[1] All twins born in Minnesota at that time were eligible to participate using birth registry data.
The focus of the MTFS is on behavioral phenotypes, such as academic outcomes, cognitive abilities, personality, and interests; family and social relationships; mental and physical health; physiological measurements.
[2] Its original goal was to establish a registry of all twins born in Minnesota from 1936 to 1955 to be used for psychological research.
It primarily conducts personality and interests tests with its 8,000+ twin pairs and family members via mail.
In 1979, Thomas J. Bouchard began to study twins who were separated at birth and reared in different families.