Though America has the highest rate of obesity, the health problem is world-wide.

The causes of Americans’ weight gain are complex, but according to Sue Saunders, an obesity researcher at New York University, the fact that Americans are getting fatter is indisputable.

"It’s partly eating habits and a lack of education," she says. "And it’s also psychological."

The definitions of obesity and overweight vary. But both are based on body mass index (BMI), a ratio of weight to height calculated by dividing weight in kilograms by height in meters squared. The National Institutes of Health defines overweight as a BMI above 25, and obesity as a BMI above 30. More than 40 million Americans are obese and 97 million are overweight, according to this definition.

The number of overweight Americans over age 20 has increased 8.6 percent since 1960, according to the National Health and Nutritional Examination Survey. It defines overweight men as having a BMI greater than 27.8, and overweight women as having a BMI greater than 27.3.