My most important scientific contribution was the research program on human individual differences, namely the Minnesota Study of Twins Reared Apart (MISTRA). A detailed report of the study and its findings up to 2012 can be found in work by Nancy Segal, but we continue to publish findings from this data set. MISTRA was a comprehensive psychological and medical assessment of monozygotic and dizygotic twins reared apart. It also included spouses of twins and additional participants such as partners and various relatives who were invited when it facilitated the participation of the twins. Launched in 1979, the program lasted for twenty years. MISTRA was funded almost entirely by private grants (with one exception: a small NSF grant, which the agency refused to renew). Federal funds were provided only for medical research, and only after the program was well established. Segal provides a history of the funding stream. We never had funds to carry us over for more than a year or so, and at times we were in considerable debt. Numerous grant applications were submitted to federal agencies, but all were rejected as many of the reviewers were hostile to the research program. The program was run on a shoestring and all my colleagues (medical and psychological) contributed their time and energy, and that of their laboratories, gratis.

There were two major findings from the study: (1) virtually all medical and psychological traits are to a notable degree heritable; and (2) shared environment was a much less important contribution to similarity between relatives in psychological traits than previously believed. These results have now been fully confirmed by a meta-analysis of fifty years of behavioral genetic research. MISTRA did lead to large-scale funding of ordinary twin research at the University of Minnesota. That psychology department now houses a leading research center for quantitative and molecular behavior-genetic research.

Our interpretation of the results of MISTRA was very straightforward. We expected that with regard to psychological traits, monozygotic twins reared apart were similar because their effective environments were similar. This was because their environments were self-selected and that selection was guided by their genotype. This idea has been operationalized as Experience Producing Drive Theory (EPD theory). According to EPD theory, genes influence the mind indirectly by influencing the choices and, consequently, the effective experiences that individuals undergo.