Obesity rates in the United States have more than doubled in the last 25 years. In 1991, only four states had obesity rates above 15 percent; by 2005, 45 states had rates above 20 percent, with many reporting rates as high as 30 percent.

Fast food restaurants have played a big role in this increase, because of their large portion sizes and because restaurant meals tend to have more fat and saturated fat than meals eaten at home.

This culpability, along with the ease of calculating calories for standardized menu items, makes chain restaurants a natural target for legislation requiring nutritional labeling. Such policies are currently being enacted or considered across the nation, including legislation implemented last year in New York City and a bill in California set to go into effect this year.

Now, House Bill 906, which was introduced in the Pennsylvania House of Representatives earlier this month, would require restaurants in the state to display dietary information on menus in the same size type as menu items.

This policy makes sense. Showing people how many calories they are consuming should give them the information they need to make an informed selection.

But the story is more complicated. People are eating more because calorie-dense foods are convenient and cheap, a state of affairs that has been facilitated by government subsidies to corn and dairy production. Furthermore, portion sizes are larger and priced to encourage overeating.

Walk into McDonalds and look at how soft drinks are priced. A "small" Coke, a third larger than the 12-ounce cans we used to drink, is $1, the medium is $1.20 and the large -- containing a whopping 32 ounces and 310 calories -- is $1.50. Why pay a dollar when, for just 50 cents more, you can get twice as much, especially given the economic hardships so many are currently experiencing?

Giving people so many inducements to consume more, and then expecting them to resist, is like holding a lit cigarette in front of an ex-smoker while informing him of the risks of lung cancer.

In our research (forthcoming in the "American Economic Review"), we have found that there may be some beneficial effects from posting calories, but that it will not roll back the obesity epidemic. Based on a study that examined fast-food choices in New York before and after the calorie-posting requirements went into effect, we found that providing calorie information seems to help those who would have otherwise underestimated how many calories they are eating, but most of the fast-food patrons didn't appear to be helped by the information.

Legislation requiring calories to be listed is a positive first step toward solving a pressing national problem, but even if implemented nationally, it is unlikely to have much impact.

If we want to get serious about helping people to lose weight, we need to take on the agricultural special interests. If we are going to subsidize agriculture, we should subsidize crops like leafy green vegetables, which enhance public health and are less likely to compete with agricultural products from the developing world. We should mandate pricing of items and portion sizes that encourage healthy eating.

We also should consider strategies to help people eat less and lose weight that take into account the psychology of human behavior. Our research suggests another, perhaps more effective, strategy for encouraging healthy eating: Make it easier for people to order healthier meals, or to make it a bit more inconvenient to get the less healthy foods.

We found that making lower-calorie sandwiches slightly easier to order led people to choose them more often, resulting in a reduction in the total calories of their fast-food meals. In this case, people changed their behavior without having to perform mental calculations, but instead with a subtle "nudge" in the right direction, a term suggested by Richard Thaler and Cass Sunstein, Mr. Obama's appointee to head the Office of Information and Regulatory Affairs.

Pennsylvania legislators should be commended for taking a first step toward tackling the thorny problem of obesity. However, their task does not end with the posting of calorie information. Our legislators also should consider other, more effective ways to propel people toward healthier eating.

As a society we have spent years making it easier and easier for people to gain pounds. Now we need to make it easier for people to lose them.