On the Twin Risk in Autism

Hallmayer J, Glasson EJ, Bower C, Petterson B, Croen L, Grether J, Risch N.
Department of Psychiatry and Behavioral Sciences, Stanford University School of Medicine, Palo Alto, CA, 94304, USA

Abstract: Am J Hum Genet. 2002 Oct;71(4):941-6. Epub 2002 Sep 12.

Findings

Findings from twin and family studies are the strongest evidence that Autism is genetically influenced. However, as of yet, no gene or genes have been found that conclusively cause a higher risk of developing the disorder. Some studies indicate that being a twin is a strong risk factor for autism—that is, there appears to be an increased rate of autism in twin pairs. Other studies have not supported this.

To investigate this, Dr. Hallmayer and his colleagues examined normal children born between 1980 and 1995, and those with autism, Asperger syndrome, or pervasive developmental disorder not otherwise specified (PDD-NOS) from their epidemiological study in Australia. Of the 465 children with a diagnosis, 13 were twin births, a rate of 30 per 1,000. This was compared with 9,640 twin births from 386,637 normal births, a rate (weighed to the number of children with a disorder) of 26.3 per 1,000.

Conclusions

These data do not support the theory that being a twin per se is a substantial risk factor in causing autism. This finding supports other recent large population-based studies in California and Sweden. The higher proportion of twins in the affected group can be explained by the fact that identical (monozygotic) twins as opposed to non-twin siblings have high rates of concordance. The rate of twins in the affected group is also influence by the proportion of families with more than one child in the population.