Conventional wisdom says Americans have gotten fat because they eat too much and exercise too little, but a new study in the International Journal of Obesity turns that notion on its head. A group of 20 researchers from eight states reviewed more than 100 obesity studies and concluded that at least 10 other factors have contributed to America's collective weight gain.

The researchers write that they found "supportive evidence for each of the factors on their list that is" as compelling as the evidence for more commonly discussed putative explanations." In interviews with the press, the researchers stressed that their study in no way minimizes the importance of how much or what types of food we eat or how much energy we burn through exercise. Instead they say it points to the oversimplification of an extraordinarily complex problem.

They study does not rank the following 10 factors based on how much they contribute to obesity, nor does it claim that the list is all-inclusive and it does not prescribe any remedies. It just provides food for thought.

1. Sleep deprivation. Studies show that not getting enough sleep increases hunger and appetite and may cause hormonal changes that lead to weight gain.

2. Pollution. Environmental toxins like PCBs can disrupt the normal activity of hormones that regulate fat metabolism.

3. Air conditioning and central heating. We spend more and more of our lives in temperature-controlled environments, which means we need to burn fewer calories to regulate our body's thermostat.

4. Decreased smoking. Studies show that smokers weight less than nonsmokers and that those who quit typically gain weight. This is of course not to say that smoking is a good weight-loss strategy.

5. Drug side effects. A whole raft of commonly used prescription drugs ? antidepressents, contraceptives, blood pressure and diabetes medicines, antihistamines, protease inhibitors, mood stabilizers ? cause people to gain weight and in some cases, a lot of weight.

6. Later-in-life pregnancies. Older mothers are more likely than young mothers to have overweight or obese children.

7. Societal changes. As our population ages and our ethnic mix gets shuffled, groups with a higher prevalence of obesity are becoming larger segments of our population.

8. Birds of a feather. Humans with similar body types tend to mate, thus passing along genes that contribute to obesity.

9. Genetic causes. Environmentally caused weight gain in earlier generations can become embedded in genes that then get passed to future generations.

10. Fertility factors. Rebenesque women tend to be more fertile than their Twiggy counterparts, meaning that future generations stand a better chance of inheriting zaftig genes.