Why Men Cheat

By Paul J. Zak

Jose and Angela have been married for two years. Two days after she gave birth their baby, Jose had an affair. He said it was the first affair since he's been married, but he had cheated many times while he was dating Angela and other women. Angela was, of course, devastated.

I met Jose and Angela when I was a guest on the Dr. Phil show that aired April 27, 2010. Jose was facing a divorce and so agreed to come on the show for some expert advice. Dr. Phil also pays for three months of therapy for those who appear on his show and would like it as Jose and Angela clearly did.

Can science tell us whey men cheat?

First we have to discuss that bugaboo word monogamy. Studies have shown that about three percent of mammals are socially monogamous. This means that males and females cohabitate to jointly raise offspring. Genetic studies have shown that few of these paired animals are sexually monogamous. Yet, many of these animals continue to be socially monogamous.

What about humans? The recent high-profile sexual escapades of Tiger Woods and Sandra Bullock's husband Jesse James are evidence that sexual monogamy in humans is also relatively rare. Why then, do so many religious and cultural traditions, not to mention much of public opinion, fall squarely against promiscuity?

Here's where the science comes in. There are three hormones that modulate monogamy: oxytocin, arginine vasopressin, and testosterone. Oxytocin has been shown in animals and humans to sustain pair bonds in males and females through its release in the brain during sex, touch, and nearly any positive social signal as I discussed in an earlier posting on generosity. Arginine vasopressin motivates mate- and offspring-guarding in male socially monogamous mammals, an important aspect of pair-bonding. Testosterone is associated with libido and many of the male characteristics like musculature and drive that are attractive to females when seeking high-quality male genes.

The distribution of oxytocin and arginine vasopressin receptors in the human brain reveals that we are a monogamous species. But is that socially monogamous or sexually monogamous? The large size of the male testicles and shape of the penis suggest that we evolved when females would have multiple sexual partners and it was a "let the best sperm win" all-out competition. We have socially monogamous brains but sexually promiscuous genitals.

Adding testosterone to the mix is like having a wild card in poker--anything can happen. Testosterone is in conflict with the bonding effects of oxytocin (literally--testosterone inhibits the uptake of oxytocin by its receptor). It motivates men and women, but mostly men who have five to ten times more testosterone than the fairer sex, to seek more sexual partners (and to be single-minded and to take more risks). Testosterone levels also respond to our social environment. Win a chess match, your testosterone goes up. Earn a million dollar bonus at your hedge fund, your testosterone goes up. Win the Master's tournament, your testosterone soars.

Pity poor Tiger with so much testosterone. Yet, our brains are constantly making cost-benefit calculations. Should you hunt for a mastodon or fish for cod? Establish an encampment on the hill or by the water? Settle down with a female who has good genes and will invest the resources to nurture your offspring, or keep an eye out for additional mating opportunities? Or...do both.

A study from my lab recently showed that administering testosterone to men changes the cost-benefit calculation in the brain to the more current, and self focused as I discussed recently. And here's the rub: Tiger Woods, Jesse James and many other philanders are dominant males. They are exciting, wealthy, famous, risk-takers and very likely have high testosterone levels. Women are attracted to them, and their testosterone inhibits the bonding effect of being with their beautiful wives and insinuates a desire for more procreation (or at least a simulacrum of it).

We disdain the cheaters because they want to have both social monogamy and sexual promiscuity (and maybe we are just jealous that they can have it both ways). But the oxytocin attachment system is pernicious. Sleep with someone enough and it is difficult not to become attached to him or her. This is why so many conventions proscribe adultery--precisely because the girlfriend now might become the wife later, leaving the first family in the lurch. This especially hurts women and children. Yes, even Tiger Woods' kids deserve a full-time dad.

Are high testosterone men to blame for cheating? Certainly. Do their physiologies and abundant opportunities to cheat provide a motivation to do so. Certainly. Cheating by males--especially high testosterone males--is unlike to disappear. We can still recognize its negative impact on women and families, but I see no reason to demonize promiscuous men, even if it feels right to do so.

In my next post I'll discuss how to identify if your partner is likely to cheat.