The Minnesota Study of Twins Reared Apart (MISTRA) stands as one of the most famous and influential research projects in the field of behavioral genetics. Launched in the late 1970s, the study’s purpose was to rigorously quantify the relative roles of genetics and the environment in shaping a vast array of human psychological and physiological characteristics. By examining individuals who shared 100% of their DNA but were raised in separate homes, the researchers sought to disentangle the age-old influences of inherited traits and life experiences.
How the Minnesota Twin Study was Conducted
The study focused on recruiting both monozygotic (identical) and dizygotic (fraternal) twin pairs, particularly those separated early in life and reared in distinct environments. Over the course of the project, more than 100 sets of reared-apart twins and triplets participated, providing a unique dataset. This approach allowed researchers to compare the similarities between individuals who shared all their genes but no common environment with those who shared genes and a common environment.
Each participant underwent an intensive, week-long assessment process totaling over 50 hours of detailed examination. This comprehensive battery of tests included medical and physiological measurements, such as brain wave patterns and heart rate, along with extensive psychological evaluations. Psychological testing included multiple measures of intelligence, personality, values, and interests.
The Dominance of Genetic Influence
The primary finding of the MISTRA research was that genetic factors account for a substantial portion of the variance in human traits, even when individuals are raised in separate circumstances. For many psychological characteristics, the genetic influence was estimated to be 50% or more. This conclusion was based on the striking observation that the similarities between identical twins reared apart were often comparable to the similarities found between identical twins reared together.
This finding introduced the concept of heritability, which, in simple terms, estimates the proportion of variation among individuals in a population that is attributable to genetic differences. When identical twins share the same genetic code but grow up in different worlds, any strong correlation in their traits can be attributed primarily to their shared genes. The data showed that the shared early environment of twins reared together added surprisingly little to their overall psychological similarity.
Specific Traits Exhibiting High Heritability
The study provided concrete heritability estimates for numerous traits, with intelligence quotient (IQ) showing one of the highest correlations. Estimates indicated that approximately 70% of the variance in IQ scores within the population could be associated with genetic factors.
Personality characteristics also showed a strong genetic correlation, including major factors like extroversion and neuroticism. Genetic influence extended to more specific domains, including vocational interests, such as selecting similar career paths, and specific leisure-time interests and social attitudes. Physiological measures like heart rate and brain wave patterns were also found to be highly similar between identical twins reared apart.
The Study’s Impact on Nature Versus Nurture
The results of the Minnesota Study of Twins Reared Apart fundamentally shifted the scientific perspective on human development. Before the MISTRA, some theories emphasized environmental determinism, suggesting that upbringing and environment were the sole forces shaping a person’s character and abilities. The twin study provided powerful empirical evidence that genetic predisposition is a strong baseline for many psychological characteristics.
The research did not suggest that environment has no role, but rather that its influence is more complex and less about the shared rearing environment of a home. Instead, the study’s findings support a view where genetic influences shape the way an individual interacts with and selects their environment, a concept known as gene-environment correlation. Ultimately, MISTRA moved the scientific community toward a more nuanced understanding, where nearly every human trait is seen as an intricate product of both genetic and environmental forces working in concert.