Sounds like the most obvious explanation in the world. But why do we believe this? What's our theoretical justification?
Ask obesity researchers. They will tell you that inactivity causes what's known as a positive caloric balance. Here is their thinking:
From the laws of physics, we get a concept of called the First Law of Thermodynamics. This tells us that energy that goes into any system (like your body) must equal the energy that comes out of it. So if you sit around playing video games, this lowers the amount of energy you burn off. That excess energy must go somewhere. And it winds up as a beer belly or double chin or fat thighs or something.
But the relationship video games and obesity may not be so clear.
An alternative explanation twists things around.
Say some primary problem causes the body to put on too much fat. This then CAUSES us to be inactive. In other words, a hormonal problem, for instance, could drain us of energy. If we feel sluggish, we will be less likely to go outside and play and more likely to sit inside and play video games.
In this case, the primary cause of fattening is not the lack of activity -- it is whatever screws up the fat tissue to begin with.
What could do that?
Well, when you eat too much sugar and carbohydrate, you produce excessive amounts of insulin. This in turn causes you to store too much fat.
So the root problem could be too much insulin: not too many video games, not too many calories.
Public health authorities tend to reject this "insulin makes us fat" argument in favor of the video games and obesity theory. But consider some of these points:
Of course, exercise can be great for many reasons.
Strengthening and toning likely drives huge benefits for the body. Conversely, playing too many video games can clearly be bad for you. For starters, it can cause you to develop carpal tunnel syndrome, limit your social life, and hurt your eyes.
But if we are trying to ascertain the relationship between video games and obesity, we at least need to consider the alternative theory, because it seems to explain the evidence we see in the real world much better.