In a huge new finding, scientists may have figured out why it’s so hard for people who get fat to lose those pounds and keep their New Year’s resolution. A study that examined overweight mice found that the brains of obese mice actually encourage inactivity, meaning that if you’re fat, you may be way less motivated than people at a normal weight to move your body, creating a vicious cycle where you can’t lose weight.

The study, pubished by Cell Press, attempted to crack the code on why pepole or animals with obesity tended to be less active. It had been thought that animals don’t move around as much because it can be disabling to carry around extra body weight, but it turns out there’s more to it than that, and that the brain chemical dopamine may play a part as well.

In the study, scientists fed normal and high-fat diets to different groups of mice. The high-fat diet mice slowed down their activity, but interestingly they did it before they actually got fat instead of after. Scientists think that the obese and slow-moving mice may have had less of a receptor for processing dopamine, and the weight gain compounded this.

“We know that physical activity is linked to overall good health, but not much is known about why people or animals with obesity are less active,” says the study’s senior author Alexxai V. Kravitz, an investigator in the Diabetes, Endocrinology, and Obesity Branch at the National Institute of Diabetes and Digestive and Kidney Diseases–part of the National Institutes of Health. “There’s a common belief that obese animals don’t move as much because carrying extra body weight is physically disabling. But our findings suggest that assumption doesn’t explain the whole story.”

“Other studies have connected dopamine signaling defects to obesity, but most of them have looked at reward processing–how animals feel when they eat different foods,” Kravitz says. “We looked at something simpler: dopamine is critical for movement, and obesity is associated with a lack of movement. Can problems with dopamine signaling alone explain the inactivity?”