Can evolution explain why some humans donate kidneys to strangers?

The standard evolutionary tools for explaining altruism run out quickly here. Kin selection explains why you would help a sibling or a child, since shared genes benefit indirectly. Reciprocal altruism explains why you would help a stranger who can help you back later, or whose community will remember the favor. Neither applies to a person who gives a kidney to someone they will never meet again and who has no way to repay them. The recipient often does not even know the donor's name.

In a conversation on Hi-Phi Nation, evolutionary psychologist Michael McCullough takes this puzzle seriously rather than dismissing it. His answer is not that evolution selected for kidney donation directly. It is that natural selection gave humans a general capacity for reasoning, and that capacity, once it existed, generated something new: an impartial moral code that treats strangers as worthy of the same consideration as kin. On this view, altruistic kidney donation and modern welfare states are both downstream effects of a moral principle like comparable sacrifice, the idea that if a stranger's need is great and your cost is small enough, you ought to help. The behavior itself was never selected for. The reasoning faculty was, and reasoning followed the logic of impartiality wherever it led, including to conclusions that are costly and non-reciprocal.

This is where the popular misreading of evolutionary explanations of altruism goes wrong. It is easy to assume that if evolutionary psychology can explain a behavior at all, it must ultimately reduce that behavior to disguised self-interest, some hidden payoff in reputation or genetic fitness. McCullough's account resists that reduction. He is not saying kidney donors are secretly getting something back. He is saying the very faculty that lets us calculate self-interest also lets us calculate what fairness demands, and sometimes those calculations point away from the self entirely.

Barry Lam and philosopher Kieran Setiya push back on this in the same conversation. Their objection is not that the moral code McCullough describes is fake, but that altruistic kidney donation is a rare outlier. Most people do not do it. If an evolutionary story is going to explain a trait, it needs to explain something closer to a species-typical pattern, not a behavior performed by a small fraction of the population who then get profiled precisely because they are unusual. A theory built to explain the exception may be straining to cover ground it was not designed for.

There is a real question underneath the disagreement. Is the impartial moral code itself the evolved trait, present in everyone in some form and merely acted upon by a rare few, or is the reasoning capacity evolved while the specific choice to donate reflects something else entirely, personality, circumstance, or a moral commitment that owes nothing to natural selection? McCullough leans toward the first. Setiya's skepticism suggests the second is at least as plausible, and that we should be cautious about assuming every human capacity, including the capacity for radical impartial sacrifice, needs an adaptive story at all.

What is useful here, whichever side you find persuasive, is the reminder that moral reasoning can outrun the narrow interests it may have originally served. Whether or not that capacity was itself selected for, the fact that a person can look at a stranger's need and treat it as a reason to act, at real cost to themselves, is worth taking seriously on its own terms rather than explaining away.