A natural experiment in human behavior
At the University of Minnesota, a long-term research project capitalized on a rare natural experiment: identical twins separated near birth and raised in different environments. The Minnesota Study of Twins Reared Apart (MISTRA), which began in 1979, sought to study the influences of genetics and environment on human psychology. Because identical (monozygotic) twins share 100% of their genes, any differences between them must be due to their environment. Any similarities, despite different upbringings, show a genetic basis.
Psychologist Thomas J. Bouchard, Jr., launched the study after reading about a pair of reunited twins. Over two decades, he and his team studied 137 pairs of reared-apart twins, including 81 identical and 56 fraternal pairs. Each pair of twins came to the university for about a week of intensive assessment. This involved approximately 50 hours of medical and psychological testing. Researchers administered multiple IQ tests, personality questionnaires, and detailed interviews about their life histories, interests, and social attitudes.
The central finding was that identical twins reared apart were remarkably similar to identical twins reared together. This suggests that growing up in the same home does not significantly increase the similarity between identical twins' personalities or interests. The study produced a famous and debated estimate that about 70% of the variance in IQ is associated with genetic factors.
The Jim Twins and heritability
The study's most well-known participants were Jim Lewis and Jim Springer, identical twins separated at four weeks old and reunited at age 39. Their story brought widespread attention to the research. Both were adopted by families in Ohio that coincidentally named them James. The parallels in their lives were extensive. Both men had a childhood dog named Toy. As adults, both worked part-time in law enforcement, were heavy smokers of Salem cigarettes, and drove a light-blue Chevrolet. They both married women named Linda, divorced, and then remarried women named Betty. One named his son James Alan, the other James Allan. When they participated in the study, their medical histories, brain-wave tests, and personality tests were nearly identical.
These anecdotal cases supported the quantitative findings of the broader study. The research suggested strong heritability for many personality traits. Traits like leadership, stress resistance, and fearfulness were reported to have a heritability ratio of over 0.50, meaning genetics accounted for a substantial portion of the variation among people. The research showed that genetics influence social attitudes and occupational interests as well. The work from the Minnesota study informs the fields of behavioral genetics and psychology, providing evidence that people's genetic makeup helps shape their own unique environments.