groups decided to not contribute (Treatment $\times$ Contribute: F(1, 77) = 4.857, p = 0.031; η² = 0.059, Figure 4).

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Next, we analyzed group-level coordination operationalized as the within-group variance in contributions, with lower variance indicating stronger coordination (De Dreu et al., 2016a). First, there was a Procedure $\times$ Role interaction (F(1, 78) = 101.978, p < 0.001, η² = 0.567) showing that sequential decision-protocol facilitated coordination for attack (F(1, 78) = 77.852, p < 0.001, η² = 0.500) and, unexpectedly, reduced coordination for defense (F(1, 78) = 39.268, p < 0.001, η² = 0.335). Importantly, as predicted, oxytocin facilitated group-level coordination (i.e. reduced within-group variance in contributions) of out-group attack but not of in-group defense, especially in earlier rounds (Role $\times$ Treatment $\times$ Round, F(14, 1092) = 1.753, p = 0.041, η² = 0.022; Figure 5A; Figure 5—figure supplement 1 for similar results of another index of coordination).

To examine what strategy attackers given oxytocin coordinated on, we first examined the contest outcomes. As noted, oxytocin may enable groups to coordinate on a peaceful ‘no-attack strategy’, in which case we should find (1) lower victories in attacker groups given oxytocin rather than placebo, (2) the oxytocin effect on the number of non-contributing attackers should not differ when attacks succeed or fail, and (3) no effect of oxytocin on tracking the rival’s defense history. This was not the case. First, the number of victories was similar in attacker groups given oxytocin (M ± SE: 24.8 ± 1.8%) and placebo (M ± SE: 26.6 ± 1.6%), (F(1, 78)=0.572, p=0.452; η² = 0.007). Thus, rather than making groups coordinate on a peaceful no-attack strategy, oxytocin may enable groups to coordinate on attacking at the right moment with the right force. Indeed, analyses of the spoils and leftovers showed that groups given oxytocin rather than placebo had higher leftovers when attacks failed (t(78) = 2.609, p=0.011, Cohen’s d = 0.581, 95% Confidence Interval (CI), 0.232 to 1.76, Figure 5B) and somewhat higher spoils when attacks were successful (t(74) = 1.819, p=0.073, Cohen’s d = 0.419, 95% CI, −0.086 to 1.888, marginal significance, Figure 5C). To illustrate the increased efficiency of attack under oxytocin, attackers’ contribution and payment under the four conditions (i.e. Simultaneous/Sequential x Oxytocin/Placebo) were plotted in Figure 5D using a bootstrapping technique (Davison and Hinkley, 1977). Specifically, a bootstrapped dataset with sample size N = 40 was resampled with replacement separately for each of the four conditions. The mean contribution and payment of the bootstrapped sample was then calculated and saved as a new data point. For each condition, this procedure was repeated for 1000 times to represent the population information. As illustrated in Figure 5D, oxytocin decreased contributions to attack (a shift toward less contribution) but increased payment (a shift toward more payment). The distribution of attacker groups under oxytocin and placebo in a space defined by the two vectors (contribution and payment) indicates a clear separation of oxytocin and placebo treatments (especially when decisions were made simultaneously, that is black dots vs. grey dots).

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Second, we conducted an ANOVA on the average number of non-contributing attackers, with Treatment (Oxytocin vs. Placebo) as a between-subjects factor, Procedure (Simultaneous vs. Sequential) and Success (Success vs. Failure) as within-subjects factors. There were more non-contributing attackers when attack failed than succeeded (Failure vs. Success: M ± SE = 1.68 ± 0.60 vs. 0.37 ± 0.05, F (1, 74) = 636.941, p < 0.001; η² = 0.891). Interestingly, we found a significant Treatment $\times$ Success interaction on the number of non-contributing attackers (F (1, 74) = 6.345, p = 0.014; η² = 0.075, Figure 5E): Oxytocin increased the number of non-contributing attackers only in failed rounds (Oxytocin vs. Placebo: M ± SE = 1.84 ± 0.09 vs. 1.52 ± 0.09; F (1, 74) = 7.036; p = 0.010; η² = 0.083) but not in successful attacks (Oxytocin vs. Placebo: M ± SE = 0.34 ± 0.06 vs. 0.40 ± 0.06; F (1, 74) = 0.448; p = 0.505; η² = 0.006).

Third, we examined how attacker groups collectively identified when to attack by creating a past defense parameter α (average defender group’s investment in the last two rounds, that is (D_j-1+D_j-2)/2 on round j) and regressed attacker group’s investments onto α (attack increased when defender groups were vulnerable rather than strong, as indicated by α approaching -1). It showed that attacker groups given oxytocin tracked their rival’s past defense and attacked especially when defenders appeared more rather than less vulnerable (i.e. attack regressed negatively on rival’s historical defense). Specifically, when decisions were made simultaneously, attack regressed more strongly on α when groups received oxytocin (M ± SE: -0.30 ± 0.05) rather than placebo (M ± SE: -0.042 ± 0.098; t(78) = -2.334; p = 0.022, Cohen’s d = -0.522, 95% CI, -0.482 to -0.038), and under oxytocin but not placebo, the regression on α was also stronger in simultaneous rather than sequential decision-making (Treatment $\times$ Procedure interaction, F(1, 78) = 8.312, p = 0.005, η² = 0.097, Figure 6A).

Combined, results suggest that groups given oxytocin created more spoils from winning and had higher leftovers when attacks failed because they better coordinated attack at the right time and with the proper force. Indeed, when decisions were made simultaneously, the more strongly attacker groups relied on tracking parameter α, the lower their within-group variance when contributing (r = 0.281, p=0.012, Figure 6B), and the lower within-group variance when contributing, the higher the attacker’s spoils when winning the conflict (r = −0.328, p=0.004, Figure 6C). Indirect mediation analyses confirmed that the attacker’s higher spoils under oxytocin was mediated by (i) increased tracking of defender group’s past investments and (ii) concomitant increased within-group coordination (indirect effect = 0.147, SE = 0.103; 95% CI, 0.019 to 0.511, Figure 6D).

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To be victorious in intergroup conflict, group members not only need to contribute to their group’s fighting capacity. They also need to coordinate collective action so that they attack when their rival is expected to be weak, and avoid wasting resources on attacking tough defenders. Here we found, using a dynamic intergroup contest between attackers and defenders, that those groups who tracked their defender’s history of play and coordinated their attacks of weak rather than strong defenders wasted less resources on failed attacks and enjoyed greater spoils when winning. In addition, we uncovered that oxytocin serves as a neurobiological mechanism underlying such well-timed and coordinated attacks. Specifically, we found that oxytocin enabled individuals within attacker groups to converge their individual contributions on each other more, to collectively refrain from attacking apparently strong defenders, and effectively attacking weak defenders. These findings emerged when groups lack an explicit coordination device; providing attacker groups with a sequential decision-making protocol as an explicit coordination device substituted oxytocin-induced tacit coordination.

Our finding that oxytocin enables within-group coordination of contributions to out-group attacks resonates with two heretofore disconnected sets of findings—that small groups of warriors engage in social bonding and cultural rites (Glowacki et al., 2016; Macfarlan et al., 2014; Schelling, 1960; Wilson and Wilson, 2007; Xygalatas et al., 2013), and that social bonding and synchronized action can trigger the release of oxytocin (Burkett et al., 2016; Carter, 2014; Samuni et al., 2017). Combined with the current results, it suggests that oxytocin might be a potential neurobiological mechanism through which social bonding and performing coordinated rituals can help groups to better coordinate their attacks. Our findings furthermore suggest that such oxytocin-mediated within-group coordination can be substituted by institutional arrangements, such as a sequential decision-making protocol. We speculate that related institutional arrangements, like appointing a leader or having open communication channels can similarly obviate the need for bonding rituals and/or oxytocin to make out-group attack well-coordinated and successful.

Neither oxytocin nor the sequential decision-protocol contributed to within-group coordination in defender groups. One possibility is that in-group defense is tacitly well-coordinated because of the stronger alignment of individual interests within defender groups. After all, when defender groups survive an enemy attack, those who did not contribute to in-group defense come out relatively wealthy. But when in-group defense fails, all group members lose regardless of whether they contributed or not. As a result of this stronger common fate in defender groups, individuals contribute spontaneously and tacitly coordinate well on avoiding collective defeat. Exogenous enhancers, whether at the neurobiological or institutional level, appear not necessary.

Previous studies provided evidence for the role of oxytocin in intergroup interaction: It can increase the positive view and benign approach of the in-group and has been linked to subtle forms of intergroup discrimination (De Dreu et al., 2010; De Dreu and Kret, 2016; Ma et al., 2015; Stallen et al., 2012; Ten Velden et al., 2017). Without exception, this early evidence was limited to individual-level decision-making, and did not clearly distinguish the distinct motives for contributing to in-group efficiency and out-group hostility. It remained unknown whether and how oxytocin differentially affects attack and defense behavior during intergroup conflict. Using a dynamic attack-defense contest we revealed selective effects of oxytocin on group attacking. As such, the present study provides a new perspective on oxytocin in intergroup conflict by highlighting its functionality for strategic attack (rather than defense). In doing so, we also obtained first-time evidence that in-group coordination for collective action can be tracked to evolutionary preserved neurobiological factors.

Our study has two potential limitations. First, it involved only Chinese males as study participants. Whereas we cannot exclude specific cultural effects, findings for the comparison between out-group attack and in-group defense and between sequential and simultaneous decision-making protocols resonate with findings obtained in Western culture, with both male and female participants (De Dreu et al., 2016a). This generates confidence in the generality of the behavioral effects for attack/defense and decision protocols. Furthermore, recent work on the role of oxytocin in in-group cooperation suggests similar effects for both male and female participants (Ten Velden et al., 2017). Thus, and given the absence of strong counter-evidence, we cautiously conclude that current findings may generalize across cultural contexts and apply to male as well as female participants. Second, it is unclear how exogenous administration of oxytocin operates at the neurophysiological level. Whereas some have questioned whether intranasal administration of oxytocin can have a direct impact on brain and behavior (Leng and Ludwig, 2016), recent studies in rodents (Neumann et al., 2013; Tanaka et al., 2018), rhesus macaques (Lee et al., 2018; Modi et al., 2014) and in humans (Paloyelis et al., 2016) collectively suggest that intranasal oxytocin elevates brain-level presence of oxytocin, and impacts behavioral decisions through neural networks involved in threat detection and reward processing (Carter, 2014; Hurlemann et al., 2010; Ma et al., 2016b; Rilling et al., 2012; Stallen et al., 2012; Wang et al., 2017; Liu et al., 2019). In addition, there is some evidence that higher levels of oxytocin in saliva, blood, or urine relate to in-group affiliation and cooperation (Madden and Clutton-Brock, 2011; Samuni et al., 2017) and intergroup discrimination (Levy et al., 2016). Nevertheless, new research is needed to uncover the neurophysiological pathways through which intranasal oxytocin impact human cognition and behavior, and how findings on intranasal oxytocin relate to endogenous oxytocin measured from saliva, urine, or blood samples.

Intergroup competition and conflict shape the economic and cultural outlook of groups and societies, and interferes with individual life-trajectories. Success and survival in times of intergroup competition and conflict depend on the extent to which individuals make personally costly contributions and, as shown here, on their ability to coordinate their contributions when attacking out-groups, or defending against threatening out-groups. Both institutional arrangements, such as leading-by-example, and neurobiological mechanisms, such as oxytocin, facilitate behavioral coordination and the effective exploitation of out-group rivals. Indeed, as shown here, providing group members with oxytocin or an explicit coordination device enables them to waste less on failed attacks, and to earn more from their victories.

The group-level statistics are plotted in Figures 3-6, Figure 5-figure supplement 1 and Figure 6-figure supplement 1.The individual data is plotted in Figures 6 and Figure 6-figure supplement 1. The source data for Figures 3-6 is publicly available on the Open Science Framework at https://osf.io/7jeas/ (doi: 10.17605/OSF.IO/7JEAS).

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Author details

1. Hejing Zhang

   State Key Laboratory of Cognitive Neuroscience and Learning, International Data Group (IDG)/McGovern Institute for Brain Research, Beijing Key Laboratory of Brain Imaging and Connectomics, Beijing Normal University, Beijing, China

   Contribution

   Software, Formal analysis, Investigation, Visualization, Methodology

   Competing interests

   No competing interests declared

2. Jörg Gross

   Institute of Psychology, Leiden University, Leiden, The Netherlands

   Contribution

   Software, Methodology

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0002-5403-9475

3. Carsten De Dreu

   1. Institute of Psychology, Leiden University, Leiden, The Netherlands
   2. Center for Research in Experimental Economics and Political Decision Making (CREED), University of Amsterdam, Amsterdam, The Netherlands

   Contribution

   Conceptualization, Visualization, Methodology, Writing—original draft, Writing—review and editing

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0003-3692-4611

4. Yina Ma

   State Key Laboratory of Cognitive Neuroscience and Learning, International Data Group (IDG)/McGovern Institute for Brain Research, Beijing Key Laboratory of Brain Imaging and Connectomics, Beijing Normal University, Beijing, China

   Contribution

   Conceptualization, Resources, Formal analysis, Supervision, Funding acquisition, Visualization, Methodology, Writing—original draft, Writing—review and editing

   For correspondence

   yma@bnu.edu.cn

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0002-5457-0354

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