I'm not sure about psychological studies as I am not familiar with them nor do I have a degree in psychology, so I am not going to try and be an arm-chair psychologist.

I can say that based on my own anecdotal experience I feel pretty good when I receive a rep, and I also feel good when I give people rep. Others who have used pplrep have reported similar experiences. And I think that's a good thing. Certainly better than dealing with the drama that occurs on other social networks.