Paternal age increases schizophrenia risk independently of IQ

By Lucy Piper, Senior medwireNews Reporter

Being born to an older father increases the risk of developing a schizophrenia spectrum disorder in adulthood but has no effect on premorbid IQ, say researchers from Denmark.

Their results showed that the risk of developing a schizophrenia spectrum disorder as a young adult decreased by about 30% with each standard deviation (SD) increase in IQ, irrespective of whether individuals were born to older or younger fathers.

By contrast, the risk of developing a schizophrenia spectrum disorder increased by 32% among young adults with each 10-year increase in their fathers’ age at birth.

The findings therefore fail to support the suggestion that the increased risk of schizophrenia associated with advanced paternal age is mediated by the effects of paternal age on offspring IQ.

The team, led by Holger Sørensen (Copenhagen University Hospital, Bispebjerg), used IQ data routinely measured on the Børge Priens Prøve (BPP) test at conscription in 138,966 Danish men who were born in 1955–1984 and in 1976–1993. The onset of a schizophrenia spectrum disorder was monitored in the year following conscription to the end of 2011.

A total of 528 men developed a schizophrenia spectrum disorder over a follow-up of 847,466 person-years, giving incidence rate ratios of 5.23 and 8.61 per 10,000 person-years for the older and younger cohorts, respectively. Schizophrenia was the primary diagnosis overall, at a rate of 64%.

Men born to fathers aged 45 years or older had a fully adjusted incidence rate ratio for a schizophrenia spectrum disorder of 1.89, compared with men born to fathers aged 25–29 years.

However, being born to an older father did not affect premorbid IQ according to the BPP tests. The unadjusted incidence rate ratio per SD increase in IQ score was 0.68, and 0.69 after parental ages had been taken into account. The fully adjusted incidence rate ratio, which also accounted for parental education level, urbanicity at birth, birth order and familial history of psychiatric disorder, was 0.72.

The researchers conclude in Schizophrenia Research that the most likely explanation for their results is “that the psychosocial benefits of having an older parent influence the premorbid general cognitive ability in persons who were on the path to develop a disorder in the schizophrenia spectrum”.

These benefits include a higher likelihood of having a planned pregnancy, having parents with a better educational and socioeconomic status and having nurturing, supportive and stable home environments, they explain.

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