Same-sex friendships are lasting coalitions between individuals who are the same biological sex.
Friendships are intriguing cross-species phenomena that clearly contribute to both fitness and survival. Friendships involve coalitions based on altruistic interactions over a long period of time. Unlike opposite-sex friendships, same-sex friendships are less likely to include sexual motives. Although affiliative bonding is common in a large number of species, humans may be unique in the extent to which this happens among unrelated individuals. The theories that have been used to explain same-sex friendships are mainly reciprocal altruism and kin selection, which will be reviewed briefly below.
Kin Selection and Same-Sex Friendships
Research that investigates proximate determinants of friendships has found that friends are genetically similar (Christakis and Fowler 2014...
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