The implications were clear. There is a reason that fat people cannot stay thin after they diet and that thin people cannot stay fat when they force themselves to gain weight. The body's metabolism speeds up or slows down to keep weight within a narrow range. Gain weight and the metabolism can as much as double; lose weight and it can slow to half its original speed.

That, of course, was contrary to what every scientist had thought, and Sims knew it, as did Hirsch.

The message never really got out to the nation's dieters, but a few research scientists were intrigued and asked the next question about body weight: Is body weight inherited, or is obesity more of an inadvertent, almost unconscious response to a society where food is cheap, abundant and tempting? An extra 100 calories a day will pile on 10 pounds in a year, public health messages often say. In five years, that is 50 pounds.

The assumption was that environment determined weight, but Albert Stunkard of the University of Pennsylvania wondered if that was true and, if so, to what extent. It was the early 1980s, long before obesity became what one social scientist called a moral panic, but a time when those questions of nature versus nurture were very much on Stunkard's mind.

He found the perfect tool for investigating the nature-nurture question - a Danish registry of adoptees developed to understand whether schizophrenia was inherited. It included meticulous medical records of every Danish adoption between 1927 and 1947, including the names of the adoptees' biological parents, and the heights and weights of the adoptees, their biological parents and their adoptive parents.

Stunkard ended up with 540 adults whose average age was 40. They had been adopted when they were very young - 55 percent had been adopted in the first month of life and 90 percent were adopted in the first year of life. His conclusions, published in The New England Journal of Medicine in 1986, were unequivocal. The adoptees were as fat as their biological parents, and how fat they were had no relation to how fat their adoptive parents were.

The scientists summarized it in their paper: "The two major findings of this study were that there was a clear relation between the body-mass index of biologic parents and the weight class of adoptees, suggesting that genetic influences are important determinants of body fatness; and that there was no relation between the body-mass index of adoptive parents and the weight class of adoptees, suggesting that childhood family environment alone has little or no effect."

In other words, being fat was an inherited condition.

Stunkard also pointed out the implications: "Current efforts to prevent obesity are directed toward all children (and their parents) almost indiscriminately. Yet if family environment alone has no role in obesity, efforts now directed toward persons with little genetic risk of the disorder could be refocused on the smaller number who are more vulnerable. Such persons can already be identified with some assurance: 80 percent of the offspring of two obese parents become obese, as compared with no more than 14 percent of the offspring of two parents of normal weight."