In recent decades, psychopathy is something that’s captured the attention of both academics and the mainstream. Psychopaths play big roles in movies and even occasionally on public radio, and there’s evidence that a few of them may be in your company’s boardroom right this minute.

But emerging research is changing how experts understand the condition. “There was a time when people thought of psychopaths as this sort of unique group of individuals — as in, there were normal people, and there were psychopaths,” said Georgetown University psychologist Abigail Marsh. “But now we’re finding that psychopathic traits work the same as other mental-illness symptoms. So with psychopathy, like almost anything else, people will have more or fewer of those traits, and so you have people at one end and most people in the middle.” Marsh calls this the “caring continuum,” and its existence, she said, “begs the question: What’s at the other end of the curve?”

New research she just published in Proceedings of the National Academy of Sciences suggests an answer: If the dark, scary end of the caring continuum is inhabited by psychopaths, way down at the other end is a group of what she calls “anti-psychopaths” — ultra-do-gooders who are extraordinarily compassionate, prosocial, and empathetic.

Marsh wanted to study the characteristics of these sorts of people, so she sought so-called “altruistic kidney donors” who offer up a kidney to anyone who needs it (as opposed to those who donate a kidney to a friend or loved one), figuring they would fit the bill.

Altruistic kidney donors fit into a debate that has long been raging among psychologists and others who study human nature: Does true altruism — good deeds for the sake of good deeds — really exist? And if so, how did it evolve? “In theory, you’d have a species where nobody wants to help anybody else,” said Marsh. “And the fact that humans do is, I think, really amazing and not well understood … So these kidney donors, they’ve done something I call extraordinary altruism, because it’s extremely unusual — it’s something most people would not do. It’s a pretty major decision to undertake, especially for a stranger.”

Altruistic kidney donors voluntarily sign up for an invasive surgery, which results in the removal of a perfectly healthy organ, all because a complete stranger (whom they’ll never meet) needs it. And, as you might guess, these donors are very rare — there are fewer than 1,400 in the U.S., Marsh said — but they help meet a huge need for donor kidneys in the U.S.(Kidney disease is among the top ten causes of death for both men and women in the U.S., according to the U.S. Centers for Disease Control.)

For this study, Marsh and her colleagues replicated several brain-scan studies that had been done on psychopaths, applying the same procedures to 19 donors to examine differences between the two groups. For example, people who score high on psychopathy tests don’t easily recognize images of fearful expressions; in fact, scans show their brains respond less strongly to those images than typical people. Would donors, on the other hand, show a hyperawareness for fearful expressions?

On this and a variety of other measures, Marsh indeed found that the brains of donors had opposite reactions to those of psychopaths. And the donors’ brains even looked structurally different from psychopaths’: While psychopaths have a physically smaller amygdala — the brain structure associated with emotion — than the average person, the donors had oversize ones.

Here’s a bleaker read of these findings: Perhaps these extreme altruists are not exactly “opposite” of a psychopath but, in fact, “rather similar to one, in that instead of being ‘addicted to evil’ they are ‘addicted to good,’” said James Fallon, a neuroscientist at the University of California, Irvine, who has studied the brains of serial killers to see how they differ from typical people’s brains. (And through this work Fallon found that his own brain resembles that of a psychopath.)

That’s one way to look at it. Still, to Marsh, the fact that there may be a neurobiological explanation for the behavior of altruistic kidney donors is valuable in its own right. “It’s surprising how many people assume that it couldn’t actually be altruism that’s motivating them to donate, [that] it must be something else,” she said. “But what this study suggests is that, cognitively and neurally, they look exactly like we would expect unusually altruistic people to look, which supports the idea that their donations were, in fact, driven by altruism.”

But this study also suggests that these ultra-altruists don’t see themselves as anything special. The researchers asked the donors to rate themselves on how empathetic they believed they were, and most gave themselves low scores. “In fact, one of our participants who was most sensitive to the fearful expressions gave himself a very low self-reported empathy score,” Marsh said. The world’s nicest people, in other words, don’t even grasp how nice they really are. Which, if you think about it, makes them even nicer.