Woman eating carrot
Photo: Hugh Kretschmer
I want to know why. And I'm not talking about external factors, like too much work and not enough time. I'm looking for what happens in our brains when we try to change, and how we can use that knowledge to actually succeed.

This is how I end up in Nora Volkow's office listening to her obsess about my chocolate. Volkow and colleagues have spent the past 15 years researching the link between drug abuse and obesity by studying one thing that makes it so freakin' hard to change a habit: dopamine, a chemical in the brain that transmits signals from cell to cell and gets us hooked on everything from food to cigarettes to shopping to sex.

Dopamine teaches your brain what you want, then drives you to get it, regardless of what's good for you. It does this in two steps. First you experience something that gives you pleasure (say, McDonald's french fries), which causes a dopamine surge. Some of that dopamine travels to the area of your brain where memories are formed and creates a memory connecting those fries with getting a reward. At that point, in sciencespeak, the fries have become "salient." And when you're exposed to something that's salient, you may think, "That's bad for me, I shouldn't,"  but your brain registers, "Dopamine jackpot!"

Which is where step two comes in: On top of creating memories, dopamine controls the areas of the brain responsible for desire, decision-making, and motivation. So once fries become salient, the next time you see or smell them, your brain releases a surge of dopamine that drives you to get more fries. When you succeed, your brain produces more dopamine, which reinforces the memory that made fries salient in the first place, etching it further into your brain. It's a never-ending cycle: The more you do something that's rewarding, the more dopamine makes sure you do it again. This is precisely how habits form. Eventually, if the fries become salient enough, your brain will release dopamine and push you to get fries anytime you see the colors yellow and red, even if you're nowhere near McDonald's.

And this is true for any behavior that results in a reward: Orgasms cause dopamine surges. So does hitting the jackpot when you gamble, winning a race, acing a test, doing cocaine or methamphetamines, smoking, drinking. "Dopamine is motivation," Volkow tells me. "If you create animals in the lab that don't have dopamine, they have no drive. They can eat food and it tastes good, but they have no motivation to actually do anything, so they won't eat, and they'll die."

As she's talking, I nod and take notes until, suddenly, her computer dings: She's got an e-mail. I am not compulsive when it comes to food, but e-mail? Forget it. Volkow doesn't share my obsession. She keeps talking about dopamine, I go back to taking notes, then there's that ding again, and I think, "She has two new e-mails." Volkow is unfazed. We go on like this until she must have 10 messages and I can barely resist getting up and reading them myself. Then it hits me: E-mail is as salient for me as chocolate is for Volkow. I often work months, sometimes years before seeing my books or articles in print, but e-mail gives me the reward of instant gratification. I tell Volkow this and she laughs. "You're right," she says. "I bet if I put you in an MRI machine and played that e-mail noise, you'd get the same dopamine surges I see in cocaine addicts when they think someone else is getting high."