The thrifty gene hypothesis is the darling of many obesity researchers. It was proposed in 1962 by the geneticist James V. Neel to explain the high rates of obesity in modern populations, particularly modernizing American Indians. It states that our species evolved under conditions of frequent starvation, so we're designed to store every available calorie. In today's world of food abundance, our bodies continue to be thrifty and that's why we're fat.
Obesity researchers love it because it dovetails nicely with the equally dim "calories-in, calories-out hypothesis", whereby calories alone determine body composition. You practically can't read a paper on overweight without seeing an obligatory nod to the thrifty gene hypothesis. The only problem is, it's wrong.
The assumption that hunter-gatherers and non-industrial agriculturalists lived under chronic calorie deprivation has been proven false. The anthropological evidence indicates that most hunter-gatherers had abundant food, most of the time. They did have fluctuations in energy balance, but the majority of the time they had access to more calories than they needed, just like us. Yet they were not fat.
The Kitavans are a good example. They are an agricultural society that eats virtually no grains or processed food. In Dr. Staffan Lindeberg's studies, he has determined that overweight is virtually nonexistent among them, despite an abundant food supply.
The cause of obesity is not the availability of excess calories, it's the deregulation of the bodyweight homeostasis system. We have a very sophisticated set of feedback loops that "try" to maintain a healthy weight. It's composed of hormones (insulin, leptin, etc.), certain brain regions, and many other elements, known and unknown. These feedback loops influence what the body does with calories, as well as feeding behaviors. When you throw a wrench in the gears with a lifestyle that is unnatural to the human metabolism, you deregulate the system so that it no longer maintains an appropriate "set-point".
Here's what Neel had to say about his own theory in 1982 (excerpts from Good Calories, Bad Calories):
The data on which that (rather soft) hypothesis was based has now largely collapsed.And what does he think causes overweight in American Indians now?
The composition of the diet, and more specifically the use of highly refined carbohydrates.RIP, thrifty gene.
For more information on bodyweight regulation, see:
Insulin Controls Your Fat
Leptin and Lectins
Thoughts on Obesity Part I
Thoughts on Obesity Part II