Games Primates Play

The evolution and economics of human relationships.

The Seven Year Itch: Theories of Marriage, Divorce, and Love

Why many couples fall in love, get married, and then get divorced

In the 1955 film The Seven Year Itch, a married man struggles with the temptation to leave his wife and small child to run off with the young woman next door, played by Marilyn Monroe. The title of the film refers to a time in a marriage when—according to the U.S. Census Bureau—a divorce is most likely to happen. One theory that explains this timing is that after seven years of marriage many couples have successfully raised one or two children through the risky infancy years and realize they don’t want to be around each other anymore, or else they haven’t had children at all and decide it’s time to look for another potential mate. Brad Pitt and Jennifer Aniston divorced after having spent seven childless years together, so the timing of Brad’s itch is consistent with this theory. Or perhaps when Marilyn Monroe or Angelina Jolie enters the picture all bets are off and, kids or no kids, married men start itching and getting ideas in their heads.

In all primates including humans the risk of infant mortality is high after birth and decreases steadily as infants grow older. Thus, if a couple splits up shortly after a baby is born, the baby’s survival or well-being could be at risk. In contemporary hunter-gatherers such as the !Kung in South Africa, mothers breast-feed their babies for approximately four years. Anthropologist Melvin Konner who studied the !Kung in the 1970s, has suggested that four years was probably the average interbirth interval for humans during much of our evolutionary history. Among the hunter-gatherers, a baby is fully weaned at four years of age, and after the transition from breast milk to solid food is complete, the risk of infant mortality drops even further. At this point, the mother typically has another child. But not all couples proceed to have a second child.

In the late 1980s, anthropologist Helen Fisher gathered divorce data from 58 different human societies around the globe and discovered that when married couples divorce, they tend to do it around the fourth year of their marriage, typically after having had a single child. One interpretation of this discovery is that many couples that end up divorcing remain married at least the minimum amount of time necessary to successfully raise one child together. Fisher took this idea a step further, however, and speculated that humans might have a predisposition to be serial monogamists, which means that people are socially bonded to one partner at a time but don’t stick to the same partner their whole life; they go from one partner to another, in succession. According to Fisher, humans are likely to switch partners every four years, after having a child. In reality, there is no strong evidence that humans are serial monogamists. Although the number of divorces keeps increasing in many societies around the world, there are many couples who remain married forever, raise children together, and aren’t convinced that their parenting job is done even when it’s pretty clear that their children can make it on their own. My own parents have been married for over fifty years and my mother still insists on buying me socks and underwear. Clearly, she doesn’t think her job is done yet.

Humans are not the only animal species in which males and females form stable pair-bonds to reproduce and raise children together. Pair-bonds and bi-parental care are common in birds and also occur in some primate species, such as marmosets and tamarins, in which adult males reproduce repeatedly with the same female and help her raise the offspring. As far as I know, however, pair-bonded birds or monkeys don’t kiss and hug each other and don’t celebrate their wedding anniversaries by having romantic candle light dinners. I suspect that romantic love exists only in our species. One of the theories that try to explain why human love exists argues that love evolved to solve the commitment problem.

As I discussed in two previous blog posts, a cooperative relationship involving reproduction and co-parenting can potentially suffer from the commitment problem, which means that when the costs of being in a relationship outweigh its benefits for one of the partners, this person may be tempted to call it quits. Economist Robert Frank has suggested—in a book titled Passions Within Reason: The Strategic Role of the Emotions—that romantic love evolved to provide the ultimate solution to the commitment problem, the only one that can ensure that two people stay together. Love is an irrational force that makes people want to be together no matter what the circumstances are, no matter how bad the cost-benefits ratios are. Frank argued that relationships motivated by irrational love are more successful than those motivated by material self-interest or exchange and cooperation.

Is Frank right? Does love really exist to provide a solution to the commitment problem? Well, I can think of at least three objections to his theory. The first problem is this: if romantic relationships present the same commitment problem as any other cooperative business partnership, why is it that business partners don’t fall in love to solve their commitment problems? Since love occurs in (some) romantic relationships but not in any other type of human cooperative partnerships, there are two possible conclusions: either love is a solution to the commitment problem but both the problem and the solution are different in romantic and business partnerships (that is, romantic partnerships present unique problems that require unique solutions), or love is not the solution to the commitment problem in romantic relationships.

Second, is it really true, as Frank maintains, that a relationship motivated by irrational feelings is inherently more stable than one motivated by rational thinking and prospects for material exchange? In fact, one could argue just the opposite - that the irrationality of love makes the romantic relationship subject to mercurial whims, while a partnership established on the basis of rational reasons is likely to persist as long as the reasons do. I don’t know if Bill and Hillary Clinton still feel passionate love for each other, but it’s clear that they have both benefited greatly, professionally and financially, from their partnership, and their relationship seems very stable. All those who expected them to divorce after the Monica Lewinsky scandal and the end of the Clinton presidency were proven wrong.

If it’s true that love provides a solution to the commitment problem, how long does the solution last? This is the third problem with the commitment model: it does not explain when, how, and why love between two people ends. If love is an irrational feeling that occurs independent of the costs and benefits of being in a relationship, it follows that changes in these costs and benefits would not cause love to end. Although true love lasts forever, the kind of love available to common mortals seems to be strongest at the beginning of a relationship, when passion is at its peak, and to gradually fade until, in some cases, it simply disappears. The commitment model, however, would predict the opposite temporal pattern. According to this model, when two people start a relationship, their partnership is advantageous to both: they have common interests and want to pursue common goals because going alone is either impossible or less effective than a joint venture. In short, love is not really necessary at the beginning. According to the commitment model, love is needed later, when circumstances change and it’s no longer advantageous for one or both partners to be together. So love should get stronger and stronger with time, to make sure that these irrational feelings hold the relationship together when rational arguments would push for a breakup. Is this what really happens?

Common sense suggests that when married couples divorce, there either wasn’t much love to begin with or love was strong at the beginning but then gradually weakened. The weakening of love is consistent with another theory, developed by a friend of mine and related to me after a few drinks, according to which love is just a stage in a relationship—an early stage. My friend didn’t elaborate on the scientific bases of this theory, but I think I can back it up with some evolutionary arguments. 

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Dario Maestripieri, Ph.D., is a professor of comparative human development, evolutionary biology, and neurobiology at the University of Chicago.

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