The United States is experiencing an obesity epidemic. The culprits? A general lack of exercise, combined with an increase in the quantity and availability of calorific food. But environment may not be the only factor at play. New research shows that genetics may also be at fault.

According to a new study published by Yale researchers and the Oregon Research Institute, the brains of obese individuals show a reduced reward response to eating compared to their leaner counterparts. The study, published in the journal Science, suggests that overweight people overeat in order to compensate for this reduced response.

With obesity on the rise in the United States, it has become more relevant to understand humans’ biological predispositions to obesity. Approximately 67 percent of male Americans and 62 percent of female Americans are overweight, according to statistics by the US State Department of Health. And last year, only one state, Colorado, had an obesity prevalence of less than 20 percent.

To study obesity, the researchers turned to a high-calorie staple: the chocolate milkshake.

In the study, adolescent girls underwent functional magnetic resonance imaging scans, fMRIs, while ingesting chocolate milkshakes.

Obese people were found to experience a reduced dopamine response in the area of the brain involved with feeding, said lead author of the study, Dana Small, associate fellow at the John B. Pierce Laboratory and an associate professor of psychology at Yale.

“Dopamine is a key chemical involved in reward and response,” she explained. “It correlates with the perceived pleasantness of a meal.”

And the fact that heavier people had a lower dopamine response suggested to researchers that some abnormal processing occurs as a function of body weight, she said.

Researchers sought to determine how and why dopamine release varies as result of body mass index.

They analyzed the genotypes of all girls in the study and discovered that there was a certain allele connected to reduced dopamine release in the dorsal striatum. This allele, TaqIA1, indicates a decreased reward pathway for food, which predicts future weight gain.

“The research supports the idea that obesity is in large part genetic,” said Alain Dagher, a neurologist and associate professor at McGill University, and a co-author on the study.

Although having the allele does not necessarily mean that the individual is overweight, it does suggest that the individual is at risk for overeating.

The risk is comparable to the risk for drug addiction. Taking drugs leads to changes in the brain, which makes an individual more likely to take drugs in the future, Small said. The data from the study suggests that similar processes are occurring in overeating.

“A factor that makes you vulnerable to addiction also makes you vulnerable to obesity,” Dagher said.

Nevertheless, researchers understand that obesity itself is a very complicated issue.

Dagher emphasized the difficulty of pinpointing one concrete cause of obesity.

“Obesity is probably multi-factorial; you can’t really attribute it to a single gene,” he said.

Small, too, recognizes the complexity of the obesity epidemic.

She attributed obesity to a number of causes — such as the fact that fewer people walk to their destinations, but instead take cars.

“It is not caused by a change in the genome because it is just too fast,” she said. “We don’t evolve that quickly.”

Small plans to continue exploring the number of causes of obesity, specifically those related to dopamine response.

She said she would like to look directly at dopamine response through positron emission tomography, which, while more expensive than fMRIs, measures dopamine activity directly instead of measuring the blood flow associated with it.