Our research shows that by 8 months of age, infants prefer to play with puppets who treat bad individuals badly, even over those who treat bad individuals well. Critically, the “bad” puppets in our studies have only been mean to other puppets —they have never been mean to the infants themselves—and yet infants still seem to appreciate new individuals who respond to these puppets’ behavior in kind by taking their toy away. By an age in which they can engage in more complex behavioral responses (toddlers around 21 months), infants themselves choose to give treats to good puppets and take them from bad puppets.

This may, indeed, indicate that infants are born with a notion of an eye for an eye, or at least that such concepts develop very early on, and certainly before we might think parents are teaching their children about the appropriateness of punishment. Reward and punishment especially seem to be a good explanation of our toddler finding—toddlers choose to direct something positive toward someone who has treated others well, and something negative toward someone who has mistreated others.

Our infants, however, may have been responding to something a bit different, and our studies cannot really tease these explanations apart thus far. Specifically, infants may be responding based on something more like “the enemy of my enemy is my friend”—they may have liked the character who apparently agreed with them that the bad guy was bad. That is, the character’s mistreatment of the bad guy could have been used as simple evidence of a shared negative opinion. While this is still impressively complex judgment for an infant, it doesn’t contain the same notions of deservingness as the phrase “an eye for an eye.”

We hope to be able to tease apart these two possibilities with future research.

J. Kiley Hamlin is a Canada Research Chair and professor of psychology at the University of British Columbia and the director of the UBC Centre for Infant Cognition.