Watching War of the Worlds reminded me of one of the big puzzles in evolutionary psychology: Why do parents and their children disagree? “I only want what’s best for you” is every parent’s slogan, and if Darwin is right, how can that be wrong? It’s one thing to say that evolution isn’t perfect. But in the real world, we often see a son defy his parents to join the guerillas, or a daughter elope with a poor but dashing suitor. Not to mention the mundane struggle to make kids do their homework.

This is one of those questions where I humbly (?) suggest that a little economics goes a long way. Suppose a headstrong daughter defies her parents to marry beneath her station. (Forgive my archaic language, I’m re-reading War and Peace.) Is she the only person whose genes are affected by this decision? Hardly. There is an obvious negative externality for her siblings, who will see their opportunities in the marriage market diminish. And since your parents are equally related to all their children, but you are only half as related to your siblings as you are to yourself, the selfishly optimal choice for your genes differs from the selfishly optimal choice for your parents’ genes.

Example: Suppose you will have 6 surviving children if you elope, but only 3 if you do not. If you elope, however, your siblings will have 12 children between them, but if you do not, their total will be 16. Then your genetic payoff from eloping (6 children plus 12 nephews and nieces who count for half equals 12) exceeds your genetic payoff from not eloping (3 children plus 16 nephews and nieces equals 11). But your parents maximize their genetic payoff if you stay, because (6+12)=18 grandchildren is less than (3+16)=19 grandchildren.

Thus, we should expect to see parent-child disagreement primarily when there are externalities for siblings. (Though the same principle holds of course if your parents are young enough to have more children, or if your behavior affects the reproduction of your extended family too). This is an elegant explanation for parents’ hostility to promiscuous daughters. But it applies just as well to parents’ hostility to sons’ engagement in high-risk activities that might make his name (hence the son’s temptation) or embarass the whole family (hence the parents’ resistance). It also accounts for the mundane fact that parents push their kids more toward financial success than happiness: Every successful child frees up resources to help other siblings reproduce too.

But what about parents of only children? Aren’t they often even more meddlesome? That’s a tough challenge. My best answer is that humans evolved in conditions when almost everyone had large families, and where everyone knew everyone. Just as we now consume sugar and salt to excess (or so I’m told) because these were scarce during our evolutionary history, we ride our kids too hard today because we evolved in conditions when one bad kid could ruin the lives of his ten brothers and sisters.

Now if I could only explain why my identical twins can’t agree on whether to watch Shrek 2 or Thomas the Train