Here is how it works. Let’s say you’re a man and you find physically attractive women sexually arousing. But you have rarefied intelligence, so you are aware that your genes have programmed you to find physically attractive women sexually arousing so that you will propagate your genes.

Each woman is a composite of about 30 faces. By Pierre Tourigny. Creative Commons license.

  You think a step ahead and realize that your genes would be even better off if you had physically attractive children, so you determine to mate not with the most physically attractive woman, but the woman with whom you would have the most physically attractive children.

For example, let’s say you have a big nose and you know that medium-sized noses are the most attractive.  You should choose to make with a woman who has a small nose in order to have children who will have medium-sized noses.  If you just blindly go for the more attractive medium-sized-nose woman, you will end up with children who have less attractive medium-large noses, which will reduce their mating prospects.

The real trick is to find a mate with whom you are equally likely to have highly attractive children of either sex.  Of course those with even more rarefied intelligence will select mates with an eye toward children with meta- rather than physical attractiveness…