Background: Two forms of alcoholism with distinct clinical features and mode of inheritance were first distinguished in the Stockholm Adoption Study. This involved a large sample of children born in Stockholm, Sweden, who were adopted at an early age and reared by nonrelatives. Type 1 alcoholism had adult onset and rapid progression of dependence without criminality, whereas type 2 had teenage onset of recurrent social and legal problems from alcohol abuse.
Methods: A replication study was carried out with 577 men and 660 women born in Gothenburg, Sweden, and adopted at an early age/by nonrelatives. The genetic and environmental backgrounds of the adoptees were classified by the exact procedures calibrated by discriminant analysis in the original study.
Results: Both type 2 and severe type 1 alcoholism were confirmed as independently heritable forms of alcoholism in male adoptees. The lifetime risk of severe alcoholism was increased 4-fold in adopted men with both genetic and environmental risk factors characteristic of type 1 alcoholism compared with the others (11.4% vs 3.0%). Neither genetic nor environmental risk factors for type 1 alcoholism by themselves were sufficient to cause alcoholism. In contrast, the risk of type 2 alcoholism was increased 6-fold in adopted sons with a type 2 genetic background compared with others; regardless of their postnatal environment (10.7% vs 2.0%). The sons with a type 2 genetic background in the replication sample had no excess of type 1 alcoholism, and vice versa. There was no increased risk of mild abuse in adopted men regardless of their genetic or environmental background.
Conclusion: Type 1 and type 2 alcoholism are clinically distinct forms of alcoholism with causes that are independent but not mutually exclusive.