Authors' reply

Vinkers et al have reported discrepant findings between their study and ours. First, their analysis is based on pre-trial reports, albeit detailed, whereas our study is based on a cross-sectional survey of current prisoners. Furthermore, additional variations that predetermine ascertainment and pathways through the criminal justice system must be taken into consideration in such comparisons. Our explanation of the higher rates of psychosis, one among a number of mental disorders we considered, is a combination of possible pre-existing morbidity and the impact of the environment on a vulnerable population. This relationship was mediated by current (defined as use while in prison) cannabis misuse. Second, our data on substance misuse are significant in terms of current use, as defined; lifelong use was similar between prisoners with and without intellectual disabilities. Third, the Quick Test may have led to over- or underestimation of the prevalence of intellectual disability, as we noted. There are additional arguments on this point, as the Quick Test has significant limitations: (a) we were quite conservative in the definition of intellectual disability, using not only a stringent cut-off for intellectual functioning but also poor educational attainment, and we excluded those not born in the UK, to avoid possible confounding by language-related problems; (b) according to Fazel et al, ¹ the pooled prevalence based on screening was 6.1% (95% CI 5.3–7.0%), ² therefore our calculations suggest that we have more or less identified the appropriate sample of prisoners; (c) the paper by Marjoram et al ³ is, in our view, erroneously cited, as its authors discuss specifically the impact of lower IQ on participant performance in theory of mind (hinting) tasks rather than psychopathology. It should be noted that all IQ tests would be compromised if administered to acutely ill individuals. Finally, the literature suggests a common pathway between psychosis and intellectual disability, particularly in early-onset cases ⁴ and this may be, to an extent, an underlying cause for the increased rates of psychosis. However, the cross-sectional nature of our study does not allow for further speculation on causality. In summary, prisoners with intellectual disabilities are vulnerable and may not receive adequate tailored input for their significant mental health needs. We agree that there should be further studies investigating these issues and we would like to thank Vinkers et al for their interest in pursuing this topic.