Boston ? Binge-eaters who say they can’t help it may be right.

A study suggests a weak gene, not feeble willpower, may be the cause for some people. The research may point the way to a future pill to tame their appetites.

The joint Swiss-German-American study makes the strongest case yet that genetic mistakes can cause an eating disorder, researchers say. Traditionally, eating behavior has been viewed as complex and cultural in its causes.

“Willpower is not always important to reduce weight. Some people can by willpower. Some cannot, and I think these patients have a hard time,” said Dr. Fritz Horber, the leader of the binge-eating study at the Hirslanden Clinic in Zurich, Switzerland.

Researchers have been trying to understand the reasons for an epidemic of obesity, which raises the risk for heart disease, diabetes and many other ailments. About 30 percent of American adults are obese, up from 14 percent 25 years ago, according to government data. The surge is widely blamed on abundant high-calorie foods and sedentary lifestyles.

However, some researchers have also begun to link several genes to obesity, implicating heredity as an important underlying factor. Increasingly, eating problems are thought to stem from a subtle interaction of lifestyle and multiple genes.

Probably the most common eating disorder, binge-eating strikes up to 4 million Americans, according to the National Institutes of Health. Binge-eaters, who are usually but not always overweight, frequently and compulsively stuff themselves — often in secret — and feel ashamed afterward.

In this study, which was published today in The New England Journal of Medicine, the researchers focused on a gene linked to obesity in earlier studies. Known as the melanocortin 4 receptor gene, it makes a protein by that name that helps control appetite in the brain’s hunger-regulating hypothalamus. If a mutated gene makes too little protein, the body feels too much hunger.

In another study in the same journal issue, a British team reported finding mutations of the same gene in more than 5 percent of 500 severely obese children.

The genetic link here was so strong that the researchers could use results from chemical tests on their genetic DNA to predict how much food the children would eat at a meal.