If this is true, then why the dramatic rise in obesity in the last few decades ?
I don't understand why you think there is a contradiction. Say there's a "hunger" gene that makes some people have a stronger desire to eat. Go back 40 years ago in Bangladesh, and I doubt you'll find any obese people because there was not enough food to eat. If you shipped them all to present-day Chicago, give them cars, apartments, and Nintendo, and I would certainly expect those
relative differences to appear.
As for the rise in obesity, I see it as a change in lifestyle and availability of inexpensive calories. Take my neighborhood from when I was a kid 30 years ago. I was there for the introduction of the Big Gulp, a 32 ounce soda at 7-11 convenience stores. The three gas stations I could walk to did not have convenience stores. They had soda machines where you could buy 12 ounce cans.
Today, 7-11 has 64 ounce sodas that are cheaper in real dollars than the cans were when I was a kid. The three gas stations also offer sodas, candy, ice cream, and all sorts of foods with a low cost per calorie. That's but one example of increased opportunity.
Lifestyles have changed. When I was coming up, I would see kids playing basketball and other pick-up games nearly every day. Kids would also play games like tag and hide-and-go-seek in yards around the neighborhood. There was lots of bike riding and hanging out with friends (which meant walking). At that time, Atari was just coming on to the scene.
Now video games are far more commonplace. Kids can communicate instantly over the computer and via cell phones. These things suck up a lot of time - time that was formerly spent in physical activity.
In broad terms I think the two combined for the genesis of increases in weight over all, and certainly more opportunity for people to become obese. As a social theory (totally out of my ass), I think that as more and more people put on weight, it became less and less of a social stigma. Vanity and social acceptance, not health, are probably the strongest factors in a person moderating their weight.
A genetic component does not preclude increases in overall obesity rates. The upside to discovering a genetic component is that maybe people will realize that maintaining their weight is more difficult for them than others, so they will accept that and work harder. The downside and more likely result is that it becomes an excuse.
It's sort of like realizing you have a bad temper because your father was an ******* and treated you poorly. If you stop there, you use it as an excuse. What you're supposed to do is realize that your "natural" reaction is not typical, so you should work harder at controlling it.