Type 2 diabetes mellitus exacts a huge toll in money and human suffering. For instance, it accounts for more than 100 billion dollars of healthcare costs annually in the United States, or 15% of costs due to all diseases combined. The number of cases worldwide is estimated at 150 million. But this is a minimum number because, for each diagnosed case, there is thought to be one undiagnosed case in First World countries and eight in the Third World.1 Despite its other name of adultonset diabetes, the disease is becoming more common in young people.2,3 At its present rate of increase, within a few decades it will be one of the world’s commonest diseases and biggest publichealth problems,2,4 with an estimated minimum of half-a-billion cases.5 This explosion in prevalence is occurring especially in the Third World, at about 50% per decade; and because the epidemic is just beginning in the world’s two most populous countries, India and China, by the year 2010 more than half of the world’s diabetics will be Asians.2,3,5There are two main forms of diabetes mellitus, and the principal characteristics are outlined in the Box. But this is the story of type 2, not only because it is much more common and rising steeply in prevalence, but because that rise is doubly puzzling. Not only would the disease seem to be highly disadvantageous in terms of natural selection, but some human populations are much more affected than others.