Autism susceptibility genes likely have effects on continuously distributed autism-related traits, yet few measures of such traits exist. The Broader Phenotype Autism Symptom Scale (BPASS), developed for use with affected children and family members, measures social motivation, social expressiveness, conversational skills, and flexibility. Based on 201 multiplex families, psychometric data on the BPASS are reported. Adequate inter-rater reliability and internal consistency were found. Parents had lower BPASS scores than affected children, after controlling for IQ. Parents and affected children showed overlapping distributions suggesting the BPASS captured variability in traits across groups. BPASS scores were not correlated with ethnicity or parent education; however, some domains were correlated with IQ. The BPASS holds promise as a quantitative phenotypic assessment for genetic studies.