The Michigan Twins Project (MTP) is a mail-in twin registry of all twins between the ages of 3 and 25 within lower Michigan.  Twin families are recruited through birth records in collaboration with the Michigan Department of Community Health.  Families complete and return a very brief questionnaire that assesses family composition and the health status of both parents and twins. Families are compensated $15 for their participation and are given the opportunity to participate in future twin studies offering additional financial compensation. Data from the MTP will offer critical new insights into the development of major health conditions (e.g., heart disease, asthma, depression, etc.) that carry significant individual and public health consequences. This study is directed by Drs. Alexandra Burt and Kelly Klump.