It all comes down to "old" vs. "new."
When Hugh Grant was arrested in June 1995 with a prostitute named Devine Brown, the whole nation gasped in disbelief and asked the same question: “Why would any straight man in the right mind cheat on Elizabeth Hurley?” The same question crossed the nation’s mind when it emerged, two years ago, that Christie Brinkley filed for divorce from her husband because he cheated on her with an 18-year-old. Why would anybody cheat on the original Uptown Girl?
Unlike the rest of the world, however, evolutionary psychologists were not surprised. In fact, we would have been aghast if it were otherwise. It all comes down to a principle in evolutionary biology called “the Coolidge Effect.” This is how Matt Ridley, the science writer extraordinaire, explains it in his 1993 classic The Red Queen: Sex and the Evolution of Human Nature, one of the books (along with Robert Wright’s The Moral Animal) from which I originally learned evolutionary psychology a dozen years ago.
The effect is named after the famous story about President Calvin Coolidge and his wife being shown around a farm. Learning that a cockerel could have sex dozens of times a day, Mrs. Coolidge said: “Please tell that to the president.” On being told, Mr. Coolidge asked, “Same hen every time?” “Oh, no, Mr. President. A different one each time.” The president continued: “Tell that to Mrs. Coolidge” (p. 299).
I am not sure if this event actually took place in history, but it is irrelevant. The phenomenon that the anecdote highlights is real. (I’m sure that, if biologists discovered the phenomenon now, it would be called “the Clinton Effect.”) Males of all mammalian species, including humans, are evolutionarily designed to seek sexual variety, and have the psychological and physiological mechanisms to be sexually invigorated at the sight of a new female with whom they have not copulated yet.
From the perspective strictly of the genes of a human male, a human female, however beautiful and desirable (like Elizabeth Hurley and Christie Brinkley), presents two problems. First, her fecundity declines with age and reaches zero as she hits menopause. (This was a particular problem for Christie Brinkley’s husband, as she was 52 when he had the alleged affair with the 18-year-old.) Second, and more importantly, she only has one womb when he has an unlimited supply of sperm. It means that, once she is pregnant, she cannot be further pregnant for at least nine months, probably for several years due to lactational amenorrhea.
This sexual asymmetry in reproductive biology makes men much more interested in sexual variety than women. If a man has sex with a thousand women in one year, he can potentially have a thousand children but more realistically (given the probability of conception per copulation of about .03) 30 children at the end of the year. If a woman has sex with a thousand men in one year, she can only have one child at the end of the year, which she can have by having regular sex with one man. (Recall the massive difference in the lifetime reproductive performance of Moulay Ismail the Bloodthirsty and Mrs. Feodor Vassilyev.) Even a woman as beautiful and desirable as Elizabeth Hurley has only one womb, the same as Devine Brown.
The comedian (and one of my personal heroes) Bill Maher puts it best in the following standup routine. I strongly recommend the entire eight-minute clip, because Maher is incredibly funny at his politically incorrect best. (Warning: It is from his 2003 HBO special and is therefore uncensored.) But the bit that is relevant to this post happens at about 04:00.
Only the Coolidge Effect can explain why Hugh Grant wanted “Marvin Hagler in a wig” when he had Elizabeth Hurley at home. From an evolutionary psychological perspective, it is not a mystery at all that Hugh Grant wanted to have sex with other women when he had Elizabeth Hurley. The only mystery is that he had to pay for it.