Editor—Procopio highlights the fact that our study included only subjects with a younger age of onset ranging from 16 to 28 and suggests that our finding of an association between paternal age and schizophrenia therefore needs to be interpreted cautiously.
This association has already been replicated in a number of studies covering much wider ranges of age of onset: the Jerusalem cohort covered ages of onset until 33,1 the case control study using the Stockholm inpatient register had ages of onset until 34,2 and Zammit et al showed such an association in their cohort with ages of onset up to 45.3 A large Danish case-control study of 7704 patients found no evidence for any interaction with age of onset.4
We also investigated possible age of onset effects by dividing our cohort according to the length of the follow up period for each individual and found no marked difference in the hazard ratios per 10 year increase in paternal age for those with less than nine years of follow up (hazard ratio 1.51, 95% confidence interval 1.25 to 1.82) and those with more than nine years of follow up (hazard ratio 1.77, 1.10 to 2.86).
One swallow doesn't make a summer, but this association has now been reported in several studies, in different settings, and over a wide range of ages. It seems to be a robust finding that may shed some light on the aetiology of schizophrenia.
The four other authors of this letter are Per Tynelius, senior statistician, Department of Public Health Sciences, Karolinska Institute; Glyn Lewis, professor of psychiatric epidemiology, Academic Unit of Psychiatry, University of Bristol; David A Leon, professor of epidemiology, Department of Epidemiology and Population Health, London School of Hygiene and Tropical Medicine, London WC1E 7HT; and David Gunnell, professor of epidemiology, Department of Social Medicine, University of Bristol, Bristol BS8 2PR.
Competing interests: None declared.
References
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