The researchers found kids raised in religious households were “significantly less sharing” than those from non-religious households. “Christian children did not differ in their altruism from Muslims,” they write. “However, both were significantly less altruistic than non-religious children.”

What’s more, kids from highly religious families (that is, those who attend services more often, and integrate religion into their everyday lives) were less likely to be altruistic than those with moderate levels of religiosity.

Decety and his colleagues found one important difference between kids from different religious backgrounds. Young Muslims were the most judgmental toward the people who displayed anti-social behavior in the short videos, and suggested harsher punishments than Christians or non-religious kids.

The researchers aren’t sure why religious kids would be less altruistic, but they suspect “moral licensing”—the idea that doing something virtuous gives us license to subsequently engage in unethical behavior—may be one explanation. Children who have said their prayers, and thus done their good deed for the day, may feel free to act selfishly later.