Archives of General Psychiatry
Recent research suggests that environmental factors may play a much greater role in autism risk than previously suspected and could even be more influential than genetic factors. These findings stem from a study of autism in twins, the largest of its kind, designed to model the genetic and environmental factors that contribute to the development of autism. Because identical twins share 100 percent of their DNA, researchers can assess the degree to which a disorder is genetic by studying the number of cases where both twins are affected (called concordance). Fraternal twins share 50 percent of their DNA, similar to siblings, so by comparing concordance rates among fraternal twins and siblings, researchers can study the influence of environmental factors, particularly those in the womb. In the study, researchers looked at concordance among 192 pairs of identical and fraternal twins and found that 77 percent of male identical twins and 50 percent of female twins were both affected by autism. Previous studies with smaller numbers of twins suggested a much greater genetic contribution, as high as 90 percent. The fraternal twins in the study had a 35 percent concordance rate – much lower than rates among identical twins but higher than rates among siblings, estimated to range from 3 to 19 percent. Using mathematical modeling, the researchers propose that environmental factors accounted for 55 percent of autism risk, while genetic heritability contributed less than 40 percent. The difference in rates among fraternal twins and siblings, who share similar amounts of DNA, suggests that environmental factors in the womb may be an important area of future study.
--IACC 2011 Summary of Advances in ASD Research