I'm probably muddling the argument - it was presented, if not endorsed in a book called "Human Natures" - by a Paul Ehrlich? I think so - I remembered that he offered a bunch of speculation - but idea that we might have evolved for competitive gang-sex was my favorite of the lot. Possibly supported by findings (no reference here - call it NPR) that dudes who watch a lot of porn with only girls have lower sperm counts than those who watch gang-sex pornos - and those individuals have abnormally high sperm counts. I think the most surprising thing about that whole business was that just seeing it in porn could cause the effect - just the visual component was enough - interesting.