If a child you know refuses to share his toys, chances are he knows he is doing wrong but cannot help it. New research published in March in Neuron reveals that underdevelopment of an impulse control center in the brain is, at least in part, the reason children who fully understand the concept of fairness fail to act accordingly.

As babies, we are inherently selfish, but as we grow, we become better at social strategy—that is, satisfying our own needs while behaving in a manner acceptable to others. Nikolaus Steinbeis of the Max Planck Institute for Human Cognitive and Brain Sciences in Leipzig, Germany, wondered how this skill develops.

Steinbeis and his team examined kids aged six to 14 performing two similar decision-making tasks that involved sharing poker chips with an anonymous recipient (the chips were redeemable for prizes). In task one, the size of a child's offering carried no consequences, but in the second task, the anonymous youngster could reject the offer, if he or she considered it unfair, and both children would get nothing. Task two thus required social strategy; task one did not.

In task one, older and younger children behaved similarly. But in task two, younger children both made worse offers and were more willing to accept bad offers even though they understood that these offers were unfair. Imaging the kids' brains while they performed the tasks revealed less activity in the younger kids' impulse-control regions in their prefrontal cortex, the seat of decision making and self-control in the brain. In addition, independent of age, less activity in this region paralleled less social strategy.

So if a kid has trouble playing fair, it is probably not because he does not understand the concept. Rather he simply cannot resist the urge to grab all the cookies and run. Steinbeis points out, however, that this finding does not excuse bad behavior. “Just because the brain is that way doesn't mean it can't be changed,” he says. “Education and setting a good example can have an enormous impact.”