What is the evolutionary explanation for why lions sometimes kill their own cubs when they acquire a new pride?
Is definitely on-topic to Biology.SE
What is the evolutionary explanation for why people pick their nails as a stress-reliever?
Would be considered sort of in the middle between Biology.SE and Psychology.SE
What is the evolutionary explanation for domestic violence being so common despite the fact that it compromises the welfare of offspring?
Would be considered "off-topic" to Biology.SE since it discusses human psychology.
Another question asked about the gray area between biology and psychology but this question is specific to evolutionary psychology, since evolution is notoriously a "biology" subject, I imagine a lot of questions that discuss "what is the evolutionary purpose (explanation) of (random psychological trait or habit)" get closed for mixing up the two, but 'evolutionary explanation' does throw a wrench in the equation, I think, because maybe the asker would get a better explanation about evolution here than over there, even if it is in the context of humans (after all, humans evolved similarly to other animals). What are the defined barriers?