Low pre-morbid IQ not associated with psychosis risk

By Ingrid Grasmo

The strong correlations currently observed between low IQ and risk for psychosis are most likely linked to the influence that psychosis onset has on cognitive functioning rather than a shared genetic background, suggest study findings.

Stanley Zammit (Cardiff University, UK) and colleagues found a low correlation between premorbid IQ and psychosis, a finding that is in contrast to the higher correlations reported in the literature with postmorbid IQ.

"This strongly suggests that shared etiological factors between IQ and psychosis are much less important that previously thought," say the researchers.

To examine the extent to which any genetic component for schizophrenia is shared with one for intelligence (as measured by premorbid IQ), the team identified IQ levels from 369,960 male siblings, 1986 monozygotic twin pairs, and 2253 dizygotic twin pairs born between 1951 and 1976. Individuals with a subsequent diagnosis of psychosis were identified via the Swedish National Hospital Discharge Register.

In total, 1.01% of included individuals were subsequently diagnosed with psychosis, while 33.9% of the entire sample were classified as having low IQ (defined as a score of 1-3 out of a maximum of 9).

Regression analysis showed that individuals with a high (a score of 7-9) and medium (a score of 4-6) IQ were 48% and 61% less likely to be diagnosed with psychosis, respectively, than those with a low IQ.

The phenotypic correlation between IQ and psychosis was found to be very low (r value of -0.11), with genetic influences accounting for 90.9% of the phenotypic correlation.

A moderately important sibling-specific influence was found for psychosis, suggesting that factors relating to the differences in experience between siblings also affect development of the condition.

Analysis of the proportion of genetic variance for psychosis independent of IQ found that approximately 93.2% of the heritability estimate for psychosis was independent of premorbid IQ, while 6.8% was shared, suggesting that most genes influencing the two traits are independent.

"A strategy of using IQ as a phenotype to identify genes that have an important role in the genetic origin of schizophrenia is likely to have a limited contribution to understanding the genetic origin of psychosis," conclude the authors in the Archives of General Psychiatry.

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