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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.
Ronald Wilson presented the first clear and compelling evidence that the heritability of IQ increases with age. We propose to call the phenomenon ‘The Wilson Effect’ and we document the effect diagrammatically with key twin and adoption studies, including twins reared apart, that have been carried out at various ages and in a large number of different settings. The results show that the heritability of IQ reaches an asymptote at about 0.80 at 18–20 years of age and continuing at that level well into adulthood. In the aggregate, the studies also confirm that shared environmental influence decreases across age, approximating about 0.10 at 18–20 years of age and continuing at that level into adulthood. These conclusions apply to the Westernized industrial democracies in which most of the studies have been carried out.
Age-related brain changes may contribute to axial features in Parkinson's disease (PD).
To determine if ventricular volume and white matter high signal changes (WMC) are related to motor signs in PD and controls independent of age.
Patients were rated with the Unified Parkinson's Disease Rating Scale (subscore A: tremor, rigidity, bradykinesia, and facial expression; subscore B: speech and axial impairment). Steps and time taken to walk 9.144 meters were measured. Total ventricular volume (TVV) and intracranial volume (ICV) were measured on T1-weighted MRI using manual tracing software. WMC were rated on axial T2-weighted, dual-echo or FLAIR MR images using a visual scale.
TVV (cm3) (PD: 36.48 ± 15.93; controls: 32.16 ± 14.20, p = 0.21) and WMC did not differ between groups (PD: 3.7 ± 4.2; controls: 3.2 ± 3.1, p = 0.55). Age correlated positively with ICV-corrected TVV and WMC in PD (cTVV: r = 0.48, p = 0.003; WMC: r=0.42, p=0.01) and controls (cTVV: r = 0.31, p = 0.04; WMC: r=0.44, p=0.003). Subscore B (r = 0.42, p = 0.01) but not subscore A (r = 0.25, p = 0.14) correlated with cTVV in PD. Steps and walking time correlated with cTVV and WMC in PD; cadence correlated with cTVV and steps with WMC in controls. Age-adjustment eliminated correlations.
Subscore B, but not subscore A correlated positively with ventricular volume in PD, though this association was accounted for by age. Age-related brain change super-imposed on PD may contribute to axial features.
Twin studies have demonstrated that personality traits show moderate genetic influence. The conclusions drawn from twin studies rely on the assumptions that twins are representative of the population at large and that monozygotic and dizygotic twins are comparable in every way that might have bearing on the traits being studied. To evaluate these assumptions, we used Multidimensional Personality Questionnaire (MPQ) data from three samples drawn from the Minnesota Twin Registry (totaling 12,971 respondents) to examine the effect sizes associated with mean differences on the 11 MPQ scales and 3 higher-order MPQ factors for singletons versus twins and MZ twins versus DZ twins. The singletons in the samples were family members of the participating twins. We also used ratios of scale variances to examine the significance of variance differences. The only mean or variance difference replicated across all three samples was greater Social Closeness (about .1 standard deviation) for twins than for singletons. This difference was obtained for both males and females. It would appear that, with respect to personality, twins are not systematically different from other people. Our results also highlight the importance of replication in psychological research because each of our large samples showed differences not replicated in other samples.
This report presents findings for the Intrinsic (IR) and Extrinsic (ER) religiousness scales from the Minnesota Study of Twins Reared Apart. The scales were shown to be internally consistent, sufficiently distinct from the scales of the California Psychological Inventory and the Multidimensional Personality Questionnaire and unrelated to a number of measures of response style to justify treating them as distinct traits. The I scales also showed considerable evidence of construct validity in its correlations with religious fundamentalism and authoritarianism as assessed by the MMPI and Altemeyer's Right-Wing Authoritarianism scale. Data on IR and ER from 35 pairs of monozygotic twins reared apart (MZA) and 37 pairs of dizygotic twins reared apart (DZA) were fitted to a biometric model and demonstrated significant heritability (0.43 and 0.39), with a model containing genetic plus environmental factors fitting significantly better than a model containing only an environmental component. Twin similarity could not be explained by placement on a self-reported measure of family Moral Religious Emphasis as measured by the Family Environment Scale.
When asked whether he would discuss man in the Origins of the Species, Darwin replied, ‘I think I shall avoid the subject, as so surrounded with prejudices, though I fully admit it is the highest and most interesting problem for the naturalist’. Galton on the other hand replied to the same question, ‘I shall treat man and see what the theory of heredity of variations and the principles of natural selection mean when applied to man’ (Pearson, 1914–30, Vol. II, p. 86).
Research on genetic influence on intelligence has a long and contentious history (Brand, 1993; Fancher, 1985; Kamin, 1974). Both the idea of a general factor of cognitive ability, Spearman's g, and the idea that genetic factors might be an important source of variance in cognitive ability have been continuously debated since they were first systematically expounded by Galton (1869, 1876). Reviews of Galton's books published in the London Times at the time of their appearance could, if slight changes were made, be published today. The debate on the nature of mental abilities and the influence of heredity on such abilities (as well as most other psychological traits) initiated by Galton continues unabated.
The current status of g
There should be no doubt that the issues of the measurability of IQ and its usefulness are still controversial issues. Consider the following recommendation regarding the measurement of abilities and other psychological traits:
Make explicit to everyone (pupils, parents, public and professionals of all kinds) that a person's abilities, activities, and attitudes cannot be measured. The public, especially, misperceive that hard data exist, and that test scores constitute these data. The public does not realize how quickly the point is reached where we do not know how to discriminate validly among people, but where data mislead us to think we do. This is what is meant by the myth of measurability
We describe six pairs of monozygotic twins, in which at least one member of five pairs were homosexual, and one of the remaining pair was bisexual, from a series of 55 pairs, reared apart from infancy; all the female pairs were discordant for homosexual behaviour. This and other evidence suggest that female homosexuality may be an acquired trait. One male pair was concordant for homosexuality, while the other was not clearly concordant or discordant; this suggests that male homosexuality may be associated with a complex interaction, in which genes play some part.