Genetics of neurosis. The effects of sampling variation upon the twin concordance ratio.

In a study of 229 neurotic same-sex adult twins admitted to all psychiatric in- and out-patient clinics and mental hospitals in Norway, a statistically significant, moderately higher concordance was found for MZ than for DZ twins. When the sample was subdivided, this higher concordance for MZ pairs was true only for male twins and for twins admitted to mental hospitals. The results are compared to those from earlier studies; different findings on the importance of genetic factors in the development of neuroses may be due to differences in sample selection.