By JOHN TIERNEY
If you’re a well-informed, health-conscious New Yorker who has put on some unwanted pounds in the past year, it might not be entirely your fault. Here’s a possible alibi: The health halo made you do it.

I offer this alibi after an experiment on New Yorkers that I conducted with Pierre Chandon, a Frenchman who has been studying what researchers call the American obesity paradox. Why, as Americans have paid more and more attention to eating healthily, have we kept getting fatter and fatter?

Dr. Chandon’s answer, derived from laboratory experiments as well as field work at Subway and McDonald’s restaurants, is that Americans have been seduced into overeating by the so-called health halo associated with certain foods and restaurants. His research made me wonder if New Yorkers were particularly vulnerable to this problem, and I asked him to help me investigate.

Our collaboration began in a nutritionally correct neighborhood, Brooklyn’s Park Slope, whose celebrated food co-op has a mission statement to sell “organic, minimally processed and healthful foods.” I hit the streets with two questionnaires designed by Dr. Chandon, a professor of marketing at the Insead business school in Fontainebleau, France, and Alexander Chernev, a professor of marketing at Northwestern University. Half of the 40 people surveyed were shown pictures of a meal consisting of an Applebee’s Oriental Chicken Salad and a 20-ounce cup of regular Pepsi. (You can see it for yourself at TierneyLab.) On average, they estimated that the meal contained 1,011 calories, which was a little high. The meal actually contained 934 calories — 714 from the salad and 220 from the drink.

The other half of the Park Slopers were shown the same salad and drink plus two Fortt’s crackers prominently labeled “Trans Fat Free.” The crackers added 100 calories to the meal, bringing it to 1,034 calories, but their presence skewed people’s estimates in the opposite direction. The average estimate for the whole meal was only 835 calories — 199 calories less than the actual calorie count, and 176 calories less than the average estimate by the other group for the same meal without crackers.

Just as Dr. Chandon had predicted, the trans-fat-free label on the crackers seemed to imbue them with a health halo that magically subtracted calories from the rest of the meal. And we got an idea of the source of this halo after I tried the same experiment with tourists in Times Square.

These tourists, many of them foreigners (they kept apologizing for not knowing what Applebee’s was), correctly estimated that the meal with crackers had more calories than the meal without crackers. They didn’t see the crackers’ health halo, Dr. Chandon said, presumably because they hadn’t been exposed to the public debate that accompanied New York City’s decision last year to ban trans fat from restaurants.

“It makes sense that New Yorkers would be more biased because of all the fuss in the city about trans fat,” Dr. Chandon told me. “It hasn’t been a big issue in most other places. Here in Europe there’s been virtually no discussion of banning trans fats.”

So might New York’s pioneering ban on trans fats have done more harm than good? Did it encourage people to eat more calories (and other fats that some scientists argue are no less harmful)? Did people start eating French fries — hey, they’re trans-fat free now! — and reward themselves with dessert? I can’t pretend to know the answers after our little experiment, which hardly constitutes peer-reviewed research. But the results were statistically significant and certainly jibe with other findings by Dr. Chandon and his frequent collaborator, Brian Wansink, the director of the Cornell Food and Brand Lab.

They’ve found that all of us, even professional dieticians, make systematic mistakes when estimating how many calories are on a plate. Experiments showed that putting a “low fat” label on food caused everyone, especially overweight people, to underestimate its calories, to eat bigger helpings and to indulge in other foods.

The researchers found that customers at McDonald’s were more accurate at estimating the calories in their meal than were customers at Subway, apparently because of the health halo created by advertisements like one showing that a Subway sandwich had a third the fat of a Big Mac. The health halo from Subway also affected what else people chose to eat, Dr. Chandon and Dr. Wansink reported last year after giving people a chance to order either a Big Mac or a 12-inch Italian sandwich from Subway. Even though the Subway sandwich had more calories than the Big Mac, the people ordering it were more likely to add a large nondiet soda and cookies to the order. So while they may have felt virtuous, they ended up with meals averaging 56 percent more calories than the meals ordered from McDonald’s.

“People who eat at McDonald’s know their sins,” Dr. Chandon said, “but people at Subway think that a 1,000-calorie sandwich has only 500 calories.” His advice is not for people to avoid Subway or low-fat snacks, but to take health halos into account.

“People need to look up calorie information, and this information needs to be clearly available on the menu or on the front of packages,” Dr. Chandon said. “If no information is available, people should say to themselves: ‘This restaurant or this brand claims to be healthy in general. Let’s see if I can come up with two reasons why this claim would not apply to this particular food.’ When we asked people to follow this ‘consider the opposite’ strategy, it completely eliminated health halos.”

More generally, Dr. Chandon advises American consumers, food companies and public officials to spend less time obsessing about “good” versus “bad” food.

“Being French, I don’t have any problem with people enjoying lots of foods,” he said. “Europeans obsess less about nutrition but know what a reasonable portion size is and when they have had too much food, so they’re not as biased by food and diet fads and are healthier. Too many Americans believe that to lose weight, what you eat matters more than how much you eat. It’s the country where people are the best informed about food and enjoy it the least.”