I always say, never underestimate the power of Spite.
This method reveals that the majority of individuals exhibit consistent (non-)spitefulness over time and that the distribution of spitefulness is bipolar: when choosing whether to be spiteful, most individuals either avoid spite altogether or impose the maximum possible harm on their unwitting victims.
To the extent that this holds up, it would be quite useful in a predictive sense.
"Most people are either good or evil"?!
http://www.plosone.org/article/info:doi/10.1371/journal.pone.0041812
Here is a rather curious paper describing psychology researchers' attempts to investigate "spitefulness" - I think they define spitefulness roughly as "hurting others without any benefit to oneself". References the Stanford Prison Experiment. Concludes, more or less, that some people are spiteful, sometimes.
I have many reservations about the methodology used in this experiment (main one: not sure if the entire process really reflects any real-world motivations, and hence results might not mean much), but I thought it might be of interest to people on this site. Also, of the 30-odd references cited at the end of the paper some sound rather interesting and many are available online.