The Michigan State University Twin Registry (MSUTR) aims to examine developmental changes in the genetic, environmental, and neurobiological influences on internalizing and externalizing phenotypes. One particular interest is eating disorders, which includes anorexia nervosa, bulimia nervosa, and disordered eating attitudes and behaviors. The MSUTR focuses on two key developmental periods for disordered eating risk: puberty and the prenatal period. We have demonstrated that genetic influences on disordered eating increase during adolescence, and that puberty, rather than age, appears to be essential in these etiologic shifts. Specifically, genetic influences account for nominal amounts of variance in disordered eating at the earliest stages of puberty, but these effects increase substantially in importance over the pubertal period. Our follow-up studies for this line of research currently investigate ovarian hormones as a mechanism for these changes in genetic effects. With regard to the prenatal period, our findings suggest that the in utero environment likely plays a role in individual and sex differences in risk for eating pathology. Using an opposite-sex twin design, we found that prenatal exposure to testosterone may masculinize the female brain, and subsequently levels of disordered eating. In sum, data from the MSUTR has been vital to our understanding of key developmental periods for disordered eating and the potential role of gonadal hormones in organizing and activating risk.