Objectives. Early-onset schizophrenia is considered to be neurobiologically similar to adult-onset forms, although it represents a more severe expression of the disorder. In the present study, we explored putative larger familial vulnerability of intellectual impairments in early-onset schizophrenia spectrum disorders (EOS) when compared to adult-onset (AOS) families. Methods. A sample of 340 subjects including schizophrenia spectrum disorder patients, their first degree relatives and age-matched healthy controls was assessed on intelligence quotient (IQ). We used linear regression analysis and intraclass correlation coefficients (ICC) to explore familial aggregation of IQ across age at onset groups. Results. The relationship between IQ level of patients and their first-degree relatives showed positive linear association (β 0.43, P < 0.01). High significant familial aggregation was found for intelligence quotient in EOS families (ICC 0.618, P < 0.01), while AOS families responded to lower estimates (ICC 0.204, P 0.26; between ICC comparison z 1.993, P < 0.05). Conclusions. High aggregation of intellectual performance in the EOS group suggests larger familial vulnerability in early-onset forms of the disease when cognitive functions are considered. Within a continuum of psychopathology in schizophrenia spectrum disorders, specific genetic effects are discussed for distinct onset forms that might be in line with a neurodevelopmental model of the disease. © 2012 Informa Healthcare.