Stigma can be a powerful force in changing behavior. Just ask smokers, whose once accepted habit is now so marginalized that the prevalence of smoking has dropped to about 19 percent of U.S. adults from nearly 24 percent just a decade ago. A lot of factors figured into the decline since smoking's mid-20th-century peak, but the sense that smoking is disgusting as well as unhealthful and socially costly has certainly contributed to many people's decision to quit.

Now that smokers have been taken care of, the obese are the new scapegoats for a lot of our ills. Last week, a letter published in the Lancet noted that the obese contribute more than their thinner compatriots to food scarcity and global warming, given that they eat more and require more transportation energy to move themselves around. While the authors' intent was probably not to make the obese feel worse, the media translations of the study in my quick Google search turned up headlines such as "Fat People Cause Global Warming, Higher Food Prices" and "Scientists Blame Fat People for Global Warming."

You might think that the obese could use some blame. As obesity increasingly becomes the norm, maybe society has grown too accepting. Perhaps what is lacking is the same thing that helped smokers lose their butts: a healthy dose of social stigma. If only there were more shame in being fat, maybe more people would be motivated to lose weight. But in fact, researchers say, stigma does very little to motivate overweight or obese people to change.

Why, first, are we increasingly intolerant of the obese even as more of us are joining their ranks? "At the same time that weight has gone up, we've had an increased emphasis on the thin ideal in society," says Janet Latner, a psychologist who studies stigma at the University of Hawaii—Manoa in Honolulu. (Think Us Weekly's celebrating Mariah Carey's recent weight loss.) People also see family and friends lose weight and believe that body weight is completely under our control. (As my colleague Sarah Baldauf has reported, that's not 100 percent true.) "In one sense, people make the final decisions over what goes in their mouths," says Kelly Brownell, director of the Rudd Center for Food Policy and Obesity at Yale University. But there are plenty of social and environmental determinants, too, he says. Immigrants to the United States tend to gain weight, for example, while people who move out of the country tend to lose it. Even rats will get overweight when given unlimited access to processed junk food, which shows the potential influence of one's surroundings. "If you change the environment in negative ways, you get obesity," Brownell says. "The solution isn't to blame people who are responding in predictable ways to the environment."

Brownell authored a 2006 study showing that when overweight people feel stigmatized because of their weight, they respond by eating more and giving up on diets. "Obese people are under such enormous pressure to lose weight," he says. "They understand the health consequences, they see fat jokes on TV, they were teased and ridiculed as kids. To think that you could add much more to that pressure is wishful thinking." The stress may make them turn to food for comfort or, as one researcher hypothesizes, might even contribute to the physiological processes of obesity through, say, stress hormones. It can also keep people out of the gym, most likely thanks to embarrassment and shame. A study published this year in the Journal of Health Psychology suggests, at least among the college-aged, that having more experience with weight stigma makes people less motivated to exercise.