To date research has been inconclusive in explaining the relationship between sociomoral reasoning and adult offending, with some researchers suggesting that this relationship is mediated by intelligence. This study investigated sociomoral reasoning among 100 mentally disordered offenders in a maximum security hospital, 50 with a primary diagnosis of personality disorder (PD) including psychopathy and 50 with severe mental illness (SMI). Participants completed a battery of measures including the Gough Socialisation Scale, the Wechsler Abbreviated Scale of Intelligence, and the Sociomoral Reflection Measure – Short Form. Between-group differences showed that, compared with the PD group, the SMI group had significantly fewer antisocial personality traits, lower IQ, and lower-level sociomoral reasoning. Hierarchical multiple regressions showed that IQ was the most powerful predictor of sociomoral reasoning after controlling for a number of other variables including social desirability. While psychiatric diagnosis may appear to predict sociomoral reasoning, intellectual function is an important mediator of the process.