The evolutionary origins of friendship.

@article{Seyfarth2012TheEO,
  title={The evolutionary origins of friendship.},
  author={Robert M. Seyfarth and Dorothy L. Cheney},
  journal={Annual review of psychology},
  year={2012},
  volume={63},
  pages={
          153-77
        }
}
Convergent evidence from many species reveals the evolutionary origins of human friendship. In horses, elephants, hyenas, dolphins, monkeys, and chimpanzees, some individuals form friendships that last for years. Bonds occur among females, among males, or between males and females. Genetic relatedness affects friendships. In species where males disperse, friendships are more likely among females. If females disperse, friendships are more likely among males. Not all friendships, however, depend… 

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