If parenthood sucks, why do we love it? Because we're addicted.

At the end of a long day, after a rotten commute filled with road rage and little accomplished at work, with chores piled up at home and the weekend nowhere in sight, my 4-year-old daughter clambered onto the sofa next to me, cuddled into my arms, and planted a moist, unasked-for kiss on my cheek.

Poof. The exhaustion disappeared, the frustrations of the day melted away. I soaked in a bath of oxytocin. Everything was right with the world.

But wait. We're getting ahead of ourselves.

In the last few months, parents and researchers have been at war. Evidence has piled up to show that becoming a parent does not make people happier; it makes them unhappier. The data show that marriage increases happiness, but children reduce it. Marriages are vulnerable to divorce shortly after the arrival of children.

People who don't have kids think studies that prove kids are stressful are about as interesting as studies that show falling off tall buildings produces injuries. "Duh," they say. If you've been on a red-eye flight where a bawling baby kept the whole cabin awake through the night, you've seen deplaning passengers muttering about how they can't wait for the day when infanticide is legal.

Parents spend endless hours commiserating with one another about the travails of parenthood. Yet when researchers present data about children and unhappiness, parents rise up in protest. Research may depict parenthood as a bile-inducing, rage-fueling, stress-producing ordeal, but parents tell us that becoming parents is the best thing they ever did. Nonparents write off this reaction as defensiveness—if you've screwed up by having a kid and don't want to admit it, you pretend to be happy—but parents regularly choose to have more than one child. If parenthood were as subjectively awful as the objective research implies, wouldn't all parents stop at one child? It's one thing to claim that a stubbed toe doesn't hurt, and quite another to aim a second kick at the chair.

The research into happiness and parenting arrives at its results by measuring how people feel at regular intervals during the day. If you asked parents every 15 minutes how they feel, the data would read:

7:15 a.m.: Max spilled water on the breakfast table and ruined my Mac. God!

7:30 a.m.: Rachel slapped Max. Max pulled Rachel's hair. I need tranquilizers.

7:45 a.m.: On way out to drop Rachel at school bus, Max has diaper accident. Rush back to change him, miss school bus. I need liquor.