Patients take part in a physical exercise lesson at the Aimin Acupuncture Weight Loss Hospital in Tianjin, China, May 2, 2005. getty

The most common New Year’s resolutions are getting healthy and losing weight. A survey done by Nielsen in 2015 showed that 37 people said “staying fit and healthy” was their top priority, followed by 32 percent who said they wanted to lose weight. But that’s easier said than done, and science may have just figured out the underlying reason why.

A study published Dec. 29 in Cell Metabolism found that a lack of willpower might not be the sole reason for physical inactivity. Researchers divided a group of mice into two groups and fed one group a standard diet while giving the other a high-fat diet for 18 weeks. The mice ingesting high-fat diets spent far less time moving and moved much slower. But the lack of movement began to set in even before significant weight gain did. Researchers then discovered that in the obese mice, altered dopamine receptors were triggering a slowdown in movement.

“There’s a common belief that obese animals don’t move as much because carrying extra body weight is physically disabling,” said the study’s lead author, Alexxai V. Kavitz, an investigator in the Diabetes, Endocrinology and Obesity Branch at the National Institute of Diabetes and Digestive and Kidney Diseases. “But our findings suggest that assumption doesn’t explain the whole story.”

Around 80 percent of people give up on their resolutions by the second week in February, according to clinical psychologist Joseph Luciani, writing for U.S. News and World Report. But figuring out just what causes a lack of movement may make it easier to address.

Kravitz suggested that pinpointing a physical cause may also reduce stigma around obesity.

“In many cases, willpower is invoked as a way to modify behavior,” said Kravitz. “But if we don’t understand the underlying physical basis for that behavior, it’s difficult to say that willpower can solve it.”