Since their separation as independent diagnostics, autism spectrum disorders (ASD) and schizophrenia spectrum disorders (SSD) have been conceptualized as mutually exclusive disorders. Similarities between both disorders can lead to misdiagnosis, especially when it comes to average-IQ adults who were not identified during childhood. The aim of this review was to examine the occurrence of SSD in average-IQ adults with ASD. Electronic and manual searches identified a total of 278 references, of which 10 were included in a meta-analysis. The pooled prevalence of SSD in the total ASD sample was close to 6%, pointing to a high co-occurrence of the two conditions. Further research is needed to determine the factors that predispose members of this population to the emergence of psychotic disorders.