A recent study conducted by Dr. Joachim Hallmayer and colleagues has received significant media attention over their findings that environmental factors may play a larger role than genetic factors in the development of autism spectrum disorders (ASDs). When evaluating the significance of genetic versus environmental influences, it is useful to examine twins. Twins are a valuable population to study because they share similar genes. Identical twins share close to 100 percent of their genetic makeup while fraternal twins share roughly 50 percent. Furthermore, twins typically share the same environment during their early development (e.g., in utero, infancy, early childhood, etc.). For these reasons, Dr. Hallmayer and colleagues set out to perform a population-based twin study on genetic versus environmental involvement in the development of ASD.
Potential participants were identified through the California Department of Developmental Services (DDS). Both identical and fraternal twin pairs were eligible to participate if they were born between 1987 and 2004, and at least one twin was reported to have an ASD diagnosis. A total of 1,156 twin pairs were identified through the DDS, however, due to limited participation from families, only 202 twin pairs were included in the study. All participants received comprehensive diagnostic evaluations to confirm diagnoses of autistic disorder or broader ASD.
ASD concordance rates, meaning the proportion of twin pairs in which both twins had an ASD diagnosis, were measured with respect to diagnosis, gender, and twin type (you can view these rates here). Using the concordance data, Dr. Hallmayer and colleagues were able to estimate the involvement of genetics versus shared environment in the development of autistic disorder and ASD. The results indicated that genetics play roughly a 37 percent role in the development of autistic disorder, while shared environment plays about a 55 percent role. Similar findings were revealed for ASD indicating that genetics play roughly a 38 percent role while shared environment plays roughly a 58 percent role.
Dr. Hallmayer and colleagues’ findings offer preliminary evidence that environmental factors may play a greater role in the development of ASD than genetic factors. Since ASD has previously been regarded as a genetic disorder, the majority of research has been focused on identifying possible genetic causes. The results of this study suggest that, in the future, more research should focus on identifying possible environmental causes in addition to genetic causes.
If you are interested in reading the full article, you can access it here.
Hallmayer, J., Cleveland, S., Torres, A., Phillips, J., Cohen, B., Torigoe, T.,… Risch, N. (in press). Genetic heritability and shared environmental factors among twin pairs with autism. Archives of General Psychiatry. doi: 10.1001/archgenpsychiatry.2011.76