IAWYC, but the reason I was interested in evolutionary explanations was twofold: first, that a lot of standard (nonevolutionary) psychologists had attacked the problem and come up with what I considered unsatisfying explanations; and second, that this seemed like exactly the sort of area evolutionary psychology had been successful at explaining in the past (ie, a universal human tendency relating to strategies in conflicts and potentially having a large impact on future success).

I don't know what to think about the proposed solutions, including Roko's. On the one hand, they all sound pretty good, including the non-evolutionary ones. On the other hand, they all sound pretty good. Although it's always possible that there was more than one pressure driving people to support the underdog, five or six separate ones working simultaneously is a bit of a stretch. That means I probably have a low standard for "sounds pretty good". Which might be your point.

Still, I don't know what you want us to do. Are you just saying keep a low probability for all untested hypotheses? That sounds like a pretty good idea.