Origins of violence: evolutionary decoupling between mild and lethal conspecific aggression in primates.

Theories on the evolutionary origins of human aggression have often implicitly assumed that conspecific aggression is a single behavioral trait. However, different types of aggression can be described, based upon their intensity, frequency, as well as the age and sex of the opponents. The phylogenetic relationships between different types of aggression remain poorly understood. We tested the strength of correlated evolution between five distinct types of aggression in primates, namely, between- and within-group mild (i.e., not life-threatening) aggression, between- and within-group adulticide, and infanticide. We collected data on 100 free-ranging, non-provisioned and group-living species, including humans. Phylogeny had a weaker effect on mild than on lethal aggression; the effect of phylogeny was greater for adulticide, especially when we partitioned our analyses by the sex of the attacker. Furthermore, we found a positive correlation between within- and between-group adulticide, and with infanticide; these results were mostly confirmed when we considered the sex of the attacker. Conversely, the two types of mild aggression were weakly related with lethal aggression. Our study highlights the importance of treating aggression as a complex set of interrelated traits in comparative analyses. Our findings indicate that mild aggression is not closely linked to killing; thus, the escalation of aggression may follow more complex patterns that what predicted by current socio-ecological models.