The demography of human warfare can drive sex differences in altruism

Culture and group-functional punishment behaviour

Humans often 'altruistically' punish non-cooperators in one-shot interactions among genetically unrelated individuals. This poses an evolutionary puzzle because altruistic punishment enforces cooperation norms that benefit the whole group but is costly for the punisher. One key explanation is that punishment follows a social-benefits logic: it is eminently normative and group-functional (drawing on cultural group selection theories). In contrast, mismatch-based deterrence theory argues that punishment serves the individual-level function of deterring mistreatment of oneself and one's allies, hinging upon the evolved human coalitional psychology. We conducted multilateral-cooperation experiments with a sample of Spanish Romani people (Gitanos or Calé) and the non-Gitano majority. The Gitanos represent a unique case study because they rely heavily on close kin-based networks and display a strong ethnic identity. We find that Gitano non-cooperators were not punished by co-ethnics in only-Gitano (ethnically) homogeneous groups but were harshly punished by other Gitanos and by non-Gitanos in ethnically mixed groups. Our findings suggest the existence of culture-specific motives for punishment: Gitanos, especially males, appear to use punishment to protect their ethnic identity, whereas non-Gitanos use punishment to protect a norm of universal cooperation. Only theories that consider normative, group-functional forces underlying punishment behaviour can explain our data.

Fitness consequences of outgroup conflict

In social species across the animal kingdom, conspecific outsiders threaten the valuable resources of groups and their members. This outgroup conflict is recognised as a powerful selection pressure, but we argue that studies explicitly quantifying the fitness consequences need to be broader in scope: more attention should be paid to delayed, cumulative, and third-party fitness consequences, not just those arising immediately to group members involved in physical contests. In the first part of this review, we begin by documenting how single contests can have survival and reproductive consequences either immediately or with a delay. Then, we step beyond contests to describe fitness consequences that can also result from interactions with cues of rival presence and the general landscape of outgroup threat, and beyond single interactions to describe cumulative effects of territorial pressure and elevated outgroup-induced stress. Using examples from a range of taxa, we discuss which individuals are affected negatively and positively, considering both interaction participants and third-party group members of the same or the next generation. In the second part of the review, we provide suggestions about how to move forward. We highlight the importance of considering how different types of outgroup conflict can generate different selection pressures and of investigating variation in fitness consequences within and between species. We finish by discussing the value of theoretical modelling and long-term studies of natural populations, experimental manipulations, and meta-analyses to develop further our understanding of this crucial aspect of sociality.

Relatedness within and between Agta residential groups

Theoretical models relating to the evolution of human behaviour usually make assumptions about the kinship structure of social groups. Since humans were hunter-gatherers for most of our evolutionary history, data on the composition of contemporary hunter-gatherer groups has long been used to inform these models. Although several papers have taken a broad view of hunter-gatherer social organisation, it is also useful to explore data from single populations in more depth. Here, we describe patterns of relatedness among the Palanan Agta, hunter-gatherers from the northern Philippines. Across 271 adults, mean relatedness to adults across the population is r = 0.01 and to adult campmates is r = 0.074, estimates that are similar to those seen in other hunter-gatherers. We also report the distribution of kin across camps, relatedness and age differences between spouses, and the degree of shared reproductive interest between camp mates, a measure that incorporates affinal kinship. For both this this measure (s) and standard relatedness (r), we see no major age or sex differences in the relatedness of adults to their campmates, conditions that may reduce the potential for conflicts of interest within social groups.

The evolution of altruism through war is highly sensitive to population structure and to civilian and fighter mortality

The importance of warfare in the evolution of human social behavior remains highly debated. One hypothesis is that intense warfare between groups favored altruism within groups, a hypothesis given some support by computational modeling and, in particular, the work of Choi and Bowles [J.-K. Choi, S. Bowles, Science 318, 636-640 (2007)]. The results of computational models are, however, sensitive to chosen parameter values and a deeper assessment of the plausibility of the parochial altruism hypothesis requires exploring this model in more detail. Here, I use a recently developed method to reexamine Choi and Bowles' model under a much broader range of conditions to those used in the original paper. Although the evolution of altruism is robust to perturbations in most of the default parameters, it is highly sensitive to group size and migration and to the lethality of war. The results show that the degree of genetic differentiation between groups (FST ) produced by Choi and Bowles' original model is much greater than empirical estimates of FST between hunter-gatherer groups. When FST in the model is close to empirically observed values, altruism does not evolve. These results cast doubt on the importance of war in the evolution of human sociality.

Modelling cultural selection on biological fitness to integrate social transmission and adaptive explanations for human behaviour

One of the difficulties with cultural group selection theory highlighted in the review by Smith (2020, Evol. Hum. Sci., 2, e7) is its inability to separate the evolutionary effects of selection of cultural traits based on biological fitness (Cultural Selection 1) from the effects of selection based on cultural fitness (Cultural Selection 2). Confusing these two processes can hinder the integration of adaptive explanations for human behaviour, which focus on biological fitness, and cultural evolution explanations, which often focus on social transmission. Recent empirical work is starting to bridge this gap, but progress in mathematical modelling has been considerably slower. Here, I suggest that modellers can contribute to achieving this integration by further developing models of Cultural Selection 1, where behaviours are influenced by culturally inherited traits selected on the basis of their effects on biological fitness. These models should build on existing social evolution theory methods and replace genetic relatedness with cultural relatedness, that is the probability that two individuals share a cultural variant.