It’s possible that the biological cards were stacked against me from the start. Researchers know that obesity tends to run in families, and recent science suggests that even the desire to eat higher-calorie foods may be influenced by heredity. But untangling how much is genetic and how much is learned through family eating habits is difficult. What is clear is that some people appear to be prone to accumulating extra fat while others seem to be protected against it.

In a seminal series of experiments published in the 1990s, the Canadian researchers Claude Bouchard and Angelo Tremblay studied 31 pairs of male twins ranging in age from 17 to 29, who were sometimes overfed and sometimes put on diets. (None of the twin pairs were at risk for obesity based on their body mass or their family history.) In one study, 12 sets of the twins were put under 24-hour supervision in a college dormitory. Six days a week they ate 1,000 extra calories a day, and one day they were allowed to eat normally. They could read, play video games, play cards and watch television, but exercise was limited to one 30-minute daily walk. Over the course of the 120-day study, the twins consumed 84,000 extra calories beyond their basic needs.

That experimental binge should have translated into a weight gain of roughly 24 pounds (based on 3,500 calories to a pound). But some gained less than 10 pounds, while others gained as much as 29 pounds. The amount of weight gained and how the fat was distributed around the body closely matched among brothers, but varied considerably among the different sets of twins. Some brothers gained three times as much fat around their abdomens as others, for instance. When the researchers conducted similar exercise studies with the twins, they saw the patterns in reverse, with some twin sets losing more pounds than others on the same exercise regimen. The findings, the researchers wrote, suggest a form of “biological determinism” that can make a person susceptible to weight gain or loss.

But while there is widespread agreement that at least some risk for obesity is inherited, identifying a specific genetic cause has been a challenge. In October 2010, the journal Nature Genetics reported that researchers have so far confirmed 32 distinct genetic variations associated with obesity or body-mass index. One of the most common of these variations was identified in April 2007 by a British team studying the genetics of Type 2 diabetes. According to Timothy Frayling at the Institute of Biomedical and Clinical Science at the University of Exeter, people who carried a variant known as FTO faced a much higher risk of obesity — 30 percent higher if they had one copy of the variant; 60 percent if they had two.

This FTO variant is surprisingly common; about 65 percent of people of European or African descent and an estimated 27 to 44 percent of Asians are believed to carry at least one copy of it. Scientists don’t understand how the FTO variation influences weight gain, but studies in children suggest the trait plays a role in eating habits. In one 2008 study led by Colin Palmer of the University of Dundee in Scotland, Scottish schoolchildren were given snacks of orange drinks and muffins and then allowed to graze on a buffet of grapes, celery, potato chips and chocolate buttons. All the food was carefully monitored so the researchers knew exactly what was consumed. Although all the children ate about the same amount of food, as weighed in grams, children with the FTO variant were more likely to eat foods with higher fat and calorie content. They weren’t gorging themselves, but they consumed, on average, about 100 calories more than children who didn’t carry the gene. Those who had the gene variant had about four pounds more body fat than noncarriers.

I have been tempted to send in my own saliva sample for a DNA test to find out if my family carries a genetic predisposition for obesity. But even if the test came back negative, it would only mean that my family doesn’t carry a known, testable genetic risk for obesity. Recently the British television show “Embarrassing Fat Bodies” asked Frayling’s lab to test for fat-promoting genes, and the results showed one very overweight family had a lower-than-average risk for obesity.

A positive result, telling people they are genetically inclined to stay fat, might be self-fulfilling. In February, The New England Journal of Medicine published a report on how genetic testing for a variety of diseases affected a person’s mood and health habits. Over all, the researchers found no effect from disease-risk testing, but there was a suggestion, though it didn’t reach statistical significance, that after testing positive for fat-promoting genes, some people were more likely to eat fatty foods, presumably because they thought being fat was their genetic destiny and saw no sense in fighting it.