It has been estimated that about one out of six adult people in the United States is classified as alcoholic; of that number, the ratio of males to females is approximately 4 to 1— a proportion that may be attributed to both environment and genetics. Research involving male relatives, both biological and adoptive, indicate that alcoholism has many sources. In other words, while sons and brothers of male alcoholics are likely to be alcoholic as well (25 percent and 50 percent, respectively), genetic inheritance is not solely responsible for all of the cases of alcoholism.