In the early 2000s, public health advocates made a bold move in their fight against the country’s obesity epidemic. Their goal was to hold fast-food chains responsible for flogging the fatty burgers and fries that were contributing to heart disease and swollen waistlines nationwide.

Drawing a comparison to cigarette companies, they argued that the fast-food industry had profited by misleading people into buying a dangerous product. To test out this legal strategy, they filed a 2002 suit claiming that McDonald’s ought to pay for making two Bronx teenagers, both habitual Mickey D’s patrons, fat and sick.

The father of one teenager told the court that he “always believed McDonald’s was healthy for my children.”

That case was eventually tossed out, but not before thoroughly spooking the food and restaurant industries, who have since spent millions lobbying for immunity from further lawsuits. So far, 26 states have enacted “commonsense consumption” laws, which prohibit people from suing food purveyors for making them fat, giving them diabetes, or adding to their high blood pressure.

Recently, economists at Vanderbilt discovered a strange, perhaps unintended consequence of these so-called “cheeseburger laws.” In states that took away the right to file obesity lawsuits, overweight residents were motivated to lose weight and eat healthier.

The mystery, still unanswered, is: Why?

Christopher Carpenter, a professor of economics, and D. Sebastian Tello-Trillo, a PhD student, used annual data from a huge national health survey to look at people living in states before and after an immunity law was enacted. To account for trends over time, they made comparisons to people in states that didn’t pass such a law, all while controlling for shifting demographics state-by-state. They also adjusted for other obesity-related laws that were passed during this time, like menu-labeling mandates and soda taxes.

In a draft report published through the National Bureau of Economic Research this week, Carpenter and Tello-Trillo showed that people behave more health-consciously after a ban on obesity lawsuits. Not everyone, though. On average, the laws did not have a detectable effect on the population at large. But they did seem to nudge people who were already overweight.

Among some of the most obese, there was a 4-6 percent increase in the number of people who said were trying to lose weight. People with high BMIs also reported eating more fruits and vegetables, about two additional servings per week.

This is a deeply weird result.

The theoretical explanation goes like this. Taking away the right to file obesity lawsuits makes people more responsible for their own weight. If can’t sue KFC for clogging your arteries, you might cut down on the fried drumsticks to protect your health in the first place. This is called the moral hazard argument for tort reform.

But obesity lawsuits have never actually succeeded in America. And who eats junk food with the expectation that, down the road, they’ll win a long-shot lawsuit making McDonald’s pay for their medical bills?

“People just don’t think that way,” said Richard Daynard, a law professor at Northeastern who specializes in public health litigation. “Think about it: Would you drive any differently depending on whether the manufacturer of your car was immunized from litigation?”

Besides, Daynard doubts that most people even know if their state bans obesity lawsuits. “The whole point was to sneak by very quietly and immunize the junk food companies from liability,” he said. “There wasn’t much noise when it happened and nobody remembers.”

Carpenter himself agrees that this explanation is unsatisfying. It presumes too much to think that people keep their litigation options in mind when they are making fast-food purchases. But what other explanations are out there?

Any alternate theory would need to propose something that changed in the 26 cheeseburger law states, around the same time each state was passing its law.

One possibility that the fast food companies themselves had something to do with it.

In the paper, Carpenter and Tello-Trillo also looked at the effect of the legislation on fast food restaurants themselves. Since these companies were the ones lobbying strenuously for such laws, they would have been well aware of which states adopted them.

The economists found that bans on obesity lawsuits increased fast-food employment by 3.7 percent while the number of restaurants stayed the same. This means that more people were working in each restaurant.

Looking specifically at McDonald’s, they also found that when states passed such laws, the company began to own and operate more restaurants itself, leaving fewer restaurants owned by franchisees.

These results suggest that fast-food companies took the risk of obesity lawsuits seriously — and that in states which protected them from litigation, they became more involved and hired more.

You could concoct a couple of stories from these facts. Maybe in cheeseburger law states, where fast-food companies felt more welcome, they were more inclined to experiment with healthy-menu options.

Or maybe the debate surrounding the passage of the cheeseburger laws raised awareness in a state about the health risks of fast food. Practically speaking, these laws didn’t change much, because obesity lawsuits are already a losing bet in America. But the laws could have sparked conversation about obesity, and made people in that state more conscious of their weight.

Then there’s the observation that most of the 26 states with obesity-lawsuit bans lean Republican. Perhaps that played a role too. Republicans who take pride in self-reliance might be more inclined to internalize messages about losing weight.

Finally, as with any study, it’s possible that the results were entirely a product of luck. This is a constant risk in social science: About one in 20 results that are statistically significant at the 5 percent level are expected to be wrong, just by chance. In the coming months, the authors plan to collect evidence to strengthen their case, including looking at states where bans on obesity lawsuits almost passed, but didn’t.

By now, public health advocates have largely turned their attention away from obesity lawsuits. It’s extremely hard to pin the blame on a particular company, when the entire industry is churning out salty, sweet, fatty ambrosia. It’s also hard to overcome the legal defense that people are responsible for their own food choices. You’re supposed to know that fries aren’t exactly good for you. In order to nail a fast-food company, you first have to prove that it willfully obscured or downplayed or distorted health information about its menu in order to sell more burgers — and then prove that someone developed weight problems specifically from eating at that chain.

One of the most recent efforts occurred in 2010, when the Center for Science in the Public Interest, an advocacy group, spearheaded a lawsuit to make McDonald’s stop using toys to market its Happy Meals to children. A San Francisco judge tossed out that claim in 2012.

These lawsuits explore an uncomfortable question about who bears responsibility for public health. Being obese is not wholly a matter of personal will. Genetics plays a role, but so does one’s environment — and these days, the public is assailed non-stop by advertisements for fattening foods.

Shouldn’t junk food companies shoulder some of the blame for making Americans fat? Shouldn’t they contribute to some of cost of treating America’s obesity problem?

Five years ago, around the same time as the Happy Meal toy lawsuit, Michelle Obama gave a stern lecture to the Grocery Manufacturers Association, the food industry trade group that represents snack food giants like Kraft, Kellogg, Nestle, and Coca-Cola.

Fighting childhood obesity was a shared responsibility, she told them. “We need you not just to tweak around the edges, but to entirely rethink the products that you’re offering, the information that you provide about these products, and how you market those products to our children.”

In those early days of the Let’s Move campaign, the first lady focused on nutrition reform. She’s had some success partnering with companies behind the scenes to make packaged foods healthier through voluntary pledges, and for getting junk food out of schools. But she has also drawn criticism for not pressuring companies harder on their marketing practices, and for not pursuing stricter laws about food labeling.

By 2011, the campaign shifted gears to emphasize personal responsibility and exercise. “The problem isn’t just what’s happening at meal time or at snack time,” she said in a November speech that year. “It’s also about how our kids are spending the rest of their time each and every day.”

Public health advocates accused her of capitulating to corporate interests. It’s politically difficult to challenging how companies sell junk food to kids. It’s uncontroversial to encourage children to exercise and eat right.

The next generation of lawsuits against junk food have set their sights on smaller goals. Daynard, who is also the president of the Public Health Advocacy Institute at Northeastern University, says his staff is working to bring cases against companies that advertise their foods in misleading ways. Consumers these days do want to eat better, but companies often make oversized claims about the healthfulness or naturalness of the foods they sell. (Or, they trumpet their fresh and non-GMO ingredients to distract from the calorie count or fat levels.)

Food companies like to say they are only giving people what they want. The proliferation of healthier choices at fast-food restaurants, and the success of health-conscious eateries like Panera Bread reflect public demand for less fattening food. These are signs of progress.

Still, people — especially children — only have so much will power. Our better selves can be sanded down by the constant temptation of juicy burger ads and sugary cereal commercials. If Americans are making an effort to cut back on the junk food, do companies have a responsibility to stop rubbing our faces in it so much?