Like Peacocks

Newish Human Evolution Theory: Our big brainy brains have many uses, but they're bigger than they have to be because they're mostly for show. Showy brains, like excessive antlers, help get potential mates interested.

Like when you see caribou, first thing you notice is it's amazing rack.

The science money shot:

His idea is that the human brain is the anthropoid equivalent of the peacock's tail. In other words, it is an organ designed to attract the opposite sex. Of course, brains have many other functions, and the human brain shares those with the brains of other animals. But Dr Miller, who works at the University of New Mexico, thinks that mental processes which are uniquely human, such as language and the ability to make complicated artefacts, evolved originally for sexual display.

But for the human animal, it goes both ways:

One important difference between peacocks' tails and human minds, of course, is that the peahen's accoutrement is a drab affair. No one could say the same of the human female psyche. That, Dr Miller believes, is because people, unlike peafowl, bring up their offspring in families where both sexes are involved in parenting. It thus behoves a man to be as careful about choosing his wife as a woman is about choosing her husband.

This Dr. Miller sounds pretty smart. And therefore sexually desirable.