Scientific American.com: Most Desirable Mates May Not Sire Prolific Offspring suggests evolutionary paradoxes like gay males may be related to gene-gender competition. A gene that helps one gender compete for mates may be "injurious" (in terms of reproductive output) to offspring of the opposite gender.
The phenomena has been demonstrated in fruit flies, now evolutionary biologists will be looking for examples in other animals, including (of course) humans.
I'll be watching to see if they're able to tie this into the peculiarity of the 'demographic transition' in which wealthy humans have fewer offspring than less wealthy humans. (Yes, I've read all the old economic explanations. I don't believe them.)
If this mechanism is in place it may contribute, says Pitnick, to maintaining a great deal of variation in the genome, opposing the conformity sexual selection seems to produce.