Be nice, and your brain will grow
Or maybe it's the other way around: if your brain grows -- a specific region of the brain which scientists have recently linked to altruistic behavior, that is -- you'll be nice. They're not sure yet. What Duke University researchers do know, however, is that the posterior superior temporal sulcus is larger in people who regularly engage in what the study calls "helping behaviors," which is to say, activities which have no obvious benefit to oneself.
Examples of "true" altruism, though, can be tough to find. According to the researchers, it's a fairly rare phenomenon in terms of human behavior; we usually engage in "reciprocal" altruism and expect something in return for our generosity. Being cheated or cuckolded is closer to true altruism than, say, giving your car to Goodwill and taking a fat tax deduction at the end of the year. While there's certainly still more research to be conducted, the implications thus far are fascinating: perhaps truly selfless acts are rare because the people who perform them regularly are, well, abnormal!