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). This project will investigate a major biological mechanism that can retain a long-lasting impression of the environment and which regulates gene expression: DNA methylation. Dr. Plomin's lab will study DNA methylation in a twin cohort called the "twins early development study" using a 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 "non-shared" 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 autism spectrum disorder (ASD), which have special value because risky environments could be prevented or reversed more easily than risky genotypes.