For these analyses, we primarily grouped families into two nested diagnostic classes (strict and spectrum ASD) based on proband diagnostic measures (45). To qualify for the strict class, affected individuals met the criteria for autism on both primary diagnostic instruments, the ADI-R (2) and the ADOS (3). In addition to individuals meeting criteria for autism, a spectrum class included all individuals who were classified as ASD on both the ADI-R and ADOS or who were not evaluated on one of the instruments but were diagnosed with autism on the other instrument.
