Elevated rates of schizophrenia in a familial sample with mental illness and intellectual disability.
BACKGROUND
It is unknown whether intellectual disability (ID) is more familially related to psychotic mood disorders or schizophrenia. L. S. Penrose's large sample of families with two or more members admitted to psychiatric hospitals provided a unique opportunity to investigate the familial relationship between mild ID, schizophrenia and psychotic affective disorders.


METHODS
There were 183 affected relative pairs comprising probands with mild ID (95 male, 88 female) and their first or second degree relatives with schizophrenia or psychotic affective disorder.


RESULTS
There were nearly twice as many relatives with a diagnosis of schizophrenia (n = 121) as relatives with affective disorders (n = 62) among the intellectually impaired probands. This excess of schizophrenia was statistically significant, even after accounting for the increased risk of hospitalization for schizophrenia (P = 0.005), and was fairly constant across the different relative types. First-degree relatives with either mental illness were more likely to be parents (n = 77) than siblings (n = 51) or children (n = 3), but there was no excess of mother-son pairs.


CONCLUSIONS
These results suggest a stronger familial relationship of ID with schizophrenia than psychotic affective disorder, and lend some support to the neurodevelopmental hypothesis of schizophrenia.
Clinical Genetics Research Program, Queen Street Site, Centre for Addiction and Mental Health, 1001 Queen Street West, Toronto, Ontario M6J 1H4, Canada.
C M T
CM
Greenwood
J
J
Husted
M D
MD
Bomba
K A
KA
Hodgkinson
A S
AS
Bassett
D000328:Adult; D002648:Child; D016241:Child of Impaired Parents / Q000523:psychology; D003072:Cognition Disorders / Q000453:epidemiology* / Q000235:genetics*; D005260:Female; D006801:Humans; D008297:Male; D019964:Mood Disorders / Q000453:epidemiology / Q000235:genetics; D010290:Parents / Q000523:psychology; D011618:Psychotic Disorders / Q000453:epidemiology / Q000235:genetics; D012559:Schizophrenia / Q000453:epidemiology* / Q000235:genetics*; D012565:Schizophrenic Psychology