Partisanship starts early. Way early. Babies, green beans, graham crackers…good and bad puppets.
"Far more surprising was that almost all the infants at both ages preferred the character who harmed the dissimilar puppet over the character who helped him. Infants’ preference for those who harmed dissimilar others was just as strong as their preference for those who helped similar ones."
“'The fact that infants show these social biases before they can even speak suggests that the biases aren’t solely the result of experiencing a divided social world, but are based in part on basic aspects of human social evaluation,' says Hamlin."
And a surprising aside for those who can't access the article: 40% of the infants preferred the green bean.