We have this perception that most cheating that goes on in marriage is among the extremely wealthy. Well, of course we do. When a man in the bottom of the income distribution discovers that the child he is raising is not his own, it doesn’t exactly make the evening news. Rationally we all know this, but the perception that cheating is positively correlated with income is fairly pervasive. So, are we correct in thinking that infidelity is a luxury good consumed by the very wealthy?The evidence about infidelity is sketchy. Not surprisingly, when asked about marital infidelity, people often don’t tell the truth. Oddly enough while women tend to understate their cheating, men tend to overstate it (yes, you read that correctly: men lie and say they have been cheating when they haven’t). One way infidelity can be measured more accurately is by population DNA testing but this only tells us about the infidelity of wives and not husbands (unless of course there are no single men around). I have been told that a medical study in the UK did just this, DNA testing one particular town, and that they eventually had to abandon their research when they discovered that 10% of the children were being parented by fathers who were mistaken in their paternal beliefs. I have also been told that a study in my country, Canada, had the same problem when they discovered that one in three of the (university age) sibling pairs they recruited were not actually full siblings. But these are just stories. DNA evidence is weak though and, I believe, often overstated because much of the public data is generated by paternity tests demanded by already suspicious fathers.
There is one US study on infidelity that appears to have some reliable results and, rather usefully, can help us answer our question about the relationship between income and cheating. While other studies that find high rates of cheating often include emotional infidelity (so sitting at your desk wondering what your co-worker looks like naked is going to put you on the cheaters list) this one sticks to a definition of sexual relations that even Bill Clinton would be comfortable with. They find that 8% of men admit to having an extramarital affair in the past twelve months compared to 3.5% of women. If you included people who are cohabitating as well as married, the number goes up to 34% for men and 23% for women. Men who cheat, cheat more; cheating men are twice as likely as cheating women to have had sex with two or more partners. Men tend to cheat with women who are younger (no news there) and women tend to cheat with men who are better educated. Very young women (say less than 26 years old) cheat more than women of any other age and while men also cheat more when they are young, the relationship between age and infidelity isn’t nearly as pronounced for men as it is for women.
Do rich men cheat more than poor men? There is no evidence to suggest that they do. In fact, after controlling for a variety of different effects, it seems there is very little connection between income and cheating in men. Interestingly, though, the real effect of income on cheating is with women; poor women are significantly more likely to cheat than wealthier women.
So who are the cheaters? Men are, but not one particular type of man. Women are too, but predominantly women who are young and poor. One possible explanation for this behavior of women is that they are using cheating as a way to shop for a better long-term partner. Another explanation, which is popular with evolutionary biologists, is that they are shopping for better genes for their children. This makes sense if they can do better in the market for sperm than they can in the market for marriage. It could also just be that the wives of poor men have less to lose if they are caught being unfaithful.Speaking of sperm, Robin Baker in his book "Sperm Wars: The Science of Sex" (which is about as close as you can get to finding academic porn) says that while 10% of all men (regardless of income) are raising children who are not their own, that number goes up to 30% for men who are in the bottom of the income distribution and down to 2% for men who are in the top. If you think that cheating imposes hardship on families, particularly on children, then you have to agree that taking a look at this particular group of cheaters has to be worth while.
Cox, Donald (2009). “The Evolutionary Biology and Economics of Sexual Behavior and Infidelity” (Working Paper)