The two members of each pair were raised in the same home environment. Telephone interviews with the twins were conducted to assess whether the participants had lost control over their drinking, were unable to cut back on drinking or had similar problems with alcohol. Twins who had an alcoholic identical twin were much more likely to be alcoholic themselves than were twins who had an alcoholic fraternal twin and this was equally true in women and in men. More surprisingly, men who had an alcoholic twin sister had very high rates of alcoholism. That wasn't the case.
Monozygotic twins share the same genetic makeup.
Dizygotic, or nonidentical, twins, who are not more alike genetically than ordinary sisters, were only three times more likely to be alcohol dependent if their twin had a drinking problem. The study also examined whether psychiatric disorders that commonly precede or coexist with alcoholism, such as childhood behaviour problems like lying and stealing or depression, were as strongly associated with risk of alcoholism in women as in men. It used to be believed that depression predicted increased risk of alcohol problems in women, whereas a history of childhood behaviour problems predicted alcoholism risk in men. The researchers acknowledge there still is much to learn about how and why alcoholism manifests itself. What they did learn, however, was that nearly two-thirds of the differences in risk of alcoholism among adults in the general population could be attributed to underlying genetic differences.