Twin studies of alcoholism have examined the increased risk of developing alcoholism in identical (i.e., monozygotic, or MZ) twins of alcoholics (who share the same genetic makeup as their alcoholic twins) and in fraternal (i.e., dizygotic, or DZ) twins of alcoholics (who, on average, share only half their genes) compared with the general population. If genetic influences or environmental factors shared by twin pairs growing up together are important, the percentage of twins of alcoholics with a current or past history of alcoholism should be much higher than the percentage in the general population. If genetic influences, in particular, are important, a significantly higher risk ratio should occur in MZ compared with DZ twin pairs.
