Cognitive deficits have been found to be more prevalent in psychotic than in other disorders. Longitudinal research has shown that these deficits were generally already existent before onset of illness and are therefore not necessarily attributable to neurodegenerative processes. This study investigated whether both low IQ and markers of premorbid cognitive dysfunction independently contribute to an increased risk for psychoses. In a cross-sectional study about 50,000 young Swiss males completed a survey of intellectual problems in childhood/adolescence and other vulnerability factors during military call-up in 2005/2006. Subsequently, military IQ assessments were carried out on the entire sample. Diagnostic assessments were conducted according to International Classification of Diseases-10th edition (ICD-10). Low, especially performance, IQ was highly associated with an increased risk for psychotic disorders after adjusting for preexisting cognitive deficits and covariates, while in other disorders this association was less marked. Furthermore, preexisting intellectual problems emerged as important risk factors for psychoses. Our results confirm the importance of low IQ as characteristic of psychoses. Although premorbid intellectual deficits are common in people who go on to develop psychosis, neurodegenerative disease processes may also precipitate further declines in fluid cognitive functions. Assessment of cognitive functioning should be taken into account in early detection of psychoses.