We are wired to have friends, not just reproductive partners, and when we form friendships, we modify the social world around us in a way that makes friendliness useful.

Friendship is a fundamental category… yet scientists have tended to neglect the role friends have played in the life of our species. An extreme focus by our species on kinship and marriage has obscured the more numerous relationships people have with unrelated friends. These friends are also, after all, the primary members of the social groups we form and live within.