We have long sought to ground our diagnostic categories in etiological processes. While not free of methodological limitations, this study nonetheless provides, for the first time, a comprehensive picture of the structure of genetic and environmental risk factors for most common DSM-IV axis I disorders and all axis II disorders. The genetic structure of these disorders, as indexed by syndromal and subsyndromal diagnoses of the axis I disorders and criterion counts for the personality disorders, was relatively simple, consisting of four factors reflecting the two major dimensions of internalizing versus externalizing and axis I versus axis II. However, for five disorders, substantial genetic influences from two of these factors were required to adequately explain their etiology. The structure of the environmental influences on these disorders, by contrast, looked quite different, indicating that the organization of common psychiatric disorders into these coherent groups was largely a result of genetic and not environmental influences. Our findings reinforce accumulating evidence over the past half century that genetic factors play a critical role not only in the etiology of individual disorders but also in the structure of disorders as they occur and co-occur in human populations.
