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Voluminous data on the familial distribution of schizophrenia in Iceland were evaluated in terms of a partly dominant genetic mechanism. The risks were found in most instances to be proportional to the probability of harbouring the proposed gene, but penetrance rates, which generally are around 25%, appear to be elevated when additional risk factors are present. This occurs in individuals with multiple ill relatives, in offspring of two schizophrenic people, or in monozygotic co-twins. Excessive risks also occur in families with unusually severe disease. [ABSTRACT FROM AUTHOR]

The more recent large scale family investigations of schizophrenia are reviewed, particularly studies in which the authors have compared the rates in relatives of index cases with those of the general population. The risk of psychotic illness is increased four-fold in first-degree relatives and two-and-a-half-fold in second-degree relatives over that of the population at large. The possible genetic mechanisms are evaluated, with the conclusion that modified dominant transmission, based on a single major gene, is a plausible system. [ABSTRACT FROM AUTHOR]

The article presents data obtained from a study on the family distribution of schizophrenia in Iceland, and to discuss relevant information on the overall subject of heredity in the aetiology of psychosis. The studies made by several investigators have showed that the elevated psychosis risk in close relatives of schizophrenic index cases has remained unchanged. The data on the families of schizophrenic index cases in the country has showed the risk of schizophrenia to be raised about four fold in the first degree relatives over that of the population at large.

Objective: Exploration of academic and creative potentials of first-degree relatives of hospital-treated psychotic patients in Iceland., Method: Relatives identified in a previously published family study of psychosis are evaluated, using records available at the National Archives and National Library in Reykjavik. Performance of groups of relatives is compared to that of the general population., Results: Healthy relatives of patients are found to excel over others in authorship of books of fiction or poetry, in general school performance at age 20 years and in mathematics. These findings apply to both schizophrenia and affective illness., Conclusion: Relatives of psychotic patients often exhibit good mental abilities.

A study of mathematically gifted Icelanders demonstrates an increased risk of mental illness in their ranks. Psychotic disorders are also frequent among their relatives. Linkage of academic records with previously published data on the family distribution of psychosis reveals a pattern compatible with the hypothesis that high arousal plays a role in reasoning ability.