I remember reading a study once that selfish behavior is evolutionarily unfavorable in the long run, so that is an argument which has merit.

Although, the psychopath thing isn't quite spot on. Rather, increased wealth has a tendency to lead to decreased empathy. People tend to compare themselves to their peers, and since the peers of the wealthy are often other wealthy people, their perspective shifts into a small bubble of society that they have a hard time seeing out of.