Following up on all the great comments from yesterday’s post, suppose we take as a working model the idea suggested by Alec’s comment — that our instincts toward the nearly-human “other” are not merely influenced by our past history with Neanderthals, but in particular both by warring with them and by mating with them.
This model would suggest that our instincts about monsters are informed by both a joy in killing them and a sexual attraction toward them. Which leads to what might be called the “Zombie Vampire Continuum” (ZVC) hypothesis: Because of our prehistoric relationship with Neanderthals, we have both a blood lust toward “nearly human” monsters and, well, the other kind of lust. A monster will therefore lie on a continuum between “a being that I want to kill” (Zombie) and “a being that I am sexually drawn to in a way that scares me” (Vampire).
Note how this parallels the repulsion/attraction that adolescents feel as they approach sexual maturity, which is generally reflected in archetypical fairy tales: Little Red Riding Hood wants the Wolf to be killed, but she also finds him seductive.
Perhaps our species’ unique relationship to sexuality is informed by instincts we acquired while interacting with rival sentient species, most recently the Neanderthal: We think of sex as dangerous, and at the same time we find killing sexy.

