Studies of identical twins, particularly if the twins are separated at birth, are thought to provide some insight into the nature versus nurture debate. Twins have the same genetic material, but may be brought up in different home environments. There have been some controversial, small studies of twins that show a striking tendency for twins to exhibit the same mental disorders. Critiques of these studies have revealed evidence that sometimes a twin exhibits the behavior of the other twin in sympathy, and even with twins separated at birth there may be controversy as to the definition of the disease in question (Joseph 2002:75). What constitutes Attention Deficit with Hyperactivity disorder versus normal behavior is controversial and often highly subjective, depending on who is diagnosing the individual, and this is of course impossible for the same individual to do so in 'separated at birth' comparisons (Joseph 2002: 76). Also, the culture at large may over- or under-diagnose certain diseases at the same points in historical time.
             Other studies consider that although genes may have an influence upon behavior, there is always an interaction between genes and the environment. For example, "the balance of evidence suggests a modest genetic effect on both normal drinking and alcoholism in men, though similar evidence for women is so far lacking," in other words, genes play a role, and twins are more likely to be alcoholic than random pairs of individuals, but the correlation is not absolute (Murray, Clifford & Gurling 1983:25). Culture, social milieu, occupation, and other influences also have a role.
             One excellent example of a class of diseases that may seem to 'run in families' but is not genetic are nutritional deficiency disorders, such as pellagra, scurvy, or rickets, caused by malnutrition and poverty. Although to a layperson there might seem to be a genetic correlation, if one twin's diet i...