WHY are Americans getting fatter and fatter? The simple explanation is that we eat too much junk food and spend too much time in front of screens – be they television, phone or computer – to burn off all those empty calories.

One handy prescription for healthier lives is behavior modification. If people only ate more fresh produce. (Thank you, Michael Pollan.) If only children exercised more. (Ditto, Michelle Obama.)

Unfortunately, behavior changes won’t work on their own without seismic societal shifts, health experts say, because eating too much and exercising too little are merely symptoms of a much larger malady. The real problem is a landscape littered with inexpensive fast-food meals; saturation advertising for fatty, sugary products; inner cities that lack supermarkets; and unhealthy, high-stress workplaces.

In other words: it’s the environment, stupid.

“Everyone knows that you shouldn’t eat junk food and you should exercise,” says Kelly D. Brownell, the director of the Rudd Center for Food Policy and Obesity at Yale. “But the environment makes it so difficult that fewer people can do these things, and then you have a public health catastrophe.”

Dr. Brownell, who has a doctorate in psychology, is among a number of leading researchers who are proposing large-scale changes to food pricing, advertising and availability, all in the hope of creating an environment conducive to healthier diet and exercise choices.

To that end, health researchers are grappling with how to fix systems that are the root causes of obesity, says Dee W. Edington, the director of the Health Management Research Center at the University of Michigan.

“If you take a changed person and put them in the same environment, they are going to go back to the old behaviors,” says Dr. Edington, who has a doctorate in physical education. “If you change the culture and the environment first, then you can go back into a healthy environment and, when you get change, it sticks.”

Indeed, despite individual efforts by some states to tax soda pop, promote farm stands, require healthier school lunches or mandate calorie information in chain restaurants, obesity rates in the United States are growing. An estimated 72.5 million adults in the United States are obese, according to the Centers for Disease Control and Prevention. Last year, about 27 percent of adults said they were obese, compared with about 20 percent in 2000, as reported in a C.D.C. study published this month. And, the report said, obesity may cost the medical system as much as $147 billion annually.