Origins of "us" versus "them": prelinguistic infants prefer similar others

A central feature of human psychology is our pervasive tendency to divide the social world into "us" and "them". We prefer to associate with those who are similar to us over those who are different, preferentially allocate resources to similar others, and hold more positive beliefs about similar others. Here we investigate the developmental origins of these biases, asking if preference for similar others occurs prior to language and extensive exposure to cultural norms. We demonstrate that, like adults, prelinguistic infants prefer those who share even trivial similarities with themselves, and these preferences appear to reflect a cognitive comparison process ("like me"/"not like me"). However, unlike adults, infants do not appear to prefer others with an utterly arbitrary similarity to themselves. Together, these findings suggest that the phenomena of ingroup bias, and enhanced interpersonal attraction toward those who resemble ourselves, may be rooted in an inherent preference for similarity to self, which itself may be enhanced during development by the influence of cultural values.