Three limitations deserve comment. First, it is important to recognize that the genetic influence on a phenotype (heritability) is not immutable; it is dependent upon the environment. Indeed, it is most accurate to characterize heritability of an illness in terms of a specific population at a particular point in time (Kendler, 2005). Since different environmental factors may be relevant for different psychiatric illnesses in particular populations, it is difficult to address this limitation effectively. Nevertheless, within cohorts, the pattern of results in Figure 1 appears consistent. For example, in studies using Swedish Twin Registry data with the same or overlapping birth cohorts, heritability estimates were as follows: bipolar disorder, 79%; Alzheimer’s Disease, 78-79%; anorexia nervosa, 56%; major depressive disorder, 38%; and generalized anxiety disorder, 27% (Bulik et al., 2006, Gatz et al., 1997, Gatz et al., 2006, Kendler et al., 2006b, Kendler et al., 1995, Mackintosh et al., 2006). Also, in studies using adult Virginia Twin Registry data, heritability estimates were as follows: drug use disorder, 66%; alcohol dependence, 49%; major depressive disorder, 39%; panic disorder, 37%; and generalized anxiety disorder, 22% (Hettema et al., 2001b, Kendler et al., 2001, Kendler and Prescott, 1999, Kendler et al., 2003).
