A second study identified some bias in infants, however. First, infants were asked to pick one of two snacks. Then, two puppets were shown enjoying one of the two snacks. Those same puppets were then used in a skit that once again featured "nice" and "mean" behavior. When the puppet perceived as "different" from the babies due to snack preference was treated in a mean fashion, the infants showed support for his attacker following the show, suggesting that the infants preferred those puppets who harmed "the other."
The scholars trace this internal bias to our species' evolution via natural selection. The researchers say that human infants are predisposed to divide the world into "us" and "them" categories.
Studies of older children suggest that society and parental nurturing can reduce this bias in children and that by the age of eight, generosity blooms and children in lab studies are likely to consider the needs of unknown others as equal, if not greater, than their own.