A group of children, of whom one parent is a monozygotic (MZ) schizophrenic twin, has been compared with a group of children of whom one parent is a normal MZ twin to a schizophrenic twin. A third group, of whom One parent is a normal dizygotic (DZ) twin to a schizophrenic twin, was examined in order to function as a sort of control group. This third group, however, was after age correction too small to supply reliable information.
In no case was the other parent psychotic.
In group 1, consisting of 47 children, 3 schizophrenic cases and one suicide were found, giving a morbidity risk of 9.4 per cent after age correction.
In group 2, consisting of 25 children, there were also 3 schizophrenic cases. The morbidity risk after age correction was 12.3 per cent (not a significant difference from group 1).
The present investigation gives no support to the hypothesis that environmental factors, associated with being reared by a schizophrenic parent, should cause more schizophrenia in the offspring, since controls reared by a non-schizophrenic had the same morbidity risk.
The result could be explained as due to factor(s) equally present in the schizophrenic MZ twins and their normal co-twin. As a pair of MZ twins are genetically identical, a hereditary factor is the most likely explanation.
However, the present material is of such limited size that no conclusion can be drawn.
Of the 6 psychiatric cases 5 were females. The twin-parents in the 6 cases were 4 female and 2 male, and 4 of these twin pairs were discordant for schizophrenia. From the precipitating factors, symptomatology and course of illness described in these cases, the question arises whether some cases of schizophrenia are (almost) only genetically determined, while in other cases both genetic and environmental factors are necessary for the outbreak of a schizophrenic psychosis.