And while children evolved to bond with us, we may have evolved to want to care for them. Research suggests that simply witnessing motherhood primes us to act as caretakers, even to babies we didn’t birth. One study from 2000, for instance, found that prolactin, a hormone associated with nurturing, increased in men when they were living with a pregnant woman. Another study from 2010 showed that fathers experienced a bump in oxytocin, a hormone associated with social bonding, after spending time with their newborns.

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