In a disease as complex as autism, both nature and nurture are known to play a role, but the latest evidence suggests that environmental factors may have a slight edge.
In an analysis involving 192 pairs of identical and fraternal twins, scientists found that environment may account for as much as 55% of the risk for autism. The chance of autism occurring in both twins in fraternal pairs was higher than expected; since fraternal twins are only as genetically similar as any two siblings, they should develop autism at the same rate as other brother or sister pairs. Their increased risk suggests that shared environmental factors--likely in the womb--are at play. The study also found that identical twins had an even higher prevalence of the disorder, confirming that genetics are important too. The authors attributed nearly 40% of the risk of autism to genes.
The researchers speculate that key environmental factors may include multiple births, older mothers and fathers and exposure to medications or infection during pregnancy. Another study, released concurrently with the twins research, linked mothers' use of antidepressants like Prozac and Zoloft during pregnancy to an increased risk of autism among their offspring.