Children as young as 3 years old are willing to punish others' bad behavior, even at personal cost, finds a new study by psychology researchers at New York University. The work adds to growing evidence that human beings distinguish between right and wrong at a very young age and are willing to pay a personal cost to encourage positive behavior in others.

"Morality is about more than just doing good oneself—it is also about encouraging good behavior in others," says lead author Daniel Yudkin, a postdoctoral researcher at Yale University and an NYU doctoral student at the time of the study, which appears in the Journal of Experimental Psychology. "These results highlight a distinctly human aspect of moral behavior."

The scientists, who included NYU psychology professors Marjorie Rhodes and Jay Van Bavel, sought to better understand a uniquely human trait: our willingness to punish, at personal cost, "bad actors" who haven't harmed us directly.

"This behavior, known as 'costly third-party punishment,' is interesting because it is believed to underlie people's conception of justice," Yudkin explains. "Specifically, it relates to justice because it involves people making sure others are acting fairly, even if their behavior doesn't impact them."

The researchers focused on young children to better understand this behavior for the following reason: seeing how we think about punishment early in life can help shed light on the underlying psychological processes driving this behavior.

Yudkin and his colleagues deployed a naturalistic experiment—one aimed at capturing the reality of children's everyday lives. In it, more than 200 children, aged 3 to 6, were recruited from the Children's Museum of Manhattan and brought one at a time into a classroom with a large red slide in the corner. The children were given the chance to try out the slide and all of them reported enjoying doing so. Next, they were shown a video of a little girl ("Stacey") tearing up someone else's drawing, then were told that Stacey planned to come back into the room later in the day to play on the slide.

The children were then given a sign—one side of which said "Open" and the other "Closed." They were told that if they put the "Open" sign on the slide, then they could go down the slide and Stacey could, too. If they put the "Closed" sign on, then they could stop Stacey from going down, but then they couldn't go down, either. In other words, punishing Stacey came at a cost to themselves, too—they would be denied the chance to do something they previously said they enjoyed (all children confirmed their understanding of the significance of their choice to the researchers).

Surprisingly, about half the children across all age groups—including some as young as 3 years old—enacted costly punishment. Rates of punishment increased with age: children aged 5 and 6 punished at about three times the rate of those aged 3 and 4.

In a follow-up experiment, the researchers wanted to test what makes children more or less likely to punish. To do this, they randomly assigned participants to different conditions. Under one, some learned that Stacey was a member of the Children's Museum and others learned she was a member of the Boston Museum (thereby manipulating Stacey's "group membership"). In another, some children wore a sheriff's badge while others did not (thereby manipulating punishers' sense of "authority"). They then examined whether rates of punishment differed according to which condition participants had been assigned.

The results showed that group membership and authority did in fact affect children's likelihood of punishment, but in an unexpected way. Typically, social science experiments show that people treat out-group members more harshly than in-group members. Thus, participants might be expected to punish Stacey more when they believed she was a member of the Boston museum rather than as a member of the Children's Museum, since the former would designate Stacey's membership to an out-group. And indeed, among the youngest children, this is exactly what happened: children were more than twice as likely to punish Stacey in the former than the latter condition.

However, this finding occurred only when the children were not wearing a sheriff's badge. When they were wearing the badge, they demonstrated precisely the opposite pattern, punishing Stacey more when she was a member of the Children's Museum than when she was a member of the Boston Museum. The researchers termed this effect "in-group policing" and concluded the following: people become more committed to ensuring that members of their own communities are behaving well when they feel a sense of responsibility.

More generally, the researchers note that we frequently encounter examples of those who perform third-party punishment—from those who risk arrest at a protest over a matter that doesn't directly affect them to others who stand up to protect a stranger being harassed on the subway.

What's behind the acquisition of this tendency is unclear. One possibility is reputational: that people do it merely to look good to others. Another possibility is that it is innate: that people are intrinsically willing to uphold moral rules.

"By showing that even some children as young 3 years of age do enact costly punishment, we provide evidence that reputation isn't the only thing driving this behavior," notes Yudkin, who adds that past research suggests that children at this age don't take into account their reputations when making decisions that affect themselves or others.

"Of course, we cannot tell for sure whether this behavior is innate or learned in the first few years of life," he concludes. "But it does add to growing evidence that, at a very young age, humans are predisposed to do good themselves and encourage good behavior in others."