Reared-Apart Twin Study Mythology: The Latest Contribution (Part One)

Jay Joseph, Psy.D.

The “Minnesota Study of Twins Reared Apart” (MISTRA) of Thomas J. Bouchard, Jr. and colleagues was conducted between 1979 and 2000, with media reports and academic papers based on it continuing to appear to the present day. In my recent book The Trouble with Twin Studies and elsewhere, I showed that the MISTRA and other “TRA” (twins reared apart) studies (also known as “separated twin studies”) are enormously flawed on several critical dimensions, and that accompanying media stories of individual reared-apart pairs provide no evidence that genetic factors influence behavioral differences in the population. Reared-apart MZ (identical) pairs, who are said to share a 100% genetic similarity, are also known as “MZA” pairs, or “monozygotic twins reared apart.”

The public’s knowledge of the MISTRA is based largely on television reports, numerous popular books, and countless articles appearing since 1979. A major theme of these reports and publications has been that the MISTRA researchers/scientists discovered, often to their astonishment, that genetic factors play a predominant role in most aspects of human behavior and abilities (including IQ and personality). The author of a 1997 popular book on twin research 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.”1 Another author suggested that the MISTRA findings showed that, behaviorally speaking, people are largely “born that way.”2 Although most commentators do not go that far, these publications, whose authors range from bloggers, to journalists, to textbook authors, to authoritative scientific commentators, have played a very important role in promoting this massively flawed and genetically biased study.

The latest endorsement of MISTRA claims comes from cancer physician and researcher Siddhartha Mukherjee, who published an article in the May 2, 2016 edition of The New Yorker  entitled “Same but Different: How Epigenetics can Blur the Line between Nature and Nurture.”3 This article dealt mainly with epigenetics, and created some controversy around that topic. Mukherjee is the author of Cancer: The Emperor of All Maladies, which won the 2011 Pulitzer Prize for General Non-Fiction.4

Here I focus on Mukherjee’s favorable description of the MISTRA in this New Yorker article. In Part 2, I will examine his discussion of the study in his 2016 book, The Gene: An Intimate History. Mukherjee is an influential author due to his medical credentials, accessible writing style, promotion by the mainstream media, and award-winning book. Yet he appears only nominally familiar with the MISTRA and its supposed findings.

Mukherjee’s description of the MISTRA is wrong on several accounts, and helps perpetuate myths about behavioral genetic and psychiatric genetic research in general. In early 2016, another case of misreporting occurred in the Wall Street Journal in relation to schizophrenia genetic research, where a former President of the American Psychiatric Association wrote an op-ed piece where he cited research that never occurred in support of genetic theories of schizophrenia.

In his New Yorker article, Mukherjee wrote the following about the Minnesota (MISTRA) twin study:

“Why are identical twins alike? In the late nineteen-seventies, a team of scientists in Minnesota set out to determine how much these similarities arose from genes, rather than environments—from ‘nature,’ rather than ‘nurture.’ Scouring thousands of adoption records and news clips, the researchers gleaned a rare cohort of fifty-six identical twins who had been separated at birth. Reared in different families and different cities, often in vastly dissimilar circumstances, these twins shared only their genomes. Yet on tests designed to measure personality, attitudes, temperaments, and anxieties, they converged astonishingly. Social and political attitudes were powerfully correlated: liberals clustered with liberals, and orthodoxy was twinned with orthodoxy. The same went for religiosity (or its absence), even for the ability to be transported by an aesthetic experience. Two brothers, separated by geographic and economic continents, might be brought to tears by the same Chopin nocturne, as if responding to some subtle, common chord struck by their genomes.”

Here I will touch only lightly on the main controversies surrounding the MISTRA and behavioral genetic research in general, and focus on areas where Mukherjee gets the basic facts wrong.

  1. Bouchard and colleagues were not, as Mukherjee implied, among the first researchers to attempt an assessment of the relative weight of nature and nurture. This has been going on since at least Galton in the 19th century. The authors of previous family, twin, and adoption studies also attempted to make such an assessment, and these studies go back to the 1920s and earlier. The first TRA study was published in 1937, where Horatio Newman and colleagues, in reference to their reared-together twin sample, wrote that they “attempt[ed] to get a measure of the relative effects of nature and nurture factors.”5
  2. The MISTRA researchers did not “scour” or use adoption records. They recruited most pairs as volunteers after twins heard about the study in media reports. As MISTRA researcher Nancy Segal described it in her 2012 book on the study, the sample consisted of “a collection of cases,” who “contacted us because they knew about the MISTRA largely through the media attention the study had attracted.”6  MZA samples recruited through media attention or appeals, however, are biased in favor of twin similarity because pairs had to have known of each other’s existence in order to respond, and they may have discovered each other because of their similarities. In the words of a different group of TRA researchers, volunteer-based studies such as the MISTRA “typically relied on identification by third parties or response to media appeals. Pairs may have come to the investigator’s (and to each other’s) attention because of their remarkable similarity.”7 The single fact that studies such as the MISTRA are biased in favor of recruiting more behaviorally similar twin pairs is reason enough to call into question genetic interpretations of their results.
  3. Contrary to public perceptions, few pairs found in TRA studies were “separated at birth.” Other than the selected information and correlations that they chose to publish, or to release to selected journalists, the MISTRA team has repeatedly denied independent researchers access to their raw data, and they have never published life history, degree-of-separation, and test score information for each studied pair.8 Most pairs in the previous three TRA studies were only partially  reared apart, and for a number of reasons it is unlikely that the Minnesota MZA pairs were any more “separated” than were these pairs.9 Due to the increasing rarity of separated twins, it is extremely difficult for independent researchers to reproduce the MISTRA findings. It is therefore critically important—actually mandatory—that independent analysts and researchers have access to the MISTRA raw data. Social and behavioral science researchers, especially those conducting studies of great social importance that are not easily reproduced, must not become “lords of the data,” permitting others to see only selected findings that they choose to release. When this occurs, the researchers’ conclusions must be automatically rejected until independent analysts are allowed to inspect, and publish findings from, their raw data.
  4. Information on each MISTRA pair relating to whether twins were reared in “different families and different cities” has never been published or reported.
  5. There is no published evidence that MISTRA pairs often grew up in “vastly dissimilar circumstances,” but there is much published evidence from earlier TRA studies suggesting that most did not.10
  6. Even in the extremely rare cases where MZA pairs were separated at or near birth, Mukherjee’s and many others’ claim that “twins shared only their genomes” is completely false. MZA pairs share a common prenatal environment, are the same age and sex, and share a striking physical resemblance. MZAs share many environmental similarities and non-genetic cohort effects. The “cohort effect” concept refers to similarities in age-matched people’s behavior, preferences, beliefs, physical condition, and other characteristics that are caused by experiencing stages of life at the same time in the same historical period and cultural milieu. In addition to sharing a common prenatal environment and experiencing similar postnatal healthcare, most reunited MZA pairs grew up sharing at least ten different behavior-molding cultural/environmental influences in common: national, regional, language, ethnic, religious, physical resemblance, adoptee status, economic class, birth cohort, and gender cohort.
  7. The “powerful correlations” reported by the MISTRA researchers can be explained by the non-genetic factors outlined above. In other words, like studies of twins reared together, in TRA studies genetic interpretations of twin correlations are confounded by environmental influences. As a pair of behavioral genetically oriented criminologists recently put it in relation to epigenetics research, but which applies perfectly to twin research, “When you can’t do experiments, you have to be very careful about something called confounding. Confounding is a pernicious problem that can make one thing look like it’s causing something else when, in actuality, it’s not.” Indeed, TRA studies are based on correlational data on twin pairs who grew up in environments that the researchers did not design, control, or observe. Environmental confounds are a “pernicious problem” in TRA studies such as the MISTRA, and can make it look as though common genes are causing above-zero MZA behavioral (“psychological trait”) correlations, when in actuality they are not.
  8. The tale about separated twins being “brought to tears by the same Chopin nocturne,” if true, is a selectively reported (“cherry picked”) story that proves absolutely nothing about genetics. But it does serve its purpose of selling the false ideology of genetic determinism to the public. Such similarities can be found between most randomly selected (genetically unrelated) pairs in the general population, if similarities are what one is looking for. According to Wikipedia, cherry picking is “the act of pointing to individual cases or data that seem to confirm a particular position, while ignoring a significant portion of related cases or data that may contradict that position.” Twins also had incentives to invent various “spooky” stories they told the researchers, and to exaggerate their degree of separation and behavioral similarity.
  9. The final Minnesota MZA sample was actually 81 pairs. The final DZA (dizygotic, or fraternal twins reared apart) sample was 56 pairs.11 Mukherjee did not mention that the MISTRA researchers to this day have failed to publish their full-sample DZA IQ correlations, even though they published such correlations for personality and most other MISTRA-studied behavioral characteristics (traits) in the 1980s and 1990s. The most likely reason, as I showed in Chapter 6 of The Trouble with Twin Studies, is that the MISTRA’s apparently high DZA IQ correlations invalidate Bouchard and colleagues’ conclusion that IQ “is strongly affected by genetic factors.”12 This has been confirmed by molecular genetic research, which has seen nearly a quarter-century of failed attempts to identify genes for “general intelligence” (IQ). In 2014, Bouchard recognized that the results of attempts to uncover such genes “have been dismal in comparison with expectation.”13

Unfortunately, Mukherjee is only one of many authoritative authors to misreport the methods and findings of human genetic research.14 In Part 2, I will highlight several additional errors in his description of the MISTRA in his 2016 book, The Gene: An Intimate History.

* * * * *


  1. Wright, L., (1997), Twins: And What They Tell Us about Who We Are, New York: John Wiley & Sons, p. 143
  2. Wright, W., (1998), Born That Way, New York: Knopf.
  3. Mukherjee, S., (2016, May 2), Same but Different: How Epigenetics Can Blur the Line between Nature and Nurture, The New Yorker.
  4. Mukherjee, S., (2016), The Gene: An Intimate History, New York, Scribner.
  5. Newman et al., (1937), Twins: A Study of Heredity and Environment, Chicago, University of Chicago Press, p. 111.
  6. Segal, N. L., (2012), Born Together—Reared Apart: The Landmark Minnesota Twin Study, Cambridge, MA: Harvard University Press, p. 36.
  7. Pedersen, et al., (1988), Neuroticism, Extraversion, and Related Traits in Adult Twins Reared Apart and Reared Together, Journal of Personality and Social Psychology, 55, 950-957, p. 955.
  8. Joseph, J., (2015), The Trouble with Twin Studies: A Reassessment of Twin Research in the Social and Behavioral Sciences, New York: Routledge, Chapter 5.
  9. Joseph, 2015, Chapter 2.
  10. Joseph, 2015, Chapter 2. See also Taylor, H. F., (1980), The IQ Game: A Methodological Inquiry into the Heredity-Environment Controversy, New Brunswick, NJ: Rutgers University Press.
  11. Segal, 2012, p. 42.
  12. Joseph, 2015, Chapter 6. The quotation is from Bouchard et al., (1990), Sources of Human Psychological Differences: The Minnesota Study of Twins Reared Apart, Science, 250, 223-228, p. 227.
  13. Bouchard, T. J., Jr., (2014), Genes, Evolution and IntelligenceBehavior Genetics, 44, 549-577.
  14. Joseph, J., (2004), The Gene Illusion: Genetic Research in Psychiatry and Psychology under the Microscope, New York: Algora; Joseph, J., (2006), The Missing Gene: Psychiatry, Heredity, and the Fruitless Search for Genes, New York: Algora, Chapter 7, Chapter 5.

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  1. The activities of the researchers gives the impression they’re more interested in protecting their and their position’s veracity after they found out their actual results were indeterminate and neither proved nor disproved their position; therefore, it was necessary to go into survival mode by concealing the actual details of their study and its findings.