If evolution chanced upon a way to bind people together into large groups, the most obvious glue is oxytocin, a hormone and neurotransmitter produced by the hypothalamus. Oxytocin is widely used among vertebrates to prepare females for motherhood. In mammals it causes uterine contractions and milk letdown, as well as a powerful motivation to touch and care for one’s children. Evolution has often reused oxytocin to forge other kinds of bonds. In species in which males stick by their mates or protect their own offspring, it’s because male brains were slightly modified to be more responsive to
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