Across studies, the ratio of males to females affected with autism is approximately 4:1. Basic research on sex differences in behavior has shown that differences in circulating levels of gonadal hormones during fetal and infant development are responsible for most sexual dimorphism in adults. For normal male development to occur, sex hormones like testosterone act through estrogen receptors, which in turn also activate other genes and proteins. The activation of certain genes through the estrogen receptor, then, may partially explain the sex difference. The body produces natural testosterone and estrogen but environmental chemicals may mimic these compounds and produce deleterious effects during development. This study will use estrogen receptor knockout mice to determine sex-differences on many autism-like behaviors in mice as well as to identify genes which are affected by bisphenol A (BPA) in order to identify the mechanism of action of this environmental chemical and whether it may be linked to autism.