Two previous epidemiological studies of autistic twins suggested that autism was predominantly genetically determined, although the findings with regard to a broader phenotype of cognitive, and possibly social, abnormalities were contradictory. Obstetric and perinatal hazards were also invoked as environmentally determined aetiological factors. The first British twin sample has been re-examined and a second total population sample of autistic twins recruited. In the combined sample 60% of monozygotic (MZ) pairs were concordant for autism versus no dizygotic (DZ) pairs; 92% of MZ pairs were concordant for a broader spectrum of related cognitive or social abnormalities versus 10% of DZ pairs. The findings indicate that autism is under a high degree of genetic control and suggest the involvement of multiple genetic loci. Obstetric hazards usually appear to be consequences of genetically influenced abnormal development rather than independent aetiological factors. Few new cases had possible medical aetiologies, refuting claims that recognized disorders are common aetiological influences.
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We are grateful to: Susan Goode for psychometric testing; Les Butler and Doreen Summers at Queen Elizabeth's Hospital, Hackney, for cytogenetic analysis; Malcolm Carruthers and colleagues for metabolic analysis; Len Dobson at Lewisham Hospital for blood group analysis; John Buckler for data on normal twins; Andrew Pickles for statistical advice; Douglas Schmidt for data entry; and Linda Wilkinson and Elaine Hunt for secretarial support. We are also grateful to the many clinical colleagues who allowed access to their patients. This study would not have been possible without the generous cooperation of all the subjects and their families. Supported by grants to Michael Rutter from The MacArthur Foundation, The Mental Health Foundation and The Medical Research Council, UK.
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