(CNN) -- Looking back on it, Wendy Fox thinks it was the M&M's that did her in.

They sat in a glass bowl on her living room table, taunting her every time she walked by, seemingly calling out for her to scoop up a few and eat them. Eat them she did, as well as anything else chocolaty that crossed her path, such as a brownie at her favorite restaurant or a mocha drink at Starbucks.

 

Fox's chocolate cravings (and three pregnancies) helped her gain 40 pounds in her 30s. "It wasn't pretty," says Fox, 41, a real estate agent in Weston, Massachusetts.

Unhappy with her growing figure -- she'd gone from a size 4 to a size 10 -- Fox watched as her mother lost 30 pounds. Her mother, who also craves sweets, had attended a program with nutritionist Susan Roberts at Tufts University to curb cravings and suggested Fox do the same.

 

"At my first meeting with Sue, I told her, 'You've met your challenge,' " Fox remembers with a laugh.

 

Roberts, author of "The Instinct Diet," explained to Fox that there's a biological reason most of us crave ice cream and not broccoli, but we can unlearn our instincts. It worked. After 14 months, Fox now craves salads instead of M&M's, has lost 36 pounds and is back down to a size 4.

 

The caveman's instinct

"For most of human history, people didn't have enough to eat, so fat was something you really needed to seek out," says Marcia Pelchat, a food psychologist at the Monell Chemical Senses Center in Philadelphia. To avoid dying in a famine, the brain is wired to pick up on signs that calorie-rich foods are nearby, which helps explain why that piece of cake on the plate in front of you is so irresistible, or why seeing a sign for a doughnut shop draws you in even when you know you need to watch your diet.

 

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