To identify the broader autism phenotype (BAP), the Family History Interview subject and informant versions and an observational tool (Impression of Interviewee), were developed. This study investigated whether the instruments differentiated between parents of children with autism, and parents of children with Down syndrome (DS). The BAP scores of parents of 28 multiplex autism families were compared with parents from, 32 DS families. The BAP measures provided good group differentiation but when considered together, the subject interview did not improve group differentiation. The differentiation was better for fathers than mothers. The measures do carry an important degree of validity; whether they can differentiate the BAP from other social disorders should be tested.