Psychology says that many of the friendships we carry into old age are the ones formed before thirty — not simply because of loyalty, but because there is a natural limit to how many close relationships the human brain can sustain.

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The friendships people carry into old age often look, from the outside, like proof of loyalty. Two people still call each other after fifty years. Three old friends still meet for lunch. A school friend still knows the family story without needing the short version. It is easy to explain this as sentiment: they stayed ... Read more