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Abstract
The scale of human cooperation is an evolutionary puzzle. All of the available evidence
suggests that the societies of our Pliocene ancestors were like those of other social
primates, and this means that human psychology has changed in ways that support larger,
more cooperative societies that characterize modern humans. In this paper, we argue
that cultural adaptation is a key factor in these changes. Over the last million years
or so, people evolved the ability to learn from each other, creating the possibility
of cumulative, cultural evolution. Rapid cultural adaptation also leads to persistent
differences between local social groups, and then competition between groups leads
to the spread of behaviours that enhance their competitive ability. Then, in such
culturally evolved cooperative social environments, natural selection within groups
favoured genes that gave rise to new, more pro-social motives. Moral systems enforced
by systems of sanctions and rewards increased the reproductive success of individuals
who functioned well in such environments, and this in turn led to the evolution of
other regarding motives like empathy and social emotions like shame.