The debate over the relative importance of nature and nurture, of heredity and environment, is centuries old. If undesirable traits are determined by heredity, there is not much anyone can do to improve human health or performance. Nowadays, most scientists avoid the debate. They say instead that heredity determines potential capacities, and it sets limits. A child who is born retarded can not grow up to be a Nobel prize winner. But scientists emphasize that both heredity and environment are important and that how a person turns out depends on a complex interaction between the two. An analogy may make the point clearer. It takes moisture and cold to make snow; you can not say that the moisture has twice as much influence as the cold in producing the snow, or half, because both elements are essential.