When you're in the throes of this romantic love, it's overwhelming - you're out of control, you're irrational, you're going to the gym at 6 A.M. every day - Why? Because she's there.

We evolved in a tropical climate where the smells of plants and flowers were all around us. We spent a lot of time in the trees with a lot of sunlight and no clothes.

That "ol' black magic" is a fickle force. The chemistry of romantic love can trigger the chemistry of sexual desire and the fuel of sexual desire can trigger the fuel of romance. This is why it is dangerous to copulate with someone with whom you don't wish to become involved. Although you intend to have casual sex, you might just fall in love.

What our grandmothers told us about playing hard to get is true. The whole point of the game is to impress and capture. It's not about honesty. Many men and women, when they're playing the courtship game, deceive so they can win. Novelty, excitement and danger drive up dopamine in the brain. And both sexes brag.

It certainly would have been adaptive for ancestral man to have a chubby wife during stressful times of famine. Not only would she have had more calories to burn, and thus more energy and endurance, but since fat stores estrogen, she would have remained fertile for longer.

In general, men are wired to notice obvious signs that convey interest in mating - a warm smile, for example - and ignore other subtleties, like if your lipstick is faded.

Men want to think women don't cheat, and women want men to think they don't cheat, and therefore the sexes have been playing a little psychological game with each other.