An invariant feature of pervasive developmental disorders (PDDs) is a relative deficit in the capacity for reciprocal social behavior (RSB). The authors acquired teacher reports of RSB in 287 schoolchildren and parent reports of RSB in 158 child psychiatric patients using a new research instrument, the Social Reciprocity Scale. Total scores on this measure of RSB were continuously distributed in all groups of subjects; children with PDDs scored significantly higher for the degree of deficits in RSB than did clinical or nonclinical controls. Latent class analysis and factor analysis failed to demonstrate separate categories of deficiency for core autistic symptomatology and more general impairments in RSB, consistent with the notion of a "broader autism phenotype." Assessments of RSB on a continuous scale may be useful clinically for characterizing the behavior of children whose social deficits fall below the threshold for a full diagnosis of autism. They may also be useful in genetic-linkage studies of autistic spectrum disorders.