EDITOR—Two twin studies of type 1 diabetes have reached opposite conclusions. In one, a population based cohort of Danish twins in which one or both cotwins had type 1 diabetes was studied for the presence of islet autoantibodies.1 High rates of autoantibody positivity were identified in twins with and without diabetes. Since positivity did not differ between the monozygotic and dizygotic twins it was suggested that a shared intrauterine or early postnatal environment might be more important than genetic factors. The second study found that the prevalence of islet autoantibodies was lower in initially unaffected dizygotic twins than monozygotic twins and did not differ from that found in unaffected non-twin siblings. The authors concluded that islet autoimmunity is determined predominantly …