Right on!

FLYERS at petrol stations do not normally ask for someone to donate a kidney to an unrelated stranger. That such a poster, in a garage in Indiana, actually did persuade a donor to come forward might seem extraordinary. But extraordinary people such as the respondent to this appeal (those who volunteer to deliver aid by truck in Syria at the moment might also qualify) are sufficiently common to be worth investigating. And in a paper published this week in the Proceedings of the National Academy of Sciences, Abigail Marsh of Georgetown University and her colleagues do just that. Their conclusion is that extreme altruists are at one end of a “caring continuum” which exists in human populations—a continuum that has psychopaths at the other end.

Biology has long struggled with the concept of altruism. There is now reasonable agreement that its purpose is partly to be nice to relatives (with whom one shares genes) and partly to permit the exchanging of favours. But how the brain goes about being altruistic is unknown. Dr Marsh therefore wondered if the brains of extreme altruists might have observable differences from other brains—and, in particular, whether such differences might be the obverse of those seen in psychopaths.

She and her team used two brain-scanning techniques, structural and functional magnetic-resonance imaging (MRI), to study the amygdalas of 39 volunteers, 19 of whom were altruistic kidney donors. (The amygdalas, of which brains have two, one in each hemisphere, are areas of tissue central to the processing of emotion and empathy.) Structural MRI showed that the right amygdalas of altruists were 8.1% larger, on average, than those of people in the control group, though everyone’s left amygdalas were about the same size. That is, indeed, the obverse of what pertains in psychopaths, whose right amygdalas, previous studies have shown, are smaller than those of controls.

Functional MRI yielded similar results. Participants, while lying in a scanner, were shown pictures of men and women wearing fearful, angry or neutral expressions on their faces. Each volunteer went through four consecutive runs of 80 such images, and the fearful images (but not the other sorts) produced much more activity in the right amygdalas of the altruists than they did in those of the control groups, while the left amygdalas showed no such response. That, again, is the obverse of what previous work has shown is true of psychopaths, though in neither case is it clear why only the right amygdala is affected.

Dr Marsh’s result is interesting as much for what it says about psychopathy as for what it says about extreme altruism. Some biologists regard psychopathy as adaptive. They argue that if a psychopath can bully non-psychopaths into giving him what he wants, he will be at a reproductive advantage as long as most of the population is not psychopathic. The genes underpinning psychopathy will thus persist, though they can never become ubiquitous because psychopathy works only when there are non-psychopaths to prey on.

In contrast, Dr Marsh’s work suggests that what is going on is more like the way human height varies. Being tall is not a specific adaptation (though lots of research suggests tall people do better, in many ways, than short people do). Rather, tall people (and also short people) are outliers caused by unusual combinations of the many genes that govern height. If Dr Marsh is correct, psychopaths and extreme altruists may be the result of similar, rare combinations of genes underpinning the more normal human propensity to be moderately altruistic.