Well before “not fair!” becomes a staple phrase of your child’s spoken repertoire, he or she might already have a fundamental grasp of right and wrong. A study published last October in PLoS One found that 15-month-old infants could identify unequal distributions of food and drink and that this sense of fairness was connected to their own willingness to share.

To measure these moral sentiments, researchers first had the children watch movies of an actor distributing food, either equally or unequally, between two people. Most of the toddlers spent more time looking at the unequal outcome, suggesting it surprised them by violating their basic sense of fair­ness. Next, every child picked his or her favorite of two new toys, and the researchers then asked the kids to share one of the toys. Of the infants who shared their favorite toy, 92 percent had also been surprised by the unfair outcome in the videos.

Scientists have typically thought that other-regarding preferences—which may have played an important role in the evolutionary history of human cooperation—emerge in early or mid-childhood, around the ages of seven or eight. This study suggests that they may develop as early as the second year of life and that those early moral judgments and behaviors are more closely intertwined than ever expected.

This article was published in print as "Baby Justice."