Acculturation drives the evolution of intergroup conflict

Abstract

Conflict between groups of individuals is a prevalent feature in human societies. A common theoretical explanation for intergroup conflict is that it provides benefits to individuals within groups in the form of reproduction-enhancing resources, such as food, territory, or mates. However, it is not always the case that conflict results from resource scarcity. Here, we show that intergroup conflict can evolve, despite not providing any benefits to individuals or their groups. The mechanism underlying this process is acculturation: the adoption, through coercion or imitation, of the victor's cultural traits. Acculturation acts as a cultural driver (in analogy to meiotic drivers) favoring the transmission of conflict, despite a potential cost to both the host group as a whole and to individuals in that group. We illustrate this process with a two-level model incorporating state-dependent event rates and evolving traits for both individuals and groups. Individuals can become "warriors" who specialize in intergroup conflicts, but are costly otherwise. Additionally, groups are characterized by cultural traits, such as their tendency to engage in conflict with other groups and their tendency for acculturation. We show that, if groups engage in conflicts, group selection will favor the production of warriors. Then, we show that group engagement can evolve if it is associated with acculturation. Finally, we study the coevolution of engagement and acculturation. Our model shows that horizontal transmission of culture between interacting groups can act as a cultural driver and lead to the maintenance of costly behaviors by both individuals and groups.

Keywords: cultural evolution; group selection; intergroup conflict.

Fig. 1.

Evolution of warrior production with constant group-level traits. (A) Increased warrior production leads to smaller groups. Five replicate simulations with constant $q=0.64,r=0$ were grouped together to calculate mean group density in groups with different mean $p$. Dark ribbon shows standard deviations, and light ribbon indicates the full range of values. The dashed line shows estimated equilibrium values (SI Appendix, Eq. S5). (B and C) Effect of engagement and assortment on warrior production. Each point indicates a group’s mean $p$ after equilibrium has been reached, when $q$ is constant and $r=0$ (no acculturation). B depicts the effect of varying $q$ (engagement), with constant $s=10$, while C depicts the effect of varying $s$ (assortment parameter), with constant $q=0.5$. For each treatment, the results of five replicate runs are shown. Whole-population extinction at high levels of $q$ has been prevented by enforcing fissions if the population drops to a single group.

Fig. 2.

Evolution of warrior production increases the risk of whole-population extinction. Increased warrior production leads to a lower number of groups, which decreases time to extinction. (A) Five replicate simulations with constant $q=0.64,r=0$ were grouped together to calculate mean group size for different values of mean group $p$. Dark ribbon shows standard deviations, and light ribbon indicates the full range of values. The dashed line shows estimated equilibrium values for group density [SI Appendix, Eq. S7, using mean group density in place of $n\left(p\right)$]. (B) Time to whole-population extinction (simulations were interrupted at time t = 1,000). For each value of $p$, we show 50 replicate simulations. All trait values ($p$, $q=1$, and $r=0$) were held constant.

Fig. 3.

Coevolution toward peace and coevolution toward conflicts. Time dynamics of a single replicate, showing the coevolution of the individual-level trait $p$ (warrior production) and the group-level traits $q$ (engagement) and $r$ (acculturation). (A) Without acculturation ($r=0$), $q$ and $p$ coevolve toward a peaceful equilibrium. (B) With maximal acculturation tendency ($r=1$), $q$ and $p$ coevolve, leading to conflicts and maximizing warrior production ($p\approx 0.514$). (C) Acculturation coevolves with the other traits, resulting in an outcome similar to B. Lines indicate population-wide rolling averages of group values ($q,r$) or of mean group values ($p$). Dark ribbons show rolling standard deviations, and light ribbons encompass the full range of values. To make the transient dynamics of $p$ and $q$ clear, mutational effect sizes in A (${\sigma }_q=0.01$) are smaller than in B and C (${\sigma }_q=0.05$).