WASHINGTON (Reuters) - People who just can’t stop eating may be able to blame their genes, at least in part, U.S. researchers said on Monday.

They said up to 50 percent of the population carries a gene variation that may give them extra pleasure from eating -- and may explain why they are willing to work twice as hard for snacks as the other half of the population.

The gene affects dopamine, an important message-carrying chemical, or neurotransmitter, associated with behavior and movement.

“This gene is not just associated with overeating,” said Jennifer Temple, a neuroscientist at the University at Buffalo in New York who worked on the study.

“It has been associated with drug addiction, alcohol abuse, gambling. It is associated with reward decisions,” Temple added in a telephone interview.

Writing in the journal Behavioral Neuroscience, Temple and colleagues described an experiment in which they gave people access to many tasty snacks, including chips and chocolate.

Their volunteers thought they were taste-testing the food, but Temple’s team watched to see who ate the most.

They also ran genetic tests on their 74 unwitting volunteers, looking for variations in two genes that affect dopamine levels.

One gene appeared to have no effect on eating. But the Taq1 A1 variation had a noticeable effect.

People carry two copies of each gene, one from the mother and one from the father. Nearly half the population has one A1 and one A2 version, while the other half has two copies of A2. It is rare for someone to have two A1s.

People with two A2s experience feelings of reward easily, while people with A1 may have to work harder to feel good, the researchers said.

FOOD REWARD

Temple’s team set up a simple test to see who would work the hardest for snacks -- a computer test in which they had to click a mouse. People could win points they could trade for treats, and could quit at any time.

“People that had the (A1 variation) and who were obese worked twice as hard as people who were either not obese or who were obese and didn’t have the gene,” said Temple. “They worked twice as hard to get the food.”

The finding may help explain why so many people will continue to eat even when they are not hungry, said Temple.

Not everyone may choose food as their reward, she added.

Her team plans to do more studies to see if some people may in fact get more of a reward from gambling, or drugs.

“We are actually repeating this study in a larger sample where we are also going to be looking at more genes,” Temple said.

It may be that either genes or environment decide what a person’s reward of choice is -- food or drugs or gambling.

The team has found evidence that chemically manipulating dopamine levels alters eating behavior, which may help in the development of better diet drugs.