By Jay Joseph

from GeneWatch 28-1 | Jan-May 2015

The Trouble with Twin Studies: A Reassessment of Twin Research in the Social and Behavioral Sciences
By Jay Joseph. Hardcover edition published in 2015 by Routledge (Taylor & Francis Group). For ordering details and other information, please see the page and the Taylor & Francis page.



Since the 1920s, twin studies have been put forward as scientifically validated "natural experiments" for assessing the relative importance of heredity and environment. Identical (MZ) twin pairs are said to share 100% of their segregating genes, whereas fraternal (DZ) pairs are said to share only 50% on average. After reaching its low point in the early 1960s, twin research began a comeback that continues to this day, as twin studies are widely cited in support of important genetic influences on a great variety of psychological characteristics, behaviors, psychiatric disorders, and common medical conditions. Almost all studies investigate twin pairs reared together in the same family home, while a tiny yet influential handful have studied what researchers and others refer to as "reared-apart" twin pairs. Most genetic researchers and their critics agree that the results of a (non-twin) family study, where a behavioral characteristic is found to "run in the family," can be explained entirely by non-genetic factors. Adoption studies are much rarer, and have their own set of methodological issues and questionable assumptions. In light of gene discovery failures, twin research continues to supply the main scientific evidence put forward by the nature (genetic) side of the "nature-nurture" debate.

The first chapters of The Trouble with Twin Studies examine the six existing reared-apart twin studies, which have focused on areas such as IQ and personality. The most often cited are the three "classical" studies published between 1937 and 1965, and the highly publicized "Minnesota Study of Twins Reared Apart," whose most frequently cited publications appeared in the 1980s and 1990s. Other topics include the "Cyril Burt scandal" and fraudulent or unethical practices in the social and behavioral sciences in general, and the controversy surrounding Berkeley psychologist Arthur Jensen's claim that IQ score differences between various ethnic groups are due mainly to genetic influences. A central problem with these studies of reared-apart identical twin pairs (MZA, or monozygotic twins reared apart) is that most pairs were reared together for periods of time, had frequent or regular contact, and/or had a close emotional bond with each other. Most twin pairs, therefore, were only partially reared apart. Behavioral similarity can also result from the fact that, although they are raised in different family environments, both members of an MZA pair are the same age and sex, are very similar in physical appearance, and usually grow up in very similar cultural and socioeconomic environments in the same eras.

The influential and widely cited Minnesota Study of Twins Reared Apart (MISTRA) is very problematic - particularly the questionable basis upon which the researchers arrived at genetic interpretations of their data. Problem areas include the many similarities of the twins' social and cultural environments, the researchers' failure to make their raw data available for inspection by independent reviewers, the questionable assumptions underlying the researchers' model fitting procedures (some of which the researchers themselves recognized are "likely not to hold"), sample size issues, recruitment bias, and a reliance on twins' potentially unreliable accounts of their degree of separation and behavioral similarity.

A major issue has been the failure of the Minnesota researchers to publish and evaluate their full-sample reared-apart fraternal twin (DZA, or dizygotic twins reared apart) IQ correlations, even though they reported full-sample DZA correlations for non-IQ behavioral characteristics in various MISTRA publications. This occurred despite the fact that IQ (cognitive ability) was the main focus area of the study, and that the researchers had designated DZA twins as the MISTRA control group. To this day, the researchers have not published and evaluated the results for all tests completed by the full sample of twin pairs they studied. Nevertheless, based on the incomplete data that have been published, there does not appear to be a statistically significant Wechsler (WAIS) IQ or Raven Progressive Matrices IQ correlation difference between the MISTRA MZA and DZA groups - a result that runs counter to genetic predictions and theories, and by itself would invalidate genetic interpretations of the MISTRA IQ studies. Because MZA pairs are more similar genetically than are DZA pairs, an MZA sample correlation not higher than the corresponding DZA sample correlation at a statistically significant level suggests that non-genetic (environmental) factors alone are responsible for raising both correlations above zero. Contrary to the way the authors of textbooks and other authoritative works usually write about the MISTRA, the study failed to provide scientifically acceptable evidence in support of genetic influences on IQ, personality, and other types of behavior.

The second section of The Trouble with Twin Studies focuses on the much more common studies comparing the behavioral resemblance of reared-together identical (MZT, or monozygotic twins reared together) versus same-sex fraternal twin pairs (DZT, or dizygotic twins reared together), which use a procedure called the "classical twin method." The evidence undermines this method's "equal environment assumption," as MZT pairs grow up experiencing much more similar environments and treatment, and experience much greater levels of identity confusion and psychological attachment, than experienced by DZT pairs. Therefore, the greater behavioral resemblance of MZT versus DZT pairs can be completely explained by environmental factors. 

The book also examines twin research in psychiatry in the context of over four decades of gene discovery failures in the field. Psychiatric twin studies are based on MZT-DZT comparisons (the twin method), and constitute the most frequently cited evidence that the major psychiatric disorders have an important genetic component. Although supporters of psychiatric twin studies argue that the equal environment assumption has been tested and upheld, the best-replicated disconfirmation of this critical theoretical assumption consists merely of all the psychi¬atric twin studies ever performed. Nine decades of such studies have shown consistently that pairs experiencing similar environments and high levels of identity confusion and attach¬ment - MZTs - resemble each other more for psychiatric disorders than do pairs experiencing less similar environments and much lower levels of identity confusion and attachment - DZTs. The results of these EEA-test studies strongly suggest that the assumption is false.

Finally, the book addresses the possibility of a "post-behavioral genetics" era, beginning with the ongoing decades-long failure to identify genes for behavioral characteristics such as IQ, personality, and psychiatric disorders. Genetic researchers have since 2008 referred to a "missing heritability problem," but I propose an alternative understanding: that the best explanation for "positive" twin study findings in combination with negative molecular genetic results is not that "heritability is missing," but that something is wrong with genetic interpretations of twin data. The numerous assumptions, decisions, claims, and conclusions made by leading twin researchers and others have been tested under the microscope of molecular genetic research, and the (negative) results are now in.

 Twin studies published to date have been unable to supply scientifically valid evidence in support of genetic influences on the characteristics studied in the social and behavioral sciences. My conclusions are that (non-molecular) behavioral genetic and psychiatric genetic research methods are, to varying degrees, unable to disentangle the potential roles of genetic and environmental influences on differences in human behavior; that the historical positions, research methods, and "landmark" studies of these fields are massively flawed and environmentally confounded; and that family, social, cultural, economic, and political environments - and not genetics - are the main causes of psychiatric disorders and differences in human behavior.

Jay Joseph, Psy.D., is a licensed psychologist practicing in the San Francisco Bay area. In addition to The Trouble with Twin Studies, he is the author of The Missing Gene: Psychiatry, Heredity, and the Fruitless Search for Genes (2006) and The Gene Illusion: Genetic Research in Psychiatry and Psychology under the Microscope (2004). For a complete list of publications, please see


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