Two fascinating, powerful articles I've read recently (available here and here) have led me to revisit the issue of who is really to blame when a collapses under the unbearable weight of what is normally an otherwise inconsequential extra-marital affair.

Are we justified in blaming the normally middle-aged man, who should have controlled himself? Why would he risk so much for so little?

Explanations are easy to come by, ranging from economics (He finally has enough money and status to be attractive to sexy young women at the peak of their reproductive power) to existential dread (he's coming to terms with his own mortality by lashing out symbolically against his own impending old age and death) to the wife's life cycle (she's nearing so he's biologically driven toward the fertility of younger women). Each of these may have some measure of truth, but none answers the most pressing question: Why do men have such overwhelming hunger for variety in their sexual partners-not just at , but always?

The Coolidge effect has been discussed elsewhere. Essentially, it's just a scientific way of saying men (like many male mammals) need variety in partners to maintain sexual interest. If the ghost of Calvin Coolidge weren't haunting him, a man would simply buy a DVD or two of his favorite actress and watch it over and over the rest of his life. Knowing how the movie ends is hardly going to ruin the experience for him If you've never been to a porn gateway web site, you'll be astounded by the variety and specificity of the offerings there: everything from "unshaved Japanese lesbians" to "tattooed redheads" to "overweight older gals."

When researchers decided to look at this issue to develop a Sexual Boredom Scale, they found that for men, sexual boredom was correlated with variety in partners (or lack thereof), while for women, it was more related to variety in activity. In other words, women were more likely to be satisfied by changes in the sexual what, while men (gay or straight) were more likely to respond to a changes in the sexual whom. It's a simple, unavoidable truth almost everyone knows to be true, but few dare to discuss: variety and change are the necessary spice of the life of the male of our species.

But even having an intellectual understanding of this aspect of many men's inner reality doesn't make acceptance any easier for many women. Writer and film director Nora Ephron has explored these issues in many of her films, including Heartburn, which was based on her own failed marriage. In a 2009 interview she explained how raising two sons had informed her view of men: "Boys are so sweet," she said. "But the problem with men is not whether they're nice or not. It's that it's hard for them at a certain point in their lives to stay true. It just is. It's almost not their fault." But then she added, "it feels like it's their fault if you are involved with any of them."

In a letter to his then-friend Sigmund , wrote, "The prerequisite for a good marriage, it seems to me, is the license to be unfaithful." (January 30, 1910.)

Is there any truth to this? If men evolved over millions of years to seek sexual novelty (thus avoiding genetic stagnation in small groups of foragers in a sparsely populated planet), is it fair to condemn them now for responding to these ancient, evolved appetites?

What do you think? In future installments, I'll look at this all too common situation from the perspective of the betrayed wife and the home-wrecking other woman.

Update: Anthropologist/Science historian Eric Michael Johnson has posted a very well-informed comment on this post at his blog here:

http://scienceblogs.com/primatediaries/2009/08/those_cheating_testicles_...