Sex differences in early aggressive experience at Kanyawara

Experts and parents alike have long debated whether human boys are more physically aggressive than girls because of nature or nurture. Long-term KCP research on young chimpanzees just published in PNAS indicates that increased physical aggression among boys and men may have deep evolutionary and developmental roots.

For humans, one argument is that boys and girls behave differently due to gender socialization. Even though violence is generally discouraged by parents and other role models, boys’ physical aggression is sometimes deemed more acceptable than girls’. Consequently, boys grow up to be more physically aggressive as men.

However, the existence of similar sex differences in aggression in other species, including our closest relatives, suggests that shared evolutionary history and shared developmental processes might also shape these patterns. Previous work by KCP researchers has documented large differences in male and female chimpanzees’ physical aggression. Not surprisingly, aggression is most frequent among adult and adolescent males as they jockey for position in the dominance hierarchy. Males also use aggression to directly contest mating opportunities. Thus, practicing physical aggression and learning how to compete during the juvenile period is likely particularly important for young male chimpanzees.

As part of her dissertation work at University of New Mexico, lead author Kris Sabbi aimed to find out how early experience might shape this sex difference. Were developing male chimpanzees more likely than females to receive aggression from, or to witness aggression among, other group members? And, if so, was this just because they were males, or was there was some aspect of males’ early social lives that put them at greater risk of receiving aggression?

To answer these questions, Sabbi followed young Kanyawara chimpanzees in Kibale National Park, Uganda, over three years. She recorded their social interactions in intricate detail, from whom they traveled and played with, to how far they moved from their mothers. Then she combined her data on social experience with long-term aggression records from KCP to test these ideas.

Young male chimpanzees did, in fact, experience higher rates of aggression from other group members during development. However, this was not a consequence of their spending more time with adult males or being more independent from mothers – these things did not differ by sex in young Kanyawara chimpanzees. Instead, the strongest predictor of how much aggression young chimpanzees received was the amount of aggression that they themselves displayed, regardless of whether they were female or male.

In other words, when male and female chimpanzees of the same age used aggression at the same rate, they received similar amounts of aggression. But young males were generally more aggressive than young females, so males received more aggression overall. The ultimate source of this particular sex difference in social experience is thus found in an early-emerging sex difference in the use of physical aggression.These findings highlight the fact that infants and juveniles actively shape their social experiences, rather than passively receiving socialization, and call for a deeper look into the ways that experience may interact with other aspects of development, like hormonal profiles, to shape individual differences in adult behavior.

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