The roots and branches of human altruism.

The authors' rigorous and ingenious programme of work documents young humans' capacity for prosocial action (Warneken & Tomasello, 2009). Their laboratory findings are corroborated by centuries of observational data, spanning cultures, and historical epochs. Two general questions are raised. Firstly, what are the rules of evidence needed in using comparative data to make claims about the evolution of human social behaviours? Secondly, what ontogenetic processes contribute to the transformation of toddlers' helpfulness into mature, cognitively informed and rule-governed altruism? These findings alone do not provide complete evidence for the phylogenetic roots of altruism. I argue that selective pressures in human evolution have favoured sociability, which could lead to aggression or altruism, depending on context and the nature of the rearing environment. Any social behaviour shown in infancy may have multiple functions, at phylogenetic, ontogenetic and episodic levels of analysis.