Background: Clinical heterogeneity of autism likely hinders efforts to find genes associated with this complex psychiatric disorder. Some studies have produced promising results by restricting the sample according to the expression of specific familial factors or components of autism. Previous factor analyses of the restricted, repetitive behaviors and interest (RRBI) domain of autism have consistently identified a two-factor model that explains a moderate amount of variance. The identification of additional factors may explain more variance in the RRBI domain and provide an additional component of autism that may help in the identification of underlying genetic association.
Methods: We conducted factor analyses of RRBI symptoms with a sample that included verbal subjects meeting full criteria for autism aged 5 to 22 years (n = 245). Among affected sibling pairs (n = 126) we examined the familial aggregation of the identified factors. We also examined the associations of the factors with autism-related personality traits in fathers and mothers (n = 50).
Results: The previously identified two-factor model – insistence on sameness (IS) and repetitive stereotypic motor behaviors (RSMB) – was replicated in our sample. Next, a second factor analysis that included the item for verbal rituals resulted in a four-factor model – IS, ‘simple’ RSMB, ‘complex’ RSMB, and a fourth factor including symptoms associated with intense preoccupations (IP). Of these four, both IS and IP were significantly familial among affected siblings, but only IP was significantly correlated with the broader autism phenotype traits of rigidity and aloofness in fathers.
Conclusions: The results support previous evidence for the IS factor, its familiality, and the identification of IP as an additional strong candidate trait for genetic studies of autism.