Background: Clinical heterogeneity of autism likely hinders efforts to find genes, but promising results were obtained when samples were restricted to subjects that shared a common familial trait.  Previous studies have analyzed the factor structure of the restricted, repetitive behavior and interests (RRBI) domain and found a two-factor model, 'insistence on sameness' (IS) and repetitive stereotyped motor behaviors,' (RSMB) that accounted for a relatively small amount of variance.  Identifying additional familial factors that account for more variance may provide an approach to reduce heterogeneity in future studies.

Objectives: To a) replicate previous findings; b) identify new factors in the RRBI domain; c) examine their familiality among affected siblings; and, d) measure the association with broader autism phenotype (BAP) traits in their parents.

Methods: Our sample included 310 verbal subjects (age 5 to 22) who met DSM-IV criteria for autism.  We factor analyzed the RRBI 'ever' items from the Autism Diagnostic Interview-Revised to replicate previous findings and then in a separate analysis we included the communication item, 'verbal rituals.'  We examined the familial aggregation of factors and their association with BAP traits.

Results: The IS and RSMB two factor model was replicated in our sample.  In the second factor analysis (RRBI + verbal rituals) we observed a four factor model: IS, ‘simple’ RSMB, ‘complex’ RSMB, and a fourth factor including symptoms associated with intense preoccupations (IP).  IS and IP were each significantly familial (p < .001 for both), but only IP was significantly correlated with the BAP traits of rigidity (r = .39, p = .007) and aloofness (r = .39, p = .005) in fathers.

Conclusions:The results support previous evidence for the familiality of IS and identify IP as an additional strong candidate trait for genetic studies of autism.