Infidelity: Nature or nurture?

Infidelity is not usually the first thing that comes to mind at a friend’s wedding. Yet, that’s exactly what happened to me over the weekend. Bride and Groom were on the dance floor, grooving to the tunes of “Love is in the air...,” she in her super-slick Prada creation, he it doesn’t matter! Both exuded an aura of elation and unconditional devotion to each other, yet there was I, a few glasses of wine later, unable to rein in the following thought:

Evolutionary biology suggests that both men and women are “natural cheaters,” each for their own reasons. So what is it that makes two human beings commit to sharing the same bed, roof and room temperature for the rest of their lives?

Naturally, I sought an answer in Economics. After all, our partners are arguably a resource; a resource for offspring, obviously, but also (depending on the dynamics of a partnership) food, housing, inter-personal communication, emotional fulfillment and, quite often, sex. As such, they are critical drivers of our “utility”—our overall happiness and satisfaction. And economics is all about identifying the choices that will maximize our utility in the presence of constraints.

Sure enough, some economists out there have written studies about this. And even though those I’ve read are not among the most outstanding pieces of literature, they served to me as inspiration for dissecting the whole “infidelity vs. monogamy” trade-off. So here is my take.

Let’s talk evolutionary biology first (biologists, feel free to attack!). Accordingly, men and women are natural cheaters—a state of mind driven by their congenital mission to reproduce. Men will cheat as indiscriminately as possible to maximize the probability their genes are disseminated. Women will cheat in order to ensure their offspring are of the best quality, while also ensuring that they themselves are provided for—the underlying assumption being that the best “provider” may not necessarily be the man with the best quality genes.

Technically speaking, this means that the utility from cheating increases with the number of partners for both men and women —though in the case of men the extra pleasure out of an extra partner would tend to be much higher than for women (We’re still talking strictly biology!)

But more partners also mean higher costs. In the case of men, the cost has to do with monitoring: The higher the number of women you sleep with, the more difficult it is to monitor that the child they actually bear is yours (and remember, that’s the only thing you care for!). You see, women tend to hide their fertility cycles pretty well and—being the natural cheaters that they are—they may well give birth to someone else’s baby, which you will be asked to care and provide for. Not good!

In the case of women, the cost stems from their bodily limitations. The “production” of a baby takes nine months of “incubation,” and then a few years of breastfeeding, spoon-feeding and brain-feeding. This means that there are only so many babies a woman can produce during her lifetime—which, of course(!), means that there are only as many partners she can mate with. Effectively, this makes the cost of mating with any additional partner beyond this limited number infinite.

So if biology were the whole story, the equilibrium would look like this: Men would marry one (any) woman and monitor her closely, while going around town spreading their genes as widely as possible at little extra cost. Women would say “I do” to a man (any man) who would offer to be the “provider,” but then cheat on him with a better quality male to ensure their offspring makes it to Harvard. So both get their cake, and eat it too.

Now, economics introduces a dose of realism into the story. I mean, when you spot an attractive woman at a bar, you don’t really think “I feel like reproducing tonight.” Instead, you say “great legs” and reach for your condoms. Likewise, if I bump into Matt Damon, it’s not exactly my fertility cycle that will spring to mind.

Technically speaking, economics introduces new “arguments” in our utility and cost functions. No, not marital arguments; I’m talking about the different parameters that determine the levels of our utility and our costs when (sorry, IF) we cheat. Mathematicians like to call these “arguments.” So let’s see what they might be.

First of all, the utility from cheating does not only depend on the number of people one sleeps with. Statisticians have found it also depends on one’s age, in a way that resembles the shape of an “inverted U.” Meaning that you tend to cheat less when you’re young and innocent, more as life shows you otherwise, and less so again when you “mature.” Secondly, your utility from cheating also depends on the length of time you’ve been married… the seven-year itch and all that. The longer you’re married, the higher the perceived benefits of extra-marital affairs—especially for those of you who have missed out on Oprah’s invaluable sex advice.

What about the costs? Economics has a say here too, by better capturing the incentives for cheating. You see, in a day an age where contraception (God forbid!) is an option, my cost of sleeping around does not stem from my bodily limitations as a child-bearer. My costs are quite different. For example, if I’m married to a guy with the physical symmetry of Matt Damon, the financial resources of Bill Gates, the unconditional devotion of my mother and a Harvard education, my incentive to cheat on him will likely be low. Likewise, men are not the blithe sperm-disseminators biologists portray them to be. They do care! (right??)

So as an economist, I would introduce two more arguments in the cost function. The first is the relative “quality” of your spouse—how much better-looking and/or smarter your partner is compared to the average in your community. For both men and women, their cost of cheating would be expected to rise, the higher the relative quality of their partner.

The second parameter is the spouse’s ability (again, relative to the community average) to act as the “provider,” not necessarily of financial resources but, more importantly, of emotional support. (A professor of Family Studies and Human Development at the University of Arizona, Bruce Ellis, went as far as creating an index for that—the “Partner Specific Investment” index—or PSI). So the higher your partner’s relative PSI, the higher the cost of cheating on him/her.

Where does this leave us? How can we stop our partners (and ourselves) from cheating? The model above seems to offer a whole range of options, the relative appeal of which I leave up to you to decide. But here they are:

• Marry the Damon/Gates/mother combo (or the female equivalent depending on your predilections);
• Raise his/her cost of cheating by vowing to break your entire crockery collection on his/her head before kicking him/her out of the door;
• Relocate with your spouse to an uninhabited isle in Turks and Caicos to minimize the “community options” for both of you;
• If you’re a woman, wait till you’re at least 40, then marry a man who is at least 55—the ages where your respective “inverted Us” begin to decline (or so they say);
• Marry into Ivy League (ok, possibly Stanford, if you’re into the west coast thing). No, seriously. Some economists found that the probability of cheating decreases with the level of education, purportedly because a college degree enhances one’s ability to assess the costs of cheating.
• Accept cheating as an evolutionary necessity and, thus, good for human kind.

Alternatively, you can ignore economic models, tune into Oprah, and join me as I take another sip of wine and sing along: “Love is in the air.. everywhere I look around...”


Glossary: Resource, utility, constrained optimization, argument, cost function, seven-year itch, love.

Reality bites! Further readings...

Infidelity: It may be in our genes

The myth of monogamy: Fidelity and Infidelity in Animals and People

Even Harvard cheats!