The second approach used a twin design to estimate the contribution of genes versus the environment in risk for AUD. Identical twins (monozygotic; MZ) share 100% of their genes while fraternal twins (dizygotic; DZ) share only approximately 50% (the same as full siblings). In addition, environmental variability can usually be equated more evenly when twin pairs are used because they are usually raised together in the same homes and experience the same developmental events at the same ages. Therefore, a higher level of similarity for a diagnosis among identical (MZ) compared with fraternal (DZ) twins indicates that genetic factors were likely to have contributed to the development of the disorder, not just childhood environment. Twin studies were carried out in the United States, Finland, Sweden, and the United Kingdom (Gurling et al., 1984; Hrubec and Omenn, 1981; Kendler et al., 1994; Murray et al., 1983; Partanen, 1972), and almost all supported a genetic influence in alcoholism with an estimated proportion of the risk explained by genes of about 60% (Edenberg and Foroud, 2006; Kendler, 2012). Taken together twin, family and
