Premorbid intellectual functioning and risk of schizophrenia and spectrum disorders

Evidence from longitudinal studies indicates that lower IQ score in childhood and early adolescence increases risk of schizophrenia and schizophrenia spectrum disorders (SSD). This study investigated the association between premorbid IQ and risk of SSD in a population-based cohort of 17-year-old conscripts. Fifty four thousand males assessed by the Israeli Draft Board during two consecutive years were followed by means of the Israeli National Psychiatric Hospitalization Case Registry for up to 11 years. Tests of verbal and non-verbal reasoning, mathematical knowledge and instructions comprehension and several psychosocial variables were recorded by the Draft Board. Risk for SSD increased with decreasing IQ score. Only poorer nonverbal reasoning conferred a significant increased risk for SSD after taking into account general intellectual ability. IQ was not associated with age of onset. These results confirm the importance of low intellectual functioning as a risk factor for SSD. This is unlikely to be due to prodrome.