While altruism may explode during the holiday season, some folks are extraordinarily generous year-round. Turns out that extreme altruists, such as folks who donate their kidneys to strangers, have brains wired differently from most of us.

Researchers at Georgetown University examined the brain structures of 19 kidney donors, comparing them with 20 people in a control group. They found that super altruists were better at reading signs of distress on people's faces, and that a certain part of their brains that processes emotion was larger than that part in the control group.

"The findings suggest that individual differences in altruism may have an underlying neural basis," wrote Georgetown psychology professor Abigail Marsh, a chief author of the study published in the Proceedings of the National Academy of Sciences.

Moral, ethical and genetic theories have been offered to explain what drives people to do good. There are also jokes about it being motivated by Catholic/Lutheran guilt or generating "good karma" or some other personal reward.

But the study shows there is a neurological basis behind the truly altruistic — folks who voluntarily help other people at some sacrifice to themselves.

Researchers found that a part of the brain associated with processing emotion — the right amygdala — was 8 percent larger on average among the kidney donors.

Conversely, this same part of the brain was smaller among psychopaths they studied earlier, using similar methods.

The two groups are at different ends of the "caring continuum" spectrum, the study said.

The study involved taking functional MRIs (magnetic resonance imaging) of participants' brains to record neural activity while they looked at pictures of faces with angry, scared or neutral expressions. The fearful images created much more activity in the right amygdala of the kidney donors than among the control group.

Researchers hope the findings prompt further studies exploring the links between biology and benevolence.

Jean Hopfensperger • 612 673 4511