Coalitionary Conflict as Ultimate Cause or By-Product of Cooperation

Abstract

Throughout nature, animals cooperate within coalitions and groups to inflict damage on other groups. Prominent theory in the behavioural and biological sciences consider this reflective of the effects intergroup conflict exerted on the evolution of cooperation. Here I address a missing link in this line of argument – how can intergroup conflict emerge ex nihilo. Experiments considering participation in coalitionary aggression on non-threatening out-groups disconfirm evolved out-group bias as an explanation. More likely is that coalitionary aggression rests on parochial solidarity – to win ‘spoils of war’ for oneself and one’s in-group. Cooperation did not evolve because of conflict. Conflict is a by-product of evolved capacity for cooperation and can be avoided.

Prologue

It was, and still is, an honour to be recipient of the 2022 McGrath Lifetime Achievement Award. How people form groups, how they manage group dynamics, and what makes groups perform or fail are questions that accompanied me throughout my career. As a PhD student at the University of Groningen and later as postdoctoral fellow funded by the Royal Netherlands Academy of Sciences, I studied team conflict and negotiation (De Dreu, 1995; De Dreu et al., 1994, 2000; De Dreu & Weingart, 2003). As professor of work and organizational psychology at the University of Amsterdam, I studied group innovation and performance in organizational (De Dreu, 2006, 2007; De Dreu & West, 2001) and laboratory settings (De Dreu et al., 2008; Nijstad & De Dreu, 2012).

This emphasis on the social and organizational dynamics of small groups changed during a sabbatical leave spent at Leiden University between 2008 and 2009. For a review on cooperation and conflict (De Dreu, 2010), I delved into the literature on intergroup conflict. It struck me that theory and research on intergroup relations largely ignored intra-group dynamics and that work on intra-group dynamics, my own included, largely ignored that groups exist next to, and have interdependencies with (individuals in) other groups (De Dreu, Aaldering & Sagyi, 2014; De Dreu, Balliet & Halevy, 2014).

It was during that sabbatical leave also, and perhaps because of my latent interest in biology and neuroscience, that it occurred to me too how little we knew about the biological underpinnings of anything I had studied. And what we knew was often not only seen in humans. It was seen too in other group-living animals that, like humans, compete and fight with and between groups. In the years thereafter, my collaborators and I increasingly focused on these poorly understood biological foundations of cooperation and conflict, within and between groups (De Dreu, 2012; De Dreu et al., 2010, 2011; De Dreu & Kret, 2016). Between 2015 and 2024, as professor of social and economic psychology at Leiden University and with significant funding from the European Research Foundation and the Netherlands Science Foundation, my collaborators and I merged biology, neuroscience, experimental economics, and social psychology to excavate the foundational mechanisms of cooperation and conflict both within and between groups, human and non-human (De Dreu et al., 2020; De Dreu & Triki, 2022). What follows reflects some insights from this recent line of work.

Before moving forward, I take the opportunity to share two lessons learned throughout my career. First, scientists need the occasional sabbatical leave, to re-orient and re-invent themselves and their science, to ask new questions and to pursue the unknown. I am thankful for the opportunities my employers offered me. Second, science is teamwork, especially when multiple methods and fields of inquiry are employed and integrated. I have been extremely fortunate with smart and creative PhD students, postdoctoral fellows, and collaborators. There are too many to mention, but without each and all of them, my sabbatical leaves would have been wasted, my office days would have been a persistent bore, and my lifetime achievements would have been meagre at best.

Bright and Dark Sides of Evolved Sociality

Thirty years ago, Szathmáry and Smith (1995) published a ground-breaking article on major transitions in the evolution of biological complexity. Chief amongst these, they proposed, is the transition from solitude to sociality. Throughout nature, and over the course of millions of years, life evolved to become deeply social. From microbes to humans, individuals live together and organize themselves in myriad social structures (Billeter et al., 2024; De Dreu et al., 2024; Dugatkin, 2024; Hatchwell & Komdeur, 2000; Kappeler et al., 2019; Leimar, & Bshary, 2024; Mäs et al., 2010; West, Diggle et al., 2007). Individuals in groups raise and care for their offspring, manage shared resources and territories, hunt together, and protect their communities against outside dangers. Such togetherness, and cooperating with others, can create levels of innovation, safety, and prosperity that individuals cannot achieve alone (Dawes, 1980; Gross & De Dreu, 2019a, 2019b; Ostrom, 2009; West, Griffin, & Gardner, 2007). Individuals, human and non-human, that live in groups where they help each other, are wealthier and healthier, and they live longer (Berkman & Syme, 1979; Hare, 2017; Holt-Lunstad et al., 2015; Testard et al., 2024;).

What often escapes the analysis is that togetherness and cooperation also help individuals and their groups in the ‘constant struggle throughout nature between the instinct of the one to escape its enemy and of the other to secure its prey’ (Darwin, 1859; D. S. Wilson & Wilson, 2007). Indeed, cooperating with others not only benefits individuals and their groups. Cooperation can also make groups dangerous to other groups, and more dangerous than any individual could be alone (Le Bon, 1895). Social insects like ants and bees, for example, launch collective raids on nearby nests and colonies, killing tens of thousands of conspecifics (Chandra et al., 2021; Cunningham et al., 2014; Mabelis, 1978). One in every 20 male chimpanzee dies because of the injuries sustained from conflict with neighbouring communities (M. L. Wilson et al., 2014; Wrangham et al., 2006). And from prehistory to modern times, human groups have raided neighbouring communities, injured and killed their people and plundered and destroyed their properties (Choi & Bowles, 2007; De Dreu & Triki, 2022; Fibiger et al., 2023; Heath, 2009). Across human societies, and in the last two decades only, millions died because of political violence, terrorist attacks, civil conflicts, and inter-state wars (Davies et al., 2024; Roth et al., 2018).

That togetherness and cooperation benefits groups and their members, and makes them dangerous enemies, raises questions about the role of conflict in the evolution of cooperation. One prominent answer is that it is because of conflict that cooperation within groups evolves. In political science, for example, conflict and enemy threat is taken as a key driver of the formation of nation states, of patriotism, and of how much people support their leaders and social institutions (Bagge, 2019; Gat, 2013; Haas, 1982; Hainmueller & Hopkins, 2014; Moffett, 2013; Olson, 2000; Spruyt, 2002; Wimmer & Feinstein, 2010). Prevailing theory in psychology and behavioural economics proceeds by assuming that competition with other groups is crucial to group formation, group solidarity, social identification and cooperation (Bar-Tal, 2007; Brewer, 1999; Cikara et al., 2011; Dovidio et al., 2007; Hewstone et al., 2002; Hogg & Turner, 1985; Shayo, 2009; Tajfel & Turner, 1979). And in the biological and evolutionary sciences, intergroup conflict is identified as an important selection pressure in the evolution of cooperation. Already Darwin (1859) noted, for example, that ‘groups with a greater number of courageous, sympathetic and faithful members, . . . ready to warn each other of danger, to aid and defend each other . . . would spread and be victorious over other tribes’ (Alexander, 1987; Bruintjes et al., 2016; Preston et al., 2021; Snijder et al., 2024; Traulsen & Nowak, 2006; D. S. Wilson & Wilson, 2007).

The problem with this line of argument – that cooperation evolved because of conflict – is two-fold. First, it ignores compelling evidence that cooperation within groups can evolve also in absence of conflict and enemy threat. Even in groups operating in social isolation, without contact with other groups, cooperation evolves and prevails (for a review see e.g., De Dreu et al., 2024). Second, the prevailing argument that cooperation evolved because of conflict leaves open where conflict and enemy threat come from in the first place. Conflict is presumed rather than explained, and what we need is an answer to the question why individuals in groups cooperate to fight other groups ex nihilo – when there are no histories of violence to revenge, and when outside groups and communities pose no threat to oneself or one’s group.

In what follows, I review findings from experiments performed in my own and others’ laboratories that probed whether individuals cooperate to fight out-groups (i) because of an evolved dislike for outside groups and societies, or (ii) to better their own groups that they depend on for social status, resources and opportunities. Findings combined suggest that human group-living, and their capacity to cooperate, did not evolve because of conflict. Instead, conflict can, but does not have to, be the result of evolved capacities to cooperate within groups. When and how group cooperation creates conflict or not is an important question for future research and theory to address.

Structures of Conflict

Conflict can be about many things such as ownership, territorial access, status and respect, or what is right and wrong (e.g., Blattman & Miguel, 2010; Bornstein, 2003; De Dreu, 2010; Deutsch, 1973; Gould, 1999; Rapoport, 1960; Schelling, 1960). What issues are at stake is, in theory, of minor relevance to the answer whether and how conflict and cooperation relate. What does matter, however, is the structure of conflict. Indeed, and regardless of what issue is at stake and whether it involves individuals or groups, conflict can be structured in two possible ways (Coombs & Avrunin, 1988; De Dreu & Gross, 2019; Pruitt & Rubin, 1986). First, conflict can be about something all parties want but only some can have (Coombs & Avrunin, 1988). Examples include politicians competing for the same senate seat, rivalling research laboratories claiming the patent ownership of some lucrative technology, or super-powers seeking world hegemony. Alternatively, conflict can be about something some parties want that others try to prevent from happening (Miller, 2009; Pruitt & Rubin, 1986). Examples include revisionist states seeking to capture their neighbour’s territory, activist rebels fighting elitist powerholders, companies launching hostile take-over attempts, or terrorists attacking civilian and military targets.

Often fuelled by leader rhetoric, people in conflict often perceive themselves to be on the defending side, themselves being benign and of good will and the other side being the threatening aggressor (Chambers et al., 2006; Plous, 1985; Snijders et al., 2025; Staub, 1996). Obviously, such perceptions must be biased as conflict would not emerge when both sides are truly benign and peaceful. And indeed, upon closer scrutiny most conflicts appear asymmetrically structured, with one ‘attacking’ and the other ‘defending’ the status quo (De Dreu & Gross, 2019; Rusch, 2013). For example, two-thirds of the 2,000 militarized interstate disputes that emerged since the Congress of Vienna in 1816 involved a revisionist ‘attacker’ and a non-revisionist ‘defender’ state (De Dreu et al., 2016; Gochman & Maoz, 1984; Wright, 2014). Likewise, 68% of community disputes involved a challenger aiming to revise and a defender aiming to preserve the status quo (Ufkes et al., 2012).

In asymmetrical conflict, one side is under enemy threat created by the other side. Defending oneself and one’s group against outside threats is jus ad bellum – morally, legally, and psychologically permissible (Russell, 1915). Enemy threat furthermore aligns the interests of individuals within groups, creating a shared fate and purpose that can strongly motivate and unite group members to fight for themselves and their group (Aktipis et al., 2018; Balliet et al., 2017; De Dreu et al., 2024; Roberts, 2005). Differently put, in itself being under enemy threat can motivate group cooperation, fitting the argument that cooperation evolved because of conflict. All this is different for the other side in asymmetrical conflict, where individuals and their groups seek to seize territory, resources, or political power from otherwise non-threatening groups. Unlike defending against enemy threat, initiating otherwise unprovoked aggression towards outsiders may be difficult to justify on moral grounds (Cushman et al., 2012). And while participating in defending one’s group serves one’s group as much as it serves one’s own survival, participating in unprovoked aggression typically offers group members few, if any, direct benefits – only when victorious there may be a share in the ‘spoils of war’ that can, but do not have to, offset the costs and risks inherent to conflict participation (De Dreu et al., 2016, 2022).

Appreciating that conflict can and often is asymmetrical reveals the problem in the prevailing argument that cooperation evolved because of conflict. Cooperating to defend the group against outside threat can be understood, and those that cooperate well to defend against outside threat more likely survive. But why would humans, and non-humans, cooperate to aggress and attack non-threatening out-groups. Why do humans cooperate to create conflict where none exists?

Coalitionary Aggression May Not Emerge from Out-Group Bias

When neighbouring groups and communities are peaceful and non-threatening, individuals may still invest time, skills, and effort in fighting them. One reason may be that individuals perceive their ‘in-group’ as more worthy than ‘out-groups,’ and out-groups as less likeable and worthy than their in-group. Such social bias – more positive about one’s in-group, and more negative about out-groups – is seen across cultures (Brewer, 1999; Cikara et al., 2011; De Dreu et al., 2011; Haslam, 2006; Hewstone et al., 2002), and it is seen in human infants less than a year old (Buttelmann et al., 2013; Hamlin et al., 2013). It is seen too in non-human primates like chimpanzees and bonobos (Brooks et al., 2021, 2024; Campbell & de Waal, 2014; but see Van Berlo et al., 2023). Perhaps out-group bias has deep evolutionary roots indeed (Moffett, 2013; Van Berlo et al., 2023).

For humans, there is good evidence that positive feelings about one’s in-group support helping in-group individuals (Aaldering et al., 2018; Rahal et al., 2020; Swann et al., 2012; Weisel & Zultan, 2021; Whitehouse et al., 2016). It would then be a small step to also assume that seeing ‘them’ as less likable in comparison might cause individuals to be predominantly hostile and aggressive towards out-group individuals (Bar-Tal, 2007; Brewer, 1999; Cikara et al., 2011; Hewstone et al., 2002; Shayo, 2009). Even if out-groups are non-threatening and harmless, evolution may have prepared individuals to dislike outsiders (Wrangham, 1999). Perhaps cooperating for conflict is explained by an evolved ‘dislike-them’ bias that sets up individuals to join coalitions to aggress out-siders.

My collaborators and I examined this possibility in a series of studies with human participants from across the globe – over 12,000 individuals residing in 51 countries worldwide (Romano et al., 2024). In our experiments, individuals were given money that they could use to benefit others (i.e., in an investment game; Van Dijk & De Dreu, 2021) and, in another task, to harm others (i.e., in a contest game; De Dreu et al., 2019). And these others were of two kinds – they were individuals from one’s own community, or they were from another community.

We expected two sets of findings. First, individuals should help someone from their in-group more than someone from out-groups. Such ‘in-group favouritism’ has been observed many times in other studies (for a meta-analytic review, see Balliet et al., 2014). It has been observed in a range of non-human animals (Clutton-Brock, 2002; Long et al., 2022; Silk, 2009), and it is what we found too – humans were ‘parochial’ cooperators. Even in our simplified and rather abstract experiments, individuals cooperate more within than between groups.

Second, if coalitionary aggression rests on evolved out-group bias, we should see that individuals are not only less cooperative, but also more aggressive with someone from an out-group. This, however, we did not find. In fact, we found the opposite. In tens of thousands of decisions made by individuals from across the globe, individuals were less rather than more competitive and aggressive towards members of out-groups. Individuals behaved as ‘nasty neighbours,’ competing more within than between groups (Romano et al., 2024).

That individuals can be more aggressive towards individuals from in- rather than out-groups seems not limited to humans. Neighbour nastiness has been observed in social insects (Newey et al., 2010), group-living birds (Christensen & Radford, 2018), and various mammals such as Eurasian beavers (Benten et al., 2020), banded mongoose (Müller & Manser, 2007), Diana monkeys (Decellieres et al., 2021), and black crested gibbons (Niu et al., 2023). One possible explanation for such neighbour nastiness is that individuals depend more on their own rather than other groups. Such dependency makes it comparatively worthwhile to cooperate within one’s group and to compete for social standing and some share of group revenues. Indeed, we have recently shown with evolutionary game theory and numerical simulations that mixtures of parochial cooperation and neighbour nastiness prevail and can become evolutionary stable when within-group cooperation is comparatively efficient and between-group interactions relatively infrequent (Romano et al., 2025).

That, all else being equal, individuals compete more with in- rather than out-group members is not only robust, generalizable and fitness functional but also, crucially, difficult to reconcile with the argument that unprovoked coalitionary attacks rest on evolved out-group bias. Individuals may like better those they know well and are close to, than unfamiliar others. But evolved dislike of out-group individuals, if at all present, cannot explain well why individuals cooperate to make their groups both strong and dangerous.

Coalitionary Aggression Likely Emerges from Parochial Solidarity

Neighbouring groups and communities may be dislikable, they may be a threat to the in-group and they may be neither dislikable not a threat. Regardless, however, neighbouring groups and communities hold and produce resources that benefit themselves but can also, to varying degrees, benefit outsiders. To serve oneself and one’s in-group, individuals may engage in inter-community trade and exchange of people, goods, and services. And they may participate and invest in coalitionary aggression to seize the resources held and produced by out-groups. If true, participation in coalitionary conflict rests on selfishness and, perhaps, on parochial solidarity – a preferential sympathy with, and concern for the welfare and well-being of the members of one’s own group, community, or society. Parochial solidarity may explain unprovoked coalitionary conflict on non-threatening out-groups when there are no histories of conflict to revenge and retribute, and when out-groups are not dislikeable. Parochial solidarity may explain why citizens and soldiers fight their leaders’ revisionist wars, in far-away countries and for little or no pay. And as parochial solidarity can be a reason for conflict to emerge ex nihilo, it can provide the missing link in the argument that cooperation evolved because of conflict.

Evidence for parochial solidarity as a root cause of conflict comes from a series of experiments that engaged participants in intergroup attacker-defender contest (De Dreu et al., 2016; Snijder & De Dreu, 2025a). The contest involves six individual participants in two groups of three, one attacker and one defender group. Individuals receive an endowment (e.g., 20€), which they can keep to themselves or contribute to group conflict. Whatever they contribute to conflict is lost. Contributions are voluntary personal sacrifices to make one’s group a stronger and more dangerous contender. The contest success function specifies that when attackers together donate more to conflict than defenders, attackers win the conflict and earn whatever money defenders did not invest in group defence (with these ‘spoils of war’ being equally distributed among victors, regardless their individual contributions to conflict). Succinctly put, investing in coalitionary aggression is personally costly yet can return revenues for the in-group and oneself (De Dreu & Gross, 2019; De Dreu et al., 2020).

We performed experiments in the Netherlands, in rural regions in Kenya, and in China. Typically, investments in coalitionary defence exceed those in coalitionary attack and attacks are successful in less than 30% of the cases (De Dreu et al., 2020). Nonetheless, on average, individuals in attacker groups donate to conflict more than what they should have done were they only interested in their personal outcomes (for analyses of game-theoretic equilibria, see De Dreu & Gross, 2019; De Dreu et al., 2016). Possibly, this excess reflects parochial solidarity among the members of attacker groups. Indeed, individuals cooperated more on out-group attacks when they, prior to the contest, socially bonded (Yang et al., 2020; also see Zhang et al., 2019). And pro-social individuals, who value group fairness and welfare, cooperated more on coalitionary attacks than individuals who prioritize their self-interests (Snijder & De Dreu, 2025b; Snijder et al., 2024).

Parochial solidarity within groups increases under ecological pressure. In response to threatening climate change, in-group norm conformity increases (Barth et al., 2018), following natural disasters, social cohesion and helping increases (Calo-Blanco et al., 2017; Li et al., 2013; Vardy & Atkinson, 2019), and crime rates reduce (Hombrados, 2020). Other studies documented increased donations to public goods following natural shocks such as floodings (Whitt & Wilson, 2007) and earthquakes (Pancotto et al., 2024; also see Lojowska et al., 2023). From this and related research follows that individuals in groups under pressure may also become more likely to participate and invest in coalitionary aggression on neighbouring groups and communities, especially when out-groups are better off. This is indeed what we found. In one study, we induced environmental stress by making resources left after investing in conflict subject to risk of destruction (versus not). This created distress and, as expected, increased participation in and coordination of unprovoked coalitionary attacks (De Dreu et al., 2022). In another study, environmental unpredictability was simulated by making the returns of group commons (un)predictable. Unpredictability reduced group cooperation and as expected, increased contributions to coalitionary attacks beyond what could be expected from rational payoff-maximizing individuals. Coalitionary attacks were also better coordinated under environmental unpredictability, suggesting that environmental unpredictability fosters parochialism, in which individuals engage in conflict with outgroups to benefit their ingroup (De Dreu, Reddmann et al., 2025).

Conclusions

Throughout nature, animals pay fitness costs to benefit others, they reward and reciprocate others trust and cooperation, and they help to protect others against outside threat. And throughout nature, animals invest time and energies in coalitionary conflict with neighbouring groups and communities. And to reconcile these two basic tendencies, scholars in the social, behavioural and biological sciences proposed that it is precisely because of conflict and enemy threat that individuals within groups and communities (learned to) cooperate. Because of conflict, cooperation and parochial solidarity evolved.

The argument fits well extant research that animals, human and non-human, are quite willing to self-sacrifice for their group to withstand enemy threat and to defend themselves and their groups against aggressive neighbours. What remains unclear is where enemy threat comes from, and why individuals risk injury and invest resources in coalitionary attacks on non-threatening out-groups. I probed and rejected the possibility that coalitionary aggression rests on evolved out-group bias and dislike for unfamiliar strangers and members from out-groups. I also probed, and could support, the possibility that coalitionary aggression rests on parochial solidarity, and evolved capacities to care for the groups individuals depend on for status, resources and opportunities.

The evidence against out-group bias, and in favour of parochial solidarity, has a non-trivial implication. Rather than that cooperation evolves because of conflict, conflict emerges because of (parochial) cooperation. For the very same reason that individuals cooperate with others in their groups, they also cooperate to make their groups dangerous enemies to outsiders. Indeed, cooperation can evolve independent of the presence of other groups (De Dreu et al., 2024; Van Dijk & De Dreu, 2021) and, as discussed here, independent of whether other groups are threatening or not. Conversely, conflict between groups cannot evolve without individuals in at least one group cooperating to become dangerous to others. One can have group cooperation without intergroup conflict. One cannot have intergroup conflict without group cooperation.

That parochial solidarity explains how individuals cooperate ‘for both good and bad’ should not be taken as if individuals are unconditionally cooperative with their in-group. First, individual animals sometimes initiate and escalate group conflict for selfish reasons (De Dreu & Triki, 2022). Group leaders, for example, can personally benefit from outside wars more than their followers – in human and in non-human societies (Dogan et al., 2018; Glowacki & McDermott, 2022; Hoenig et al., 2025; Massaro et al., 2022; Sankey et al., 2022; Snijder et al., 2025). Second, as discussed above, individuals sometimes even compete more within that between groups. Leaders and followers depend on their groups not only for safety and prosperity, but care too about their own status, their share in group resources, and personal opportunities for growth. We need theory that delineates when, how and with whom individuals decide to connect, cooperate, and compete (Romano et al., 2024, 2025).

There are two further implications of the conclusion that conflict rests on parochial solidarity, rather than the other way around. First, intergroup conflict is not inevitable. As group cooperation does not require intergroup conflict, groups may peacefully co-exist, each minding their own business. We often see such co-existence of groups, in human and animal societies alike. From the present analysis, it follows that such peaceful co-existence prevails and stabilizes when individuals can serve themselves and their groups also, or better, without fighting out-groups. Second, and relatedly, interactions with outsiders can be or become constructive and cooperative. There indeed is a long history of mutually beneficial trade in both human (Pisor & Ross, 2022) and animal societies (Rodrigues et al., 2022). The present analysis suggests that inter-community trade and cooperation emerge when it best serves individuals and their groups. Our recent work supports this idea. Individuals quickly learned how much sacrifice was needed to strike mutually beneficial deals with individuals from distinctly different cultural out-groups (Rojek-Giffin et al., 2023). And when individuals could benefit from their own group and from out-groups, parochial solidarity faded and cross-group cooperation rapidly evolved (Gross et al., 2023, 2024).

Parochial solidarity supports group cooperation, and this can make groups and its members strong and sometimes dangerous. Sometimes group cooperation is used for conflict, and sometimes it is used to strike lucrative deals with other groups and communities. How evolved capacities for cooperation are engaged depends on what best serves individuals and the groups they depend on.

My conclusion that conflict evolves because of cooperation rests mainly on laboratory experiments with human participants from the Netherlands, from China, and from rural parts of Africa. The questions I raised, however, are more general and pertain also to other animal species like social insects, birds, meerkats, lemurs, and chimpanzees. Like humans these animals live in groups and like humans these animals attack and defend against groups of conspecifics (De Dreu & Triki, 2022). Like humans, other animals have evolved capacities for group cooperation (Lemoine et al., 2022; Rilling & Young, 2014; Triki et al., 2022). Not excluded is that also in other animals, parochial solidarity makes groups strong and sometimes dangerous (De Dreu, Herrmann et al., 2025; De Dreu & Triki, 2022). To identify the basic laws that govern sociality across animal species, we need to examine present conclusions on cooperation and conflict in other species.

A final set of open questions pertains to the fact that insights about cooperation and conflict were obtained for individuals in groups of three – the smallest possible group size. The questions raised and the observations that provoked them often involved much larger groups. Ant colonies, like human societies, can contain thousands and thousands of soldiers. And in our experiments, attacker and defender groups were equally sized and equally powerful. Often, conflicts are between groups and societies that differ in size, wealth, and power. Such inequalities may weaken or strengthen parochial solidarity and, with that, weaken or strengthen the link between group cooperation and intergroup conflict (Hauser et al., 2019; Redhead, 2025; Ross & Pisor, 2025; Stallen et al., 2023). We need theory that considers how inequalities contribute to the rise and fall of cooperation and conflict, of groups and organizations, of societies and nation states. In the ultimate, groups and organizations, societies and nation states are built on parochial solidarity and cooperation. And so is conflict.