Shared Genetic Variation for 65% of People with Autism

The Los Angeles Times is reporting on a new study published in the journal Nature:

The first of two Nature studies released Tuesday found that 65% of autistic participants shared a variation between cadherin 10 and cadherin 9, a region of the genome that controls cell-adhesion molecules in the brain. Those molecules help brain cells connect, and autism researchers have long suspected that trouble there may be linked to the disorder.

However, as Science News reports on the study, the variation is also shared by more than half of of people without autism:

One of the new studies encompasses more than 14,000 people and has uncovered a variant found in about 65 percent of people with autism. More than half of people without autism or related disorders also have the variant, indicating that other genes and environmental factors are necessary to trigger autism. Still, the variant may account for up to 15 percent of autism cases, a team led by researchers at the Children’s Hospital of Philadelphia and the University of Pennsylvania School of Medicine, also in Philadelphia, reports online April 28 in Nature.

Some people have suspected that the mercury was used as a preservative in some children's vaccines was the environmental factor triggering the increase in the number of autistic children. This theory has been discredited, but no competing theory has been provided. What explains the high incidence of autism among Somali children in Minnesota?