In his previous life, João—a chubby man with pointy ears and arched black eyebrows—had been stern and serious, prone to squirreling money away. But after suffering a health crisis in 1990, at age 49, he wanted to live differently. “I saw death from close up,” he would often say. “Now I want to be in high spirits.” And nothing made him happier than giving. To those who didn’t know him well, he must have seemed like the embodiment of selflessness—the Saint Francis of Rio de Janeiro.

What’s most interesting about João’s story, though, is that his new outlook resulted not from a spiritual awakening but from brain damage caused by a stroke. Among other symptoms, he became a chronic insomniac and lost his sex drive; he started forgetting things and had trouble focusing; his movements slowed. And, his neurologist says, he became “pathologically generous”—compulsively driven to give. His carefree attitude toward money led to confrontations with his family, especially his brother-in-law, who co-owned the french-fry cart. But even when his family berated him, and the cart went out of business, and he was reduced to living on his mother’s pension, João refused to stop. Giving simply made him too happy. (João died of kidney failure in 1999. His doctor provided only his first name, to protect the family’s privacy. )…

This work does raise uncomfortable questions, though. We normally think of generosity as pure and noble—evidence of the soul, not evidence of brain damage. But what if giving is largely a reflex or an instinct or even, sometimes, a sign of mental derangement? We also think of generosity as uniquely human. If other species evolved to be generous too, does that devalue the trait?