Children as young as age 4 express dislike of and are willing to punish those who freeload off the work of other group members, a new Yale University study has found.

But kids also make a clear distinction between those who freeload intentionally and those who have good reasons why they can’t contribute.

“This expectation of cooperation emerges by pre-school and maybe even earlier,” said Yarrow Dunham, assistant professor of psychology and senior author of the paper published in Psychological Science.

Psychologists have long wondered why humans tend to be cooperative and are willing to make sacrifices to sustain cooperation. We pay taxes, stop at red lights, help elderly neighbors — and dislike and are willing to punish those who violate these social norms. Psychologists have debated whether this occurs naturally or arises during socialization.

Dunham and Fan Yang, a Yale postdoc, designed a series of experiments to determine whether the very young — who by definition are not contributing members of a group — share this feeling about freeloaders.

Children from ages 4 to 10 were presented with scenarios in which they had to give up chocolates in order to get a cake or plant seeds in a garden to get tomatoes. All children expressed dislike for those who did not contribute and were even willing to give up stickers to punish them. The youngest subjects exhibited a stronger aversion to free-riders than 9- and 10-year-olds.

However, when a freeloader has a good excuse for not contributing — e.g. her pet ate her chocolate — the aversion was greatly reduced, the researchers report.

“Even young children expect cooperation and are willing to work to sustain it even at cost to themselves,” Dunham said. “I find this very positive. The seeds that sustain cooperation seem to emerge early on, and while as a society we need to sustain and nurture these values, we may not need to instill them in the first place.”