The Minnesota Twin Family Study (or MTFS) is a longitudinal study of twins conducted by researchers at the University of Minnesota. It seeks to identify the genetic and environmental influences on the development of psychological traits.
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Principal investigators are David T. Lykken, Matt McGue, William Iacono, and Kevin Haroian. It involves several independent but related projects:
Registry
The Minnesota Twin Registry was established in 1983. Its original goal was to establish a registry of all twins born in Minnesota from 1936 to 1955 to be used for psychological research. Recently, it has added twins born between 1961 and 1964. It primarily conducts personality and interests tests with its 8,000+ twin pairs and family members via mail. From this project, it was able to confirm that twins and their families are representative of the population and that a poll of their opinions would be more accurate than polls in the newspaper.
Study
MTFS was established in June 1989 using same-gendered twin pairs age 11 or 17. All twins born in MN at that time were invited to participate using birth registry data. 500 additional 11-year-old twin-pairs were added in 2000. Twin studies are valuable to researchers because identical twins share 100% of their genes and fraternal twins share, on average, 50% of their genes. Both identical and fraternal twins share certain aspects of their environment (e.g. religious practices in the home). This allows researchers to estimate the heritability of certain traits. Participants are asked about academic ability, personality, and interests; family and social relationships; mental and physical health; physiological measurements. Of interest to researchers are prevalence of psychopathology, substance abuse, divorce, leadership, and other traits and behaviors related to mental and physical health, relationships, and religiosity.
Twins reared apart
In 1979, Thomas J. Bouchard began to study twins who were separated at birth and reared in different families. He found that an identical twin reared away from his or her co-twin seems to have about an equal chance of being similar to the co-twin in terms of personality, interests, and attitudes as one who has been reared with his or her co-twin. This leads to the conclusion that the similarities between twins are due to genes, not environment, since the differences between twins reared apart must be due totally to the environment. Psychologist now refer to studies such as this as an Adoption Strategy.