Distinguishing between marginal and total values is crucial to understanding many human activities and decisions. Almost all the decisions we make are made at the margin, but there are exceptions. We are sometimes faced with decisions that force us to compare the total value of one option to the marginal value of another. These decisions can be far more agonizing than decisions made entirely at the margin, which require sacrificing a little bit of one thing to have a little bit more of another.

Consider love and marriage, a topic not commonly thought to be one that economics has much to say about. But decisions that lead to love, marriage, and sometimes divorce always involve choices between competing alternatives, and therefore involve costs and benefits. These costs and benefits are personal and subjective, but so are all costs and benefits. And if people consider the relevant costs and benefits when making relatively trivial decisions, say on whether to acquire or discard a pair of socks, then surely we should expect them to consider the costs and benefits of vital decisions such as getting married or divorced.

Wives and Mistresses

My wife reads these columns, so I want to emphasize that economic analysis allows one to acquire insights into activities without having firsthand experience. Also, nothing important would be altered if I reversed the sexes’ roles.

After a few years of marriage a husband has had lots of experience with his wife. He has seen her almost every day, most of their time together being spent in rather routine activities. He knows her habits, so most of her behavior is predictable and not very exciting or even particularly interesting. He spends hours with her and never says a word to her or notices her presence, even if the football game is less interesting than the beer commercials. And she experiences him in much the same way, so her responses to him typically lack enthusiasm, which reinforces his own lack of enthusiasm. In other words, before long the husband doesn’t find his wife very valuable at the margin.

In contrast, the woman he meets at work or on a business trip seems far more interesting. He has not known her long, and likely doesn’t see her often, so she is less predictable and more exciting than his wife. Also, he likely sees her in more interesting situations than in a messy kitchen with screaming kids. And if his interest in her is reciprocated, it will probably be with far more eagerness than he has experienced at home in a long time. If she becomes his mistress, with occasional liaisons in romantic settings, he can find himself exhilarated at the thought of the next encounter. His mistress is far more valuable to him than his wife at the margin.

The passion of the love affair causes the husband to think about leaving his wife for his mistress. But leaving his wife involves a different calculus from comparing the value of a little more time with his wife with that of a little more time with his mistress. A divorce is not a decision made at the margin but one that forces the husband to confront competing total values. Here the advantage can easily shift to the wife. Her marginal value may be small, but her total value can be very large. The husband’s relationship with his children, his parents, and many of his friends; his standing in the community; his sense of permanence and place; and his financial prospects are all inextricably connected with his wife and marriage. Plus there is the genuine fondness he likely has for his wife and their shared memories and experiences.

The wife is like water and the mistress like diamonds. Given a marginal choice between the two, the husband readily sacrifices a little time with the wife for a little more with the mistress. But when the choice is between the total value of the wife and the total value of the mistress, the wife wins.

Obviously the analogy of the wife as water and the mistress as diamonds is not perfect. Men do leave their wives. A marriage can fail because the wife (remember, the sexes can be reversed) ceases to provide value both at the margin and in total. But the important distinction between total value and marginal value explains why so many men who are genuinely enthralled by their mistresses give them up rather than sacrifice unexciting marriages.

Hurting Those We Love the Most

As this discussion indicates, there can be a lot of pain in a marriage, even a strong marriage. Obviously in many strong marriages there is little pain, and certainly this is what most of us would consider the ideal. But there is an old saying that “we hurt the ones we love the most,” and marriages (especially strong marriages) are often good examples of this.

We often do things to aggravate and hurt those we love that we would never consider doing to casual acquaintances. The distinction between total and marginal value is important here. Those we love (who provide us with a lot of total value) are generally those who love us (we provide them a lot of total value), so we can impose some marginal costs on them with hurtful comments and behavior without eliminating all the total value they receive from us. If we did the same thing to casual acquaintances at work, for example, our total value to them would quickly become negative and we would find ourselves isolated or worse. Of course, even with loved ones, there are limits to how much pain they will take from you, but the stronger the marriage the more latitude there is. In a weak marriage there will not be much pain, at least for long, since there is little total benefit sacrificed by divorce.

To reinforce the point, consider how much aggravating behavior parents will endure from their children. There is probably no love stronger than that which parents have for their children. And children often take advantage of this love by behaving in ways that inflict tremendous pain on their parents.

I am not recommending that you take advantage of the love people have for you by behaving badly. Far from it. Neither am I arguing that economics gives a complete explanation of the behavior observed in marriages and families. That behavior is influenced by many factors best considered by those trained in other fields. But unless you distinguish between total and marginal effects of behavior, you will leave unopened an important window of understanding on marriage and families.

Dwight R. Lee

Dwight R. Lee is the O'Neil Professor of Global Markets and Freedom in the Cox School of Business at Southern Methodist University.