Anyone with a post-Thanksgiving turkey-and-stuffing hangover knows that diet arithmetic is pretty simple: If you eat more calories than you burn, you gain weight. But what if the math isn't that easy after all? David B. Allison, who studies obesity at the University of Alabama at Birmingham, set out to discover why exactly it is that human beings, as a species, consistently eat too much and exercise too little, even when we know it's bad for us. Allison's research lead him to a bunch of studies of animals who live in close proximity to people. Some of these were laboratory research animals (monkeys, chimpanzees, and rodents); others were domestic pets, like dogs and cats. There was one thing they all had in common, though: despite the fact that their diets and activity levels were closely monitored and controlled, all the animal groups got fatter over time. In other words, too much food or too little activity didn't explain the weight gain—there's something about being around humans (or just being human) that makes you fat. Allison and his colleagues don't know the answer to the mystery yet, but they have a couple of working theories.

__First is light:__Studies have shown that subtle changes in the amount of time spent in light or dark environments changes eating habits. We all spend a lot more time in front of light bulbs and computer screens and other forms of ambient light than our ancestors. That may have fundamentally changed our makeup. The second explanation is a virus. There are certain viruses that are associated with obesity. Could these be more widespread than we thought?

So does this make you feel better or worse about overeating on Thanksgiving and skipping a workout—or are you still going to do your best to be on the right side of the diet equation?