There has been much research into the genetic causes of autism, driven in part by the concordance rate in twins. However, the fact that some pairs of identical twins differ in autistic symptoms makes clear that there must be an important non-genetic (i.e., environmental) component as well that can differ even within a family (called non-shared environment). There is much speculation on what these non-shared environmental factors might be, and need a way to identify them and the mechanisms by which they contribute to autism. This project will move this field forward by investigating a major biological mechanism that can retain a long-lasting impression of the environment and which regulates gene expression: DNA methylation, a form of epigenetics similar to the work in Dr. Jiang's lab. Dr. Plomin's lab will study DNA methylation in a twin cohort called the “twins early development study” using new technology to study the whole genome. First, this lab will examine differences in DNA methylation across identical twins discordant for autism. These differences can be caused by a “nonshared” environment within a family. In addition, they will study whether these differences are seen between those affected with autism and unrelated cases who are not diagnosed. Finally, the Plomin lab will examine epigenetic markers that differ in individuals who show a social vs. non-social phenotype. The proposed biological index of non-shared environmental influence will be a vital starting point for mapping out the environmental causal pathways that lead to ASD, which have special value because risky environments could be prevented or reversed more easily than risky genotypes. What this means for people with autism: The proposed biological index of non-shared environmental influence will be a vital starting point for mapping out the environmental causal pathways that lead to ASD, which have special value because risky environments could be prevented or reversed more easily than risky genotypes. Because identical twins show identical DNA, areas of methylation on the DNA are an increasingly researched area to determine gene x environment interactions in autism and other disorders, and how one twin may manifest symptoms differently than the other.