Infants as young as nine months old prefer individuals who are nice to people like them and mean to people who aren’t like them, according to a new study published in Psychological Science, a journal of the Association for Psychological Science.

In our social lives, we tend to gravitate toward people who have things in common with us, whether it’s growing up in the same town, disliking the same foods, or even sharing the same birthday. And research suggests that babies evaluate people in much the same way, preferring people who like the same foods, clothes, and toys that they like.

This preference helps us to form social bonds, but it can also have a dark side. Disliking people who are different than us may lead us to mistreat them, and excuse — or even applaud — cases in which others mistreat people who are different than us.

Are the roots of such tendencies present in infancy?

To find out, psychological scientist Kiley Hamlin, now a professor at the University of British Columbia, conducted two studies as a graduate student at Yale University with her advisor Karen Wynn and colleagues.

The researchers had 9- and 14-month-old infants choose which food they preferred: graham crackers or green beans. The infants then watched a puppet show in which one puppet preferred graham crackers, while another preferred green beans. That is, one puppet demonstrated that its food preference was the same as the infant’s, while the other demonstrated that its food preference was different from the infant’s.

After the puppets chose their foods, infants then watched another puppet show, in which either the similar puppet or the dissimilar puppet dropped its ball and wanted it back. On alternating events, infants saw that one character always helped the ball-less puppet by returning the ball to him, while another character always harmed the ball-less puppet by stealing the ball away.

Finally, infants were given the chance to choose between the helper (giving) and harmer (stealing) puppets (see videos of the procedure) .

Unsurprisingly, infants’ choices revealed that almost all the infants in both the 9- and 14-month-old groups preferred the character who helped the similar puppet over the character who harmed the similar puppet. Previous research has shown that infants like people who are nice to totally unknown individuals, so it makes sense that they would also like people who are nice to individuals who are similar to them.

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.

According to Hamlin, these findings suggest that “like adults, infants incorporate information about not only what people do (e.g., acting nicely or meanly) but also whom they do it to (e.g., a person who is liked or disliked) when they make social evaluations.”

The researchers confirmed these results in a second experiment, which included a neutral puppet that had demonstrated no food preference and no helpful or harmful behaviors.

This time, the 14-month-olds — but not the 9-month-olds — preferred the character that harmed the dissimilar puppet over the neutral puppet, and the neutral puppet over the helper of the dissimilar puppet. These results suggest that when a dissimilar individual is in need, 14-month-olds generate both positive feelings toward those who harm that individual and negative feelings toward those who help him. The researchers suggest that between 9 and 14 months, infants develop reasoning abilities that lead to these more nuanced social evaluations.

These results highlight the fundamental mechanisms that underlie our interactions with similar and dissimilar people.

“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.

But the exact reasons for infants’ biased evaluations are still unknown.

“Infants might experience something like schadenfreude at the suffering of an individual they dislike,” Hamlin notes. “Or perhaps they recognize the alliances that are implied by social interactions, identifying an ‘enemy of their enemy’ (i.e., the harmer of a dissimilar puppet) as their friend.”

Hamlin emphasizes that even if these kinds of social biases are “basic,” it doesn’t mean that more extreme outcomes, like xenophobia and intergroup conflict, are inevitable.

“Rather, this research points to the importance of socialization practices that recognize just how basic these social biases might be and confront them head-on,” she concludes.

Co-authors on this research include Neha Mahajan of Temple University, Zoe Liberman of the University of Chicago, and Karen Wynn of Yale University.

This research was supported by National Science Foundation Grant BCS-0921515 and National Institutes of Health Grant R01-MH-081877 to Karen Wynn.

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Videos of the experimental procedure are available at: http://cic.psych.ubc.ca/Media_Videos.html

For more information about this study, please contact: Kiley Hamlin at kiley.hamlin@psych.ubc.ca or Karen Wynn at karen.wynn@yale.edu.