In what’s being called the largest study of its kind, researchers say that the likelihood of developing autism is overwhelmingly rooted in a person’s genetic makeup. Nearly 81 percent of autism risk is hereditary, according to findings published this week in the journal JAMA Psychiatry.

Researchers looked at data on over 2 million people born between 1998 and 2011 in five countries. Individuals were followed up to age 16, at which point more than 22,000 of the children had been diagnosed with autism. Through the national health registries, researchers were able to analyze records on the children’s parents, siblings and cousins to look for autism diagnoses in other family members.  Overall, however, the study found that the vast  majority of autism risk was associated with genetics. Environmental factors “contributed minimally” and maternal effects such as the mother’s weight played a “nonexistent or minimal” role.