This is why it's so hard to change. Doing so means fighting one of the most fundamental neurological systems in the brain. "Think about it," Volkow says. "If you're designing a species and you want to make sure it does things that are crucial for survival—like eating and reproducing—you create a system that's all about pleasure so they want to repeat those things. Then you have dopamine make those behaviors become automatic. It's brilliant, really."

Although she hasn't proved it yet, Volkow has a theory about why diets often fail: Based on animal studies, she thinks people may experience withdrawal when they try to kick certain foods their brains have become dependent on. "This makes it hard for them to eliminate those foods," she tells me, "because they may feel depressed or sluggish or generally horrible." If this turns out to be the case, she says, perhaps changing your diet more slowly will help.

But my big question for Volkow is this: How do you get yourself hooked on something that's not inherently pleasurable to you—like living on salads and broccoli or, in my case, exercising? Many people get a natural high from working out. I, however, am not one of them. "Isn't there some way to trick the dopamine system?" I ask her. "Some way to fool my brain into craving exercise?"

Sure, she says: The secret is thinking up rewards. My payoff for working out could be a pedicure or a new pair of shoes. For someone trying to diet: Maybe you get a massage after a week of good eating, or have a friend dole out gift certificates if you stay on track (you pay, but she controls the vouchers). "Giving yourself rewards for a behavior engages the dopamine system so your brain will associate the positive outcome with it, which will help you form the habit."

When I get home, I try it. I make a deal with myself: If I exercise every day for a week, I get a new mini MP3 player. I wake up in the morning and it's raining. I remind myself about the MP3 player. After several confused minutes of figuring out what a person wears to exercise in the rain (a poncho? an umbrella?), I end up in waterproof hiking boots and my boyfriend's hooded sweatshirt, which is three times my size. I leash the dog and we start running, but my boots are too heavy and my lungs burn, plus I can't see because the hood keeps falling over my eyes. And, of course, there's the rain. So we drop to a speed walk. An hour later we get home looking like we've been dunked in a river. I strip off my wet clothes and tell myself, "Do that six more times and you get an MP3 player." Then I think, "Yeah, right, you can't possibly exercise again without music." So I buy an MP3 player and tell myself I really need exercise clothes before I try something like running again.

The next day, I find myself in a very green and blue cafeteria at the Kennedy Krieger Institute in Baltimore, the renowned center for children and adolescents with developmental disabilities. I'm sitting across from Michael Schlund, PhD, a research psychologist who divides his time among several scientific institutions where he explores areas of the brain involved in learning and behavior change. For Schlund, this work is part of a larger project aimed at helping people with developmental disabilities, such as autism, learn. But what I'm interested in is a study he recently finished at the University of North Texas, where he spent months observing the brains of healthy adults as they learned new behaviors based on rewards.


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