In this posting I am announcing, and making available, my new 2018 critique of the “Minnesota Study of Twins Reared Apart” (MISTRA). This famous and very influential “separated twin study” was performed by Thomas J. Bouchard, Jr. and colleagues between 1979 and 2000. In the new article that I have prepared for online publication, I describe in detail what I consider to be the major invalidating problems and biases found in this study. The article is entitled “Twenty-Two Invalidating Aspects of the Minnesota Study of Twins Reared Apart (MISTRA),” and comes in two forms: the full version (about 30,000 words), and the abridged version (about 6,000 words). Although the abridged version covers most of the main points, readers seeking a more in-depth analysis should consult the full version. In the context of the larger “nature-nurture” question, here I will briefly explain why it is important to look very closely at this study, and at its authors’ claims that the study found major genetic influences on human behavioral differences.
Headlines and titles send strong messages to the public about the reported findings from twin research: “Twins Separated at Birth Reveal Staggering Influence of Genetics,” “How Genes Shape Personality,” “Born That Way,” “Are We Hardwired?,” “Mean Genes,” “Twin Brothers Separated at Birth Reveal Striking Genetic Similarities,” “Life of Crime Is in the Genes, Study Says,” “Study Raises the Estimate of Inherited Intelligence,” “The Gene Bomb,” and so on. Pulitzer Prize winning author Lawrence Wright, in his 1997 book about twin studies and behavioral genetic research Twins: And What They Tell Us About Who We Are, wrote that “the science of behavioral genetics, largely through twin studies, has made a persuasive case that much of our identity is stamped on us from conception; to the extent that our lives seem to be pre-chosen—all we have to do is live out the script that is written in our genes.”1 The potential impact of these messages on social relations and political policy decisions is enormous. The 2018 movie Three Identical Strangers has brought the study of reared-apart twins (in this case triplets) to a higher level of public awareness.
I have dedicated much of the past two decades to critically examining behavioral genetic and psychiatric genetic theories, which hold that genetic factors play an important role in causing human behavioral differences. Given the lack of gene discoveries, these theories continue to be based on the results of family, twin, and adoption studies, and on other behavioral comparisons among genetically related and non-related family members. A sizable portion of my 2015 book The Trouble with Twin Studies: A Reassessment of Twin Research in the Social and Behavioral Sciences looked into the invalidating problems of so-called “separated” or “twins reared apart” (TRA) studies, which are widely seen as providing the definitive method of separating the potential influences of genes and environment (nature and nurture) on human behavior. Among the many problem areas in these studies, I attempted to show that like other behavioral genetic methods, the MISTRA and the earlier TRA studies were unable to disentangle these potential influences, and that genetic interpretations of the results of these studies are therefore invalid. I summarized the main problems in TRA research in a 2014 online article, which included the important point that most pairs found in TRA studies were only partially reared apart, and in another article I described the main problems with anecdotal stories of individual pairs of reunited twins reported in the media (such as the “Jim Twins,” “Oskar Stöhr and Jack Yufe,” and the “Fireman Twins”). I listed many important MISTRA problem areas and biases in a 2018 tribute to the pioneering critic of reared-apart twin studies, the psychologist Leon J. Kamin (1927-2017). Estimating the “heritability of IQ” on the basis of IQ test score correlations has been the main area of reared-apart twin study focus, with “personality” and other behavioral characteristics playing secondary roles.
The MISTRA has been referred to as “arguably, the most famous social science project in the last quarter of the twentieth century,” and its findings are reported in most psychology textbooks.2 In a 2012 book about the study, Born Together—Reared Apart: The Landmark Minnesota Twin Study, MISTRA researcher Nancy Segal wrote that the study “forever changed the way people think about the roots of human behavior” in the genetic direction. 3 The public’s knowledge of the MISTRA is based largely on textbook descriptions and the works of authoritative experts, television reports, numerous popular books, and countless print and online articles that have appeared since 1979. A major theme of these reports and publications has been that the researchers discovered, often to their amazement, that genetic factors play a major role in most aspects of human behavior and abilities. Although for the most part the MISTRA researchers did not study psychiatric disorders, their claims have helped strengthen genetic theories in psychiatry.
The researchers studied “MZA” twin pairs (monozygotic or identical twins reared apart, who share a 100% genetic resemblance), and “DZA” twin pairs (dizygotic or fraternal twins reared apart, who share an average 50% genetic resemblance). The final 2000 MISTRA sample consisted of 81 MZA and 56 DZA pairs. Bouchard decided at the beginning of the study in 1979 to use DZA pairs as the MISTRA control group, which he planned to compare to the MZA experimental group.4 He and his colleagues reaffirmed this plan in a 1986 publication.5 “MZA and DZA twin pairs,” Bouchard et al. wrote four years later in the opening paragraph of their widely cited 1990 article published in the prestigious journal Science, “are a fascinating experiment of nature. They also provide the simplest and most powerful method for disentangling the influence of environmental and genetic factors on human characteristics.”6 However, although their famous Science article was the main MISTRA original-data IQ publication, and although the 1990 MISTRA sample consisted of 56 MZA and 30 DZA pairs, this “fascinating” and “most powerful” “experiment of nature” did not appear in this article, and no DZA control group results of any kind were reported.
Amazingly, the researchers’ 1990 Science article conclusion that “general intelligence or IQ is strongly affected by genetic factors” (about 70% heritability) was based on their decision (a) to assume that the numerous non-familial behavior-shaping environmental, adoptee-placement, cultural, same-sex, and cohort influences shared by MZA pairs do not exist, or that they should be counted as genetic influences7; (b) to suppress and bypass their DZA control group IQ score correlations, and to base their conclusions about genetics on their MZA correlations alone, which they assumed “directly estimate heritability”; (c) to make other questionable assumptions about genetics and twins, some of which they admitted “are likely not to hold”8; and (d) to temporarily abandon their “model-fitting” statistical procedure (which used MZA, DZA, and other data), even though they used this procedure in most non-IQ MISTRA publications starting in 1988, and continuing into the 21st century.
Bouchard and colleagues never published their full-sample DZA IQ correlations, even though they published full-sample DZA correlations for personality, “special mental abilities,” and most other MISTRA-studied psychological characteristics. To this day, they have prohibited independent researchers from inspecting the closely guarded MISTRA raw data. I show in my new article that the likely reason that they did not publish, share, or make available their full-sample DZA control group IQ data was that—based on the near full-sample DZA IQ correlations that were published in 2007 and 2012—the results would have revealed their failure to find a significantly higher MZA group versus DZA group mean correlation for any of the three IQ measures they used. An “important first step” requirement in the process of determining whether genetic factors influence IQ scores is finding that the MZA correlation is higher than the corresponding DZA correlation at a statistically significant level. The researchers bypassed this required step in their 1990 Science study, most likely because the hidden results failed to confirm their pre-existing belief that IQ was (strongly) influenced by genetic factors.9 Their strong genetic biases, it seems, led them to omit, bypass, and suppress their DZA IQ correlations in order to obtain the desired results.10
Despite the obvious false assumptions upon which they are based, studies of twins reared apart, and studies of twins reared together (the “twin method”), have been cited by various commentators as supplying “scientific evidence” in defense of economic inequality and the social status quo, in support of racism and other forms of oppression, as an explanation for socially disapproved behavior (such as criminality), and in support of cutting back or eliminating needed social programs. Historically, the position that social problems and poverty are mainly the result of bad heredity has been promoted by the economically and politically powerful in support of their interests. The MISTRA findings have been cited in support of genetic (biological) determinism, which predates twin research and refers to the belief that genetic factors play a predominant role in causing differences in human behavior and mental abilities, and that environmental factors play only a minor role, if any. The MISTRA was largely financed (about 60% or $1.42 million, roughly $3 million in 2018 U.S. dollars) by the Pioneer Fund, an organization created in the late 1930s to support eugenics, anti-Semitism and racial segregation, and racial differences research.11 The study is sometimes cited by other white-nationalist groups in support of their own appalling agendas, whose leaders predictably make completely false statements about the twins’ degree of separation and other aspects of the study.12
The huge impact of the MISTRA, in addition to the harmful and regressive social and political policy implications that flow from it, necessitates a detailed analysis of the “science” behind the study’s major claims and conclusions. For those interested in downloading and reading “Twenty-Two Invalidating Aspects of the Minnesota Study of Twins Reared Apart (MISTRA),” please select the appropriate link below:
The full-version summary of “Twenty-Two Invalidating Aspects of the Minnesota Study of Twins Reared Apart (MISTRA)” reads as follows:
“The 1979-2000 ‘Minnesota Study of Twins Reared Apart’ (MISTRA) by Thomas J. Bouchard, Jr. and colleagues is often cited in support of the claim that genetic factors play a major role in causing differences in human behavior such as IQ and personality. Because the study used reared-apart monozygotic (identical) twin pairs (‘MZA’ pairs), who are genetically identical and supposedly experienced no environmental influences in common, it is widely seen as having provided definitive evidence in favor of the nature side of the ‘nature-nurture’ debate. The present analysis closely examines how the MISTRA researchers arrived at their conclusions, and shows that these conclusions depended on the acceptance of many questionable or false assumptions, some of which Bouchard recognized ‘are likely not to hold.’ The researchers’ strong biases in favor of genetic explanations had a major impact on the methods and comparisons they used, and on the conclusions they reached. Remarkably, they omitted their reared-apart dizygotic (fraternal) twin pair (‘DZA’ pair) correlations from their IQ study, even though they previously had designated DZA pairs as the MISTRA control group. Based on the near full-sample correlations that have been published, the MISTRA MZA and DZA group IQ correlations did not differ at a statistically significant level, which the study required as an important step in the process of determining whether genetic factors influence IQ scores. To this day, the MISTRA full-sample DZA group IQ correlations have not been published. At the same time, the researchers have prohibited independent analysts from inspecting and reviewing the MISTRA raw data. Twenty-two major problem areas in the study are discussed, including that the methods used to obtain the MZA sample led to the inclusion of behaviorally similar pairs, and that most pairs used in studies of this type were only partially reared apart. In addition, even perfectly separated MZA pairs experience many non-familial behavior-shaping environmental influences in common. The researchers, however, either denied that such environmental influences exist, or counted them as genetic influences. This created a genetic ‘heads I win, tails you lose’ type of study that guaranteed that genetic interpretations of above-zero MZA group psychological test-score correlations would prevail. Highly publicized anecdotal stories of individual reunited twin pairs provide no evidence in support of genetic theories of human behavior. These selectively reported stories have been used mainly to sell such theories to the general public. Due to the many environmental confounds and other types of biases found in the study, most of which are also found in the ‘twins reared apart’ studies that came before it, the MISTRA was unable to disentangle the potential influences of genes and environments. Therefore, its findings in favor of genetic influences on human behavioral differences, major or otherwise, must be rejected.”
- Wright, L., (1997), Twins: And What They Tell Us About Who We Are, New York: John Wiley & Sons, pp. 143-144. ↩
- Tucker, W. H., (2013), (Review of the Book Born Together—Reared Apart: The Landmark Minnesota Twin Study, by N. L. Segal), Journal of the History of the Behavioral Sciences, 49, 337–341, p. 337. ↩
- Segal, N. L., (2012), Born Together—Reared Apart: The Landmark Minnesota Twin Study, Cambridge, MA: Harvard University Press, p. 400. ↩
- Segal, 2012, p. 12. ↩
- Bouchard et al., (1986), “Development in Twins Reared Apart: A Test of the Chronogenetic Hypothesis,” in A. Demirjian (Ed.), Human Growth: A Multidisciplinary Review (pp. 299-310), London: Taylor & Francis, p 300. ↩
- Bouchard, T. J., Jr., Lykken, D. T., McGue, M., Segal, N., & Tellegen, A., (1990), Sources of Human Psychological Differences: The Minnesota Study of Twins Reared Apart, Science, 250, 223-228, p. 223. ↩
- The MISTRA researchers recognized that “for most psychological, physiological, and medical variables there are substantial age and sex effects.” See McGue, M., & Bouchard, T. J., Jr., (1984), Adjustment of Twin Data for the Effects of Age and Sex, Behavior Genetics, 14, 325-343, p. 325. They devised a questionable and complicated statistical procedure, which they claimed corrected their psychological test-score correlations for these effects. In any case, age- and sex-effects on test scores are a small component of the many potential reared-apart twin study environmental confounds that I listed and described in “Twenty-Two Invalidating Aspects of the Minnesota Study of Twins Reared Apart (MISTRA).” ↩
- McGue, M., & Bouchard, T. J., Jr., (1989), “Genetic and Environmental Determinants of Information Processing and Special Mental Abilities: A Twin Analysis,” in R. Sternberg (Ed.), Advances in the Psychology of Human Intelligence (Vol. 5, pp. 7-45), Hillsdale, NJ: Erlbaum, p. 23. ↩
- According to MISTRA researcher Nancy Segal in Born Together—Reared Apart (Segal, 2012, p. 62), “The simple comparison of the MZ (or MZA) and DZ (or DZA) intraclass correlations is an important first step in behavioral-genetic analysis because this demonstrates whether or not there is genetic influence on the trait” (italics added). And on page 336 she wrote, “Genetic effects are shown if the correlation for MZ or MZA twins exceeds the correlation for DZ or DZA twins” (italics added). The final MISTRA full sample consisted of 81 MZA and 56 DZA pairs. Based on a near full sample of 74 MZA and 52 DZA pairs, the MISTRA WAIS (Wechsler Adult Intelligence Scale) full-scale mean IQ correlations were MZA = .62, versus DZA = .50. The Raven’s Progressive Matrices mean IQ correlations were MZA =.55, versus DZA = .42. The Raven test was part of the larger MISTRA “Raven’s Progressive Matrices /Mill-Hill Vocabulary Scale composite.” The WAIS IQ correlations are taken from Segal, 2012, p. 286. The Raven IQ correlations are found in Johnson et al., (2007), Genetic and Environmental Influences on the Verbal-Perceptual-Image Rotation (VPR) Model of the Structure of Mental Abilities in the Minnesota Study of Twins Reared Apart, Intelligence, 35, 542-562, p. 552, Table 3, Test #21. Tests of statistical significance, using the VassarStats website, show that both the MISTRA WAIS and the Raven MZA versus DZA correlation comparisons fail to differ below the conventional .05 level of statistical significance (.17 and .18, respectively), meaning that the differences are assumed to have occurred by chance, and that there is no difference between the correlations. I am unaware of any publication reporting the MISTRA “First Principal Component of Special Mental Abilities” DZA group IQ correlation, which was the third MISTRA IQ measure. ↩
- For more on these points, see Joseph, J., (2015), The Trouble with Twin Studies: A Reassessment of Twin Research in the Social and Behavioral Sciences, New York: Routledge, Chapters 5 and 6. ↩
- Segal, 2012, Appendix A. ↩
- For example, see American Renaissance leader Jared Taylor’s completely false claim that most MISTRA twin pairs were separated at birth, and that they did not know of each other’s existence until they were studied. This claim is seen in the video “Heritability: Where Nature meets Nurture.” ↩
Mad in America hosts blogs by a diverse group of writers. These posts are designed to serve as a public forum for a discussion—broadly speaking—of psychiatry and its treatments. The opinions expressed are the writers’ own.