The Moral Molecule

Neuroscience and economic behavior

Five tests to determine if your partner will cheat

Predict your man's tendencies

In my last post I discussed how the interaction of three hormones, oxytocin, arginine vasopressin, and testosterone create a chemical soup that can lead men to stray. I presented evidence that though the male brain is made to bond with a mate and raise children, the male genitalia suggest a high degree of sexual infidelity in our lineage. Many social, religious and philosophical traditional prohibit adultery because they seem to have implicitly understood that having sex, which releases the bonding hormone oxytocin, can cause us want to be with a sexual partner more and more, leading to family disharmony.

Insufficient oxytocin release is one reason for cheating. In the nearly one thousand men and women I've tested for oxytocin release, about two percent have highly dysregulated oxytocin release patterns. These people have impaired romantic attachments and many more sexual partners than most others. Dysregulated oxytocin release can be caused by either a genetic disorder or an insufficient amount of nurturing in the first ten years of life.

In other men, insufficient arginine vasopressin can make one lackadaisical about defending a mate from other men. Arginine vasopressin is a hormone cousin to oxytocin and is part of the "keep the couple together" system. A variant of the arginine vasopressin receptor gene was recently found to be associated with having an unhappy marriage. In several studies from my lab, we have found little effect of arginine vasopressin on partner attachment, but it still may play a role. Jealousy can be adaptive as evolutionary psychologist David Buss argued in his 2000 book on the topic.

Lastly, my lab has shown that high levels of testosterone change the brain's cost-benefit calculation toward the current and self, rather than taking a long-term view that includes others' needs. Testosterone has a potent effect on libido, too. High testosterone males cheat more, divorce more, spend less time with their children, but are also highly focused and driven. Testosterone is sensitive to a man's social position--testosterone rises in men that attain high status.

My recent appearance on the Dr. Phil show prodded me to develop some simple tests for infidelity that expose the impact of this soup of chemicals. While there is no perfect test for whether a man will cheat, I will give you a list of things to look at. Most are subtle and below our conscious awareness. If enough of these factors line up, it might be time to worry. You could even make this a fun game to play with your mate as a way to discuss the topic of cheating. Please give me feedback and let me know how well it works.

1. Measure resting heart rate. Jose, who was on the Dr. Phil show with me because he had cheated on his wife, was also an extreme sports athlete and had many speeding tickets. Men who seek excitement like him tend to engage in all kinds of arousing events, including illicit affairs. These men have low resting heart rates. Bomb disposal professionals and parachutists also have low resting heart rates. These under-aroused individuals also commit more crimes--violating social norms gets them the excitement they crave. If your partner has a heart rate less than 70 beats per minute while resting, give him one point. Exceptions are serious endurance athletes who may have low heart rates due to their cardiovascular training.

2. Measure testosterone. We do this by taking blood or saliva samples and analyzing them--this approach is not recommended for you. Instead, you can use some indirect (and noninvasive) measures. The first is a measure of the testosterone a man was exposed to before birth. The ratio of the fourth (ring) finger to the second (index) finger is a reasonable measure of this. Long ring fingers mean more prenatal testosterone. Prenatal testosterone is only weakly associated with a man's current testosterone levels but it identifies a tendency. A man's present testosterone can be assessed by looking at his face (jaw jutting out, prominent cheekbones), body hair (hairy means more testosterone, but very high testosterone causes male pattern baldness--yes, baldies are alpha males), deep voice, and muscularity. One point each for a longer fourth to second finger, more than average hairiness, and a long jaw. The picture shows a high testosterone long jaw.

3. See a dramatic movie. In a previous post I confessed that once I had kids I became an embarrassingly unmitigated movie-crier. Kids are powerful oxytocin releasers--the nurturance enabled by oxytocin ensures their survival so they have to be good at it. How does your man respond to chick-flicks? If you spot a misty eye when the guy gets the girl at the end of the movie, or the little boy succumbs to cancer, he's got an intact oxytocin system. If not, give him one point. Note, though, that oxytocin release is situation-dependent. If you have kids, does he dote on them? If not, give him one point. If you have a dog, how is he with it? Poor interaction with the dog earns him one point.

4. Sex it up in public. A rough test for the action of the protecting hormone arginine vasopressin is whether a man defends his partner from other potential suitors. Try this test. Go early to the bar or restaurant where you are planning to meet your man and be dressed in sexy clothes. Sit at the bar and see if any men start to flirt with you. If so, how does your man react? Little response, not good, give him one point. If his response is over the top though, he may have very high testosterone so this also earns him one more point.

5. Observe his family relationships. How are your partner's nonromantic relationships? On the Dr. Phil show, Jose mentioned that his father had cheated on his mother, and had told Jose that cheating is "just what men do." Bad news, as Jose had a license to cheat. If you mate's dad cheated on his mom, he gets one point. How often does he talk to his mother? Are they close? If not, he gets a point. More generally, does he interact well with his siblings, cousins and friends? And your friends, too? Poor family relationships and few friendships suggest he may not be good at bonding to people, including romantic attachments. If he hasn't had a romantic relationship for more than a year, there is no reason to think you will be the first.

Remember that high testosterone males are attractive sexual partners, they clearly have good genes. But, women tend to prefer them only during the six days of ovulation. The rest of the time, most women prefer lower testosterone/higher oxytocin men who will stick around and love them for more than a few weeks. Again I caution that there are many other factors that affect whether a man will cheat, especially the opportunity to do so undetected. The tests here are only meant to illustrate hormonal factors that may motivate men to cheat that they themselves may not be aware of. If your partner gets 70% of more of the available points, it might be a good idea to have a discussion about cheating with him.

Paul J. Zak is a neuroeconomist at Claremont Graduate University in Claremont, CA.  His book The Moral Molecule will be published in 2012.

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