A new study published in JAMA Psychiatry found 81 percent of autism risk comes from genes kids inherit from their parents, according to Spectrum News. The study, with a sample size of more than 2 million children from five countries, analyzed multiple generations of families to come to its conclusions. Researchers also found about 18.1 percent of autism risk stems from “environmental factors” that aren’t shared among family members, including non-inherited mutations. Environmental factors that were shared among family members did not contribute to autism risk, the study concludes.

However, estimates of heritability varied throughout the five countries—with Finland showing that 51 percent of autism risk came from inherited genetics, and Western Australia data showing 54 percent. Researchers said the number could be explained by the small autistic sample in those countries, or there might be a difference in how autism is diagnosed.