Is Tiger Woods a victim of his DNA?  Science suggests yes....and no.   Some 20%-40% of heterosexual married men and 20%-25% of heterosexual married women in the US will have an extramarital affair during their lifetime--although this gender difference has vanished among those under age 40. And Americans aren’t alone in their philandering. I have examined adultery in 42 other societies and found it in every one—even where you can be stoned to death for cheating.  Moreover, infidelity is also prevalent in over 100 species of monogamous birds and all mammalian species examined.  The eggs or infants that a “husband” tends are not all his own.  In fact, sexual betrayal is so widespread in nature that scientists now refer to monogamous species as practicing “social monogamy,” in which partners “play house” with one  individual…and cheat with others.  

There are lots of reasons people give for their philandering.  Some want to get caught in hopes of patching up a flagging marriage; some want to get caught in order to depart; some just want to supplement a marriage.  Some want more attention or independence.  Some want to feel special, desired, more masculine or feminine, more attractive or better understood.  Some seek intimacy.  Some want to solve a sex problem or have more sex.  Some crave drama or adventure.  Some want a partner of a different ethnic, socio-economic or religious background.  Some want revenge.  Some want to stay young, known as the “last chance affair.”   Some are just plain bored.   And your childhood, educational level, length of marriage, age and myriad other factors have been linked with philandering.   But what I find most interesting is a study by Glass and Wright (1985) in which these scientists report that some 56% of men and 34% of men who philander rate their marriage as “happy” or “very happy.”  

Why do men and women stray when they are comfortably tucked into a happy partnership?  More and more evidence suggests that biology plays a role.  One culprit is a gene in the brain’s vasopressin system.  In a study of 552 Swedish men and their partners, all either married or co-habiting for at least five years, Walum and colleagues (2008) recently reported that those men with two copies of this particular allele had lower scores on a test of partner bonding (which measured levels of affection, consensus and cohesion); these men also experienced more marital crises and more threats of divorce, and more of these men were involved in a relationship without being married.  Those with only one copy of this gene suffered considerably less relationship discord and those with no copies were the most securely bonded.  This study did not measure infidelity directly, but it measured several factors likely to contribute to adultery.

Another biological system may contribute to infidelity.  In the now classic “sweaty t-shirt” experiment, women sniffed the t-shirts of several anonymous men and selected the t-shirts of those they felt were the sexiest.  Interestingly, they selected the t-shirts of men with different genes in a specific part of the immune system.   In a subsequent investigation, women married to men with similar genes in this part of the immune system were also more adulterous.And the more of these genes she shared with her husband the more extra partners she engaged (perhaps because partner similarity in this part of the immune system can lead to complications in pregnancy and fertility).

Human brain architecture may also make us susceptible to adultery. Humanity has evolved three distinct yet overlapping brain systems for mating and reproduction: the sex drive, romantic love, and feelings of deep attachment. These three basic neural systems interact with one another and many other brain systems in myriad flexible, combinatorial patterns to provide the range of motivations, emotions and behaviors necessary to orchestrate our complex human drive to love, mate and reproduce. But these neural systems aren’t always well connected.  In fact, you can lie in bed at night and swing from feelings of deep attachment for a long term partner to feelings of romantic love or lust for someone else.  In short, we can "love" more than one person at a time.

Do all these new data from biology excuse Tiger Woods?  Not at all.  Along with our many human propensities, we evolved a huge cerebral cortex with which we make decisions.   Some people are more susceptive to alcoholism, yet they choose sobriety.  The world is filled with fatty foods, yet not all of us are over weight. And most people have plenty of opportunities to cheat.  Yet many say "no" to adultery—even the rich and famous.

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