Reading Tim Harford’s “Is Divorce Underrated?” in The Logic of Life got me wondering about the root causes of divorce. I want to create a list of “main” root causes, not partition logical space; at the same time, I want the categories to have some economic content, not just be a laundry list of bad things. Here’s what I came up with:

1. Initial mismatch. Some divorces arise because they were a bad idea from the start. If people only stay married for six months, they were probably incompatible all along.

2. Emergent mismatch – or in plain English, “We grew apart.” Some divorces arise because a marriage was a good idea for a while, but eventually ceased to be in the interests of either party.

3. Defection due to expected divergence in mate value. As evolutionary psychologists will tell you, female mate value peaks and starts to decline at a much earlier age than male mate value: It’s a lot easier for a 45-year-old man to remarry than a 45-year-old woman. This creates a big incentive for men to promise lifetime fidelity, then jump ship.

4. Defection due to unexpected non-culpable divergence in mate value. Remember the part of the contract that says “for richer or poorer, in sickness and in health”? When one partner experiences an unexpected rise in mate value (e.g. one becomes a successful novelist) or experiences an unexpected fall (e.g. develops a horrible disease), one of the parties has a temptation to back out – and some do.

5. Punishment for clear-cut breach of contract. Adultery’s the obvious example, but I’m sure you can think of more.

6. Punishment for unexpected and culpable decline in mate value – or in plain English, “You let yourself go!” The marriage contract may not explicitly say that you can’t become a bum or morbidly obese or perpetually bitter. But you’ve heard about incomplete contracts, right? When one party falls far short of expected mate value due to deliberate action or inaction, divorce is not only likely, but easy for neutral outside observers to understand.

(Just to make a men’s rights aside, it strikes me that people are a lot more forgiving when a women divorces her husband for becoming a bum than when a man divorces his wife for gaining a hundred pounds. When I see a man whose wife has let herself go, I often think “Poor guy – how could she do this to him?!”)

Anyhoo, it’s admittedly hard to catalog the reasons for divorce without passing judgment, so I might as well go ahead and pass it. (1) and (2) are hard to question if the couple has no minor children, and (5) and (6) hard to question at all. (I should add, though, that I also embrace the right of couples to make non-standard marriage contracts).

In any case, you can accept my list of root causes without making these normative judgments. Are there any big root causes of divorce that I’m missing? If so, let me know.