Altruists don’t just have bigger hearts; they may have bigger brains, according to emerging research into organ donation from Georgetown University. A new study finds people who signed up to donate a kidney had significantly larger amygdalae on the right side of their brains.

Lead author of the study, psychologist Abigail Marsh, suspects the reason for the enlargement has little to do with people disregarding their chances of survival, as one evolutionary argument suggests. Nor does it indicate people’s latent expectation of getting something in return, she says. The real reason most likely involves our primitive necessity to read babies’ facial expressions, and natural selection’s goal of keeping the necessity around.

“Because we are mammals that give birth to these very helpless young, we’re predisposed to respond to anything that reminds us of a vulnerable, helpless infant,” Marsh told the Los Angeles Times.

She and her research team uncovered the finding as part of an ongoing investigation into extreme altruism. They collected 39 registered kidney donors and, one by one, put them inside an fMRI scanner. Marsh was delighted to find not even a litany of questions designed to assess psychopathy was capable of souring the subjects’ moods. “They could not have been more delightful study participants,” she said.

After a bevy of tasks related to face-emotion processing skills and a series of brain imaging scans, the neuroscientists found altruists’ brains were larger than the average person’s, by about nine percent, and primarily in the region known as the amygdala. The region is responsible for fear and emotional processing. It’s what talks to the hippocampus, where memories are stored, to know when you should be afraid.

While the altruists were at the high end for compassion and empathy, they weren’t necessarily outstanding. They were human — fallible and sometimes reluctant. But what Marsh and the team showed was there was a profound gap between people who choose to give and people who may be interested in giving, but don’t. And it’s in that leap — from desire to action — that ultimately separated the altruists from the controls.

“The results of brain scans and behavioral testing suggests that these donors have some structural and functional brain differences that may make them more sensitive, on average, to other people's distress,” Marsh explained in a statement.

Kidney disease is the eighth-leading cause of death in the United States, according to the American Kidney Fund, with diabetes remaining the top cause for kidney failure, at 38.4 percent of all cases. Heart disease ranks as the leading cause of death among chronic kidney disease (CKD) sufferers. But while 26 million adults suffer from CKD, in 2013 only 16,896 kidney transplants were made.

Luckily, that number is rising. In 2000, the rate hovered around 14,000. And one of them, as Marsh explains, was a man by the name of Harold Mintz.

A former northern Virginian who volunteered with the Washington Regional Transplant Community and agreed to participate in the Georgetown study, Mintz says his decision to donate came as the result of years of wanting to give back somehow. In 1988, he and his wife stumbled upon a needy family in a mall, and even though they both donated blood trying to qualify for a bone marrow transplant, to help the girl’s leukemia, she ended up dying. Twelve years later, he finally got the opportunity to help an Ethiopian refugee who had settled in Washington, D.C.

“All these stories just kind of stuck inside my head,” said Mintz, a tried-and-true altruist, referring to the moments before he eventually divorced the organ from its partner. “And every time I’d see a story about a medical story of distress, it would just kind of get put away in a file inside my heart.”

Source: Marsh A, Stoycos S, Brethel-Haurwitz K, Robinson P, VanMeter J, Cardinale E. Neural and cognitive characteristics of extraordinary altruists. Proceedings of the National Academy of Sciences. 2014.