I got an email the other day from a journalist who was writing an article about why men cheat. "I'll probably be paying the most attention to Cheryl Cole and her recent marital problems," she wrote. "As I don't really know the reasons that men cheat (I wish I did; that would have saved a few months of crying and overindulging in ice cream), I was wondering if you could provide me with some quotes and pointers. Are men in the spotlight more likely to cheat than other men? Is there anything women can do to stop it?"

Some interesting questions. Now I look at these things primarily from an evolutionary perspective (although obviously learning and cultural factors come into it as well). But I immediately felt a bit reluctant about outlining the evolutionary psychologists' view of infidelity. Why? Because it's surprisingly tricky to convey this view without people getting the wrong end of the stick! So, what I did in my response, and what I'll do now in this blog, is start by pointing out what I think two of the biggest mistakes people make when they look at this issue from an evolutionary point of view. Then I'll answer the question.

Mistake #1: Exaggerating How Much Men Cheat. Most men don't cheat. Most surveys find that fewer than 50 percent of men report ever having cheated. (An exception is research by Kinsey; he found that 50 percent of married men and 25 percent of married women had cheated by age 40. But there are reasons to think his sample was atypical and that the rates of infidelity in the population in general are lower than this.) That said, not all men have the "pulling power" to cheat, and maybe more men would cheat if they could. Still, let's not characterize men in general in terms of behaviour that most men don't engage in.

Mistake #2: Polarizing the Differences Between the Sexes. It is not only men who cheat; women sometimes do as well. Plenty of men spend months crying and overindulging in ice cream, or doing other, more manly alternatives! Admittedly, there is a sex difference in the likelihood of cheating: More men than women cheat. But that doesn't mean that men are the cheating sex and women the faithful sex, anymore than the fact that there are more male homosexuals than female homosexuals means that men are the gay sex and women the straight sex.

With those cautions in mind, let's ask the question: Why do men (and women) sometimes cheat? One useful way to think about it is to imagine that there is an average optimal number of sexual partners for each sex. I don't mean optimal in the sense that this is the number we should have or that it's good for us to have; I mean it only in the strict technical sense that having this number of partners would produce the greatest number of offspring for a person (or would have done in the past, before we invented reliable methods of birth control). Why would we care what would produce the most offspring? Simple: because traits that lead people to produce more offspring are likely to be passed on to those offspring, and, as a result of the fact that offspring with these traits are being produced at a greater rate, the traits in question will come to occupy a larger and larger proportion of the population as the generations go by. In time, they may crowd out other variants entirely. This is natural selection in action.

Now, for both sexes, there were selection pressures pushing the average optimal number of sexual partners up, and selection pressures pushing it down. The final optimal number for each sex is the net result of these countervailing selection pressures operating over vast periods of time in our evolutionary past. We can expect that natural selection will have calibrated our sexual motivations and emotions so that, on average, members of each sex act as if they are deliberately aiming at their optimal number. They're not actually aiming at any such thing, of course; they're just acting on their drives and motivations. But we've got these pesky motivations today because people in the past who had them tended to have more babies than their neighbours.

So, what are the selection pressures that determine the optimal number of partners for each sex? The most important is captured in what's called the Bateman principle. This refers to the fact that the maximum number of babies a man can have is much higher than the maximum a woman can have. Consider this: If a man has sex with ten women in a year, he could potentially get all ten pregnant; in contrast, if a woman has sex with ten men in a year, she'll still only have one pregnancy at the most - same as if she'd had sex with just one man. This sex difference pushes the optimal number of partners up for males but not for females, and it largely accounts for the fact that, on average, men are keener on casual sex and sexual novelty than women, more interested in having multiple partners than women... and more likely to cheat than women. Men in the past who were this way inclined had more offspring than those who weren't, and as a result these inclinations became more common among men. In other words, they were selected.

The Bateman principle is all that people usually think about when they look at the issue of cheating from an evolutionary perspective, so they end up puzzled about why women sometimes cheat and why men are often faithful. To understand these things, we have to look at the selection pressures pushing the optimal number of mates up for females, and the selection pressures pushing the optimal number down for both males and females. We also have to look at why pair bonding and male parental care evolved in our species. (These things - pair bonding and male parental care - tend to go hand in hand throughout the animal kingdom.)

There are a number of selection pressures that may have pushed the optimal number of partners up for females. These include: (1) the benefits of genetic variability of offspring (which also applies to males); (2) fertility backup; (3) resource extraction; and (4) the best-of-both-worlds strategy. You can look these up in any good evolutionary psych textbook; I won't say too much about them except that some are more plausible than others.

The selection pressures pushing the optimal number of mates down for females and males include: (1) the risk of life-threatening STDs; (2) the risk of getting a bad reputation and thus lowering one's chances of obtaining a long-term mate (generally a bigger deal for women than men); and (3) the risk of violence from jealous spouses or partners.

On top of this, there was a strong selection pressure in our species for males to invest in their offspring. Among chimps and bonobos (our closest living relatives), males don't invest in their young, and males and females don't form pair bonds. But in our species, men and women fall in love and form long-lasting pair bonds, and men usually do invest in their offspring (although they usually invest less than women). Why are we different than the chimps and bonobos? It's because our young are so much more dependent, particularly in the early years. In species with highly dependent young, male parental care and pair bonding is much more common. The fact that, for most of our history, our offspring were much less likely to survive without paternal care made it less profitable, evolutionarily speaking, for our male ancestors to spend all their time pursuing new sexual conquests. In other words, it lowered the average optimal number of mates for men.

These competing selection pressures have resulted in a species in which men and women both sometimes cheat (men more than women), but in which both sexes often form relatively durable and generally monogamous pair bonds.

That's not the whole story, of course; there are also individual differences and situational differences that predict cheating. Here's a brief sampling: (1) High status men may cheat more because their status makes them more attractive to women and they therefore have more opportunity to cheat. (2) People are more likely to cheat if they are more attractive than their partners, and less likely to cheat if their partners are more attractive than them. (3) Modern western societies create evolutionarily-novel conditions that make cheating more likely. Alcohol is one factor that might increase cheating; long-distance relationships are another. (4) Cheating is less common in some societies because powerful sanctions are put in place to prevent it. In some Islamic countries, for instance, the penalty for adultery (if you're a woman) is death. Not surprisingly, cheating is less common in these countries! But that doesn't mean human nature is different there; it just means that people are generally smart enough and self-controlled enough not to act on aspects of human nature that are likely to get them killed.

Last question: How can you stop your partner from cheating? Well, there's nothing you can do to guarantee a faithful mate. But one thing you might want to remember is that the most reliable way to predict what someone's going to do in the future is to look at what they've done in the past. So, if you don't want a partner who'll cheat on you, don't give a cheat a second chance!

-I'll be dealing with this and related issues in my next book, The Ape that Understood the Universe. My first book, Darwin, God, and the Meaning of Life, is available now now from Amazon.com, Amazon.ca, and Amazon.uk.

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Steve Stewart-Williams

Steve Stewart-Williams is a lecturer at Swansea University and author of the book Darwin, God, and the Meaning of Life.

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