The Massachusetts Twin Registry is a group of adult twins who are permanent residents of Massachusetts. Enrollment in the Registry is voluntary. In the future, we will invite members of the Registry to participate in behavioral and biomedical research projects. Twins offer a unique opportunity to examine the influences of genetic and environmental factors on complex human characteristics. If you enroll, you are not obligated to participate in any study. We will contact you about any study that is proposed and invite you to conisider participation.