Here is someone who did do a scientific survey, at least. This can't be called a "theory" just yet, but it does give us an indication of why some are overweight and others not, why some can successfully lose weight, while others can't:
Most successful dieters lose weight on their own
Hmm... eating right and exercising regularly. Here, I thought that was an old wives tale, like cats sucking the breath out of babies.
How about people who were never overweight?
There may be other factors, of course, but diet and exercise seem to be the main ones.
Quite unlike the stories of night blindness being caused by the moon.
Of course, there are other, less common factors as well, or else that 3% of the "always thin" group who ate what they wanted and never exercised wouldn't be thin. Sure, those factors need to be researched.
But, in the final analysis, if a person wants to lose weight, the way to do it is to eat less and exercise more. Those scientific breakthroughs you see on TV infomercials telling people that they only have to take whatever snake oil they are hawking without changing their lifestyles are just so much bunk.