Madison "Mad" Nena nibbles on a tangerine picked from his garden on Kosrae, a tiny volcanic island in the Pacific Ocean some 4,670 kilometers southwest of Hawaii. The 53-year-old Nena is a rarity here. He's thin in a place where fatty, sugary foods imported from the United States have caused an alarming number of people to inflate like dirigibles; obesity-related diseases such as diabetes and heart disease have struck the island's 7,600 residents hard. Why Nena has stayed thin, and others have not, has drawn American researchers from Rockefeller University in New York City to this 109-square-kilometer patch of jungles, white beaches, mangrove swamps, and quiet villages for more than a decade, in a quest to tease out the genetic and molecular mechanisms of why humans are compelled to eat. And sometimes to eat and eat, far beyond what is healthy.

The Rockefeller team suspects that the proclivity of a person's body to approach a certain weight is determined far more by genes than was previously thought -- specifically, genes that control the impulse to eat. Growing evidence indicates that an individual's weight is 40 to 70 percent decided by genes, which makes it about as heritable as height. (Height, however, is determined during infancy and childhood, whereas weight can continue to fluctuate throughout life.) Some people appear to be hardwired to be particularly ravenous. When access to food is unlimited, say hunger-gene experts, these people can will themselves to eat less, but their efforts will almost inevitably be overridden by the far more powerful force of genetics.