The broader autism phenotype (BAP) was assessed in parents who have two or more children with autism spectrum disorder (ASD) (multiplex (MPX) autism), parents who have no more than one child with ASD (simplex autism), parents who have a child with developmental delay without ASD, and parents who have typically developing children. Clinicians, naive to parent group membership status, rated BAP characteristics from videotaped administration of the Broader Autism Phenotype Symptom Scale (BPASS). Differences among groups in BPASS scores in the four assessed domains (social motivation, conversational skills, expressiveness, and restricted interests) were examined using multivariate ANOVA and post hoc comparisons. Further, ratings of videotapes by observers naïve to family status were compared with live, non-naive ratings by observers who were aware of family status (non-naïve). Findings demonstrate that the BPASS is an instrument resistant to rater bias. Parents from MPX autism families showed significantly more autism phenotype characteristics than the parents in the other groups. Moreover, the parents from simplex autism families did not differ from the parents of children with developmental delay or typical development. Finally, no differences between live, non-naive ratings and videotaped, naive ratings were observed. These findings suggest that characteristics of the BAP, specifically in the social and communication domains, are present in MPX autism parents to a greater degree than simplex autism and control parents. Further, the results provide support for the notion that genetic transmission mechanisms may differ between families with more than one child with autism and families with only one child with autism. Autism Res2012,5:13–20. © 2011 International Society for Autism Research, Wiley Periodicals, Inc.