 This is only one example of one way in which biological altruism can evolve: there are others, which are not restricted to kin and are explained using game-theoretic or group dynamic models (again, see the entry on  biological altruism  for details). The crucial point is just that any kind of genetically-based trait can evolve if it happens to have the right kind of feedback effect on the genes that influence it. This brings us, then, to the sort of trait we are more interested in: a disposition for psychological altruism, as defined above.