Evolutionary Psychology cares much about mate choice. E.g. according to EP it makes a difference if you choose an intelligent or a stupid mate, a beautiful or an ugly mate, an agreeable or a disagreeable mate, etc. But until now EP doesn't care much about the genetic distance between mates. So my personal hypothesis would be: the genetic distance of mates matters, both on an individual and on a group level (also see: hbd chick). Probably EP would be much more interesting, if it would focus more strongly on questions about outbreeding and inbreeding (and I don't mean just incest avoidance by that).