Getty Images File

Jill, Ann, and Kimberly go off to college with warnings from their parents about sex and the “Freshman 15” ringing in their ears. Months later, Jill has gained 15 pounds and Ann has become a sexual adventurer. Kimberly, on the other hand, has not only maintained her weight, she's been too busy studying in the library stacks to hook up.

What accounts for the differences?

It could be the way each one’s brain reward center responds to food and sexual cues, reports a new study.

According to research out of Dartmouth College, in some people, hyperactivation of the nucleus accumbens, a key reward structure buried within the brain's striatum, predicted the eating and sexual behaviors of people (in this case, a group of freshmen women).

This suggests one’s ability to say “no” is not just a matter of willpower, but brain wiring.  

The study, published this week in the Journal of Neuroscience, used fMRI brain imaging and pictures depicting food, erotica, landscapes, and people to gauge how the test subjects' accumbens reacted to each stimulus. (The 48 women who completed the study had no idea what it was actually about.)

Six months later, the women returned to the lab where they were weighed and asked to fill out a questionnaire. Those whose accumbens reacted especially strongly to food cues had gained more weight. And those who reacted to sexual cues most strongly were more likely to have had sex and report stronger sexual desire.

Interestingly, their "appetites" did not cross over. The women with hyperactive responses to sex cues did not have a hyperactive response to food and vice versa.

Bill Kelley, associate professor of Dartmouth's department of psychological and brain sciences, says the study shows that the activation of one brain region proved to be a strong predictor of later behavior, demonstrating that the stronger the “liking” response to a stimulus, the less able we are to “hear” our rational brain saying “no.” 

But are we born this way, or do we acquire stronger craving for specific rewards?

“That’s a great question,” said Kathryn Demos, who led the study and is now an assistant professor of psychiatry and human behavior at Brown University.

Kelley thinks that since different women were tempted by different things, their brain wiring has developed through experience, aided by a genetic component.

Luckily, there are tools that can help people blunt the power of their brain wiring. Behavioral therapies, for example, have had some limited success in people who seem strongly stimulated by food. 

People can also try to replace various cravings with something more healthful, for instance, going for a run whenever they're tempted to eat a cheeseburger.

As for the findings, Demos says the idea that all people are equally capable of self-control is naïve.

Reward, she says, “is a very powerful system.”

Brian Alexander (www.BrianRAlexander.com) is co-author, with Larry Young PhD., of The Chemistry Between Us: Love Sex and the Science of Attraction, to be published September 13.

Related: 

• All that stress is shrinking your brain, study finds
• Facebook takes a toll on your mental health