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.

8:00 a.m.: Gas needle points to “empty,” but I keep driving to get Rachel to school on time. Car stalls in major intersection. Drivers curse me. I discover I left my cell phone at home. I bang my head repeatedly on steering wheel.

And so on.

One of my favorite psychologists, Dan Gilbert at Harvard, often quotes a study conducted by Daniel Kahneman that found that spending time with children makes mothers about as happy as vacuuming. *

And yet. Let’s go back to the sofa and that cuddle and kiss. It was a fleeting moment, but it genuinely changed how I felt about the day.

It was at that moment of bliss that I realized how the objective parenting research and the subjective parenting experience could both be right. Parenting is a grind, and most parents are stressed out much more than they are happy. But when parents think about parenting, they don’t remember the background stress. They remember the cuddle and the kiss. Parenting is a series of intensely high highs, followed by long periods of frustration and stress, during which you go to great lengths to find your way back to that sofa and that kiss.

We have a name for people who pursue rare moments of bliss at the expense of their wallets and their social and professional relationships: addicts.

Children regularly give parents the kind of highs that only narcotics can rival. The unpredictability of those moments of bliss is an important factor in their addictiveness. If you give animals a predictable reward—say, a shot of sugar every time they press a lever—you can get them to press that lever quite regularly. But if you want irrational and addictive behavior, you make the reward unpredictable. Pressing the lever produces sugar, but only once every 10 tries. Sometimes, the animal might have to go 20 or 30 tries without a reward. Sometimes it gets a big jolt of sugar three tries in a row. If you train an animal to work for an unexpected reward, you can get it to work harder and longer than if you train it to work for a predictable reward.

We’ve all seen those sad people sitting at slot machines in a casino, methodically feeding coin after coin into the slots. If you made their reward predictable—after precisely every 20 attempts, they would always get a prize—you would lower the addictive power of slots. It’s the unpredictability that drives them. Or, to put it another way, it’s the hope for reward, not the reward itself, that drives them.

I suspect oxytocin works the same way. The unexpected, kind, and loving things that children do produce chemical surges in their parents’ brains like the rush of the pipe or the needle. Like addicts, parents will sacrifice anything for the glimpses of heaven that their offspring periodically provide.

I don’t know if there is empirical evidence to back me up, but it’s conceivable that the neurological mechanisms of addiction—in all their irrational and self-defeating pathologies—are based on underlying mechanisms in the hidden brain that are designed by natural selection to make us seek out—and enjoy—parenthood.

So spare a moment of compassion for the next junkie you see. And spare a smile for the next parent you run into. Junkies we are—and proud of it.

Correction, Nov. 15, 2010:This article originally said that psychologist Dan Gilbert found that spending time with children makes mothers about as happy as vacuuming. Gilbert has cited that study, but it is not his own. ( Return to the corrected sentence.)