A substantial number of prisoners have intellectual disabilities. We
analysed data on a sample drawn from all prisons in England and Wales.
Intellectual disability was defined as Quick Test scores equivalent to an IQ
of ⩽65. We found a significantly higher prevalence of probable psychosis,
attempted suicide and cannabis use in prisoners with intellectual
disabilities. Presence of intellectual disability was twice as likely to be
associated with probable psychosis but the relationship was fully mediated
by self-rated health status. It is important to identify this group as early
as possible in order to provide timely interventions to cope in adverse
environments and manage substance misuse.