• The Sexual Peak - an oft-cited statistic is that men reach their sexual peak at 18, and women at thirty. So, by the time a woman's sexual engine is really revving up, her husband's engine is already starting to run out of gas. Mark Twain even referred to this as God's jest. But, there's no real evidence that this is in fact true. Certainly there is no biological data supporting this. I think that this myth has more to do with the fact that, in our society, young women have not traditionally been taught to embrace their as young men are taught to do. It often takes a few years of adult independence and maturity, and perhaps sexuality within a marriage, before women feel comfortable and safe enough to begin asserting and exploring their sexual needs.

• The Back-Up Plan - The only place there is some evidence of a female sexual peak later in life is that there is a spike of female infidelity in the late thirties and forties, particularly as the woman reaches the end of her reproductive years. Evolutionary researchers have suggested that this might reflect women pursuing a "backup plan" or another man to care for them, and who children she can have, if her current mate abandons her as she can longer have children. I don't know about this argument - for instance, in my book, I talk about a Roman tradition where men who had had multiple children with their wives sometimes loaned her to other childless men, so that she could bear them children. I'm just not sure that in humans, the male desire to obsessively and jealously procreate is as strong as some theorists suggest. I also suggest that these same older women are likely reaching a point of economic independence, in their own careers and finances, a factor that strongly predicts female infidelity.

• Feeling Attractive - If there is any reason for female infidelity that I have heard and seen the most, it is the woman's experience of a sense of from the of men other than her husband. One woman told me, "Of course my husband thinks I'm beautiful, he's supposed to. Or at least supposed to say that he does. But when a man I don't know thinks I'm beautiful? That sends a tingle down my spine." In so many marriages, relationships settle into a domestic comfortableness, as the exciting neurochemicals of the early stages of a relationship begin to subside. Sadly, one of the things that often subsides with it is the degree to which husbands and wives stop much of the vocal praise and admiration they gave each other early on. Couples stop talking about the things in each other that they appreciate and love. Even worse, when they do talk, it is mostly about the things that aren't working so well - finances, children, in-laws, etc. So, when a woman starts being tempted by another man, it's often because when she's with him, she feels beautiful, sexy, and desired.

• Freedom - Even as societies become more sexually equal, women continue to face tremendous pressures of and huge burdens of responsibility. Sometimes, the thrill of a sexual affair An Indian woman offered the following comment, in response to an earlier blog of mine. Frankly, I think she describes this better than I could:

I am a middle-class Indian woman who had an arranged marriage; one that is not particularly satisfying. I have realized that respectable middle-class Indian society will give me the opportunity and the space to be unfaithful; as long as I am discrete and keep up the appearance of being a chaste, devoted wife. Living as I do in a society that doesn't offer women a lot of validation outside of marriage and motherhood; and which gives husbands a disproportionate amount of authority over wives; I find that being unfaithful is often the only way I can reclaim a little bit of my personhood. For a few hours, I can be a free being, not just someone's wife and daughter-in-law. I wonder if there are other women like me, particularly in traditional, patriarchal societies like India's for whom infidelity is often a means of finding autonomous self-expression.

• - Sadly, many women enter into infidelity, seeking to redress wrongs they feel from their husbands.

• Stranger - In studies of the sexual behavior of primates, it's been found that females will often seize the opportunity to mate with a male of another troop or group. In contrast to theories that females are selective of their sexual partners, seeking out "the best," some findings suggest that females may actually prefer variety and diversity. One evolutionary explanation is that this increases the genetic diversity of the child. In humans, studies of the major histocompatibility complex (MHC), which governs the development of the immune system, it's been found that women who are married to men whose MHC is too similar to their own are prone to be sexually unfaithful (or at least tempted) around their ovulation. And they are more tempted by men whose MHC is different. Interestingly enough, to bring this blog around to where I started, the MHC variables did not predict relationship satisfaction - despite the temptations or incidents of cheating, the marriages were happy and healthy.