Psychologist Jay Joseph outlines the fallacies of the twin studies in the context of the ongoing decades-long failure to discover genes for human behavioral differences, including IQ, personality, and the major psychiatric disorders. This volume critically examines twin research with particular emphasis on reared twin studies and incorporates new and updated perspectives, analyses, arguments and evidence.
PREFACE
I have done my best to review and analyze the research and molecular genetic literature in a way that can be followed and understood by people unfamiliar with these fields. This is a book about the fallacies of twin studies and the inability to discover genes for human behavioral differences, and the relationship between the two.
ACKNOWLEDGMENTS
MICTAR Minnesota Center for Twin and Adoption Research MISTRA Minnesota Study of Separately Reared Twins. SNP Single Nucleotide Polymorphism SRQ Self-Rating Questionnaire TEDS Early Developmental Twin Study TRA Adult Twins (study) WAIS Wechsler Adult Intelligence Scale WMH World Mental Health.
STUDIES OF
REARED-APART TWINS
INTRODUCTION
In this book I examine the use of twin research in the social and behavioral sciences. Twin research was initiated in the nineteenth century by the British statistician and founder of the eugenics movement, Francis Galton.
STUDIES OF REARED- APART TWINS
Shields acknowledged that "the possibility of twins comparing notes could not always be ruled out" (p. 66). The grandmother is adopted by the mother"It was at the beginning of the war that the twins met for the first time. The main objection that can be made to this analysis of the differences and similarities between the early environments of the twins lies in the fact that the whole investigation was done by the same person” (Juel-Nielsen Part I, p. 103).
Of the 121 cases reported in the past fifty years,” Farber wrote, “only three are 'twins reared apart' in the classical sense” (p. 60).
STUDIES OF REARED-APART TWINS
It is based on the mistaken assumption that a quantitative analysis of genetic and environmental contributions to individual differences sheds light on the developmental process of individuals. The obvious conclusion is that the use of heritability estimates as a measure of the relative importance of genes and environment should be abandoned (for an opposing view, see Sesardic, 2005). Therefore, the use of heritability estimates in the social and behavioral sciences should be discontinued.
This "ACE Model" is widely used in behavioral genetics research in the social and behavioral sciences. For example, the twin design estimates genetic influence based on the difference between MZ and DZ correlations. Thus, a pioneer of model fitting in the behavioral sciences and his colleagues admitted that their method was based on assumptions that might not be true.
In the CPI, for example, the "Responsibility" (Re) scale is based on 36 out of a total of 434 questions. Based on these influences, we should expect that any two newborns of the same sex (whether 100% genetic MZA pairs or genetically unrelated pairs) placed in different families at about the same time will later share many adult characteristics. - characteristics and behaviors simply because they are of the same gender and grow up in the same era (Joseph, 2004; McGue & Bouchard, 1984; Rose, 1982). In the next two chapters, I will further evaluate the assumptions, methods, results and conclusions of MISTRA.
The following description of the sample as it stood in 1990 is taken from the MISTRA Science publication (Bouchard, Lykken, McGue, Segal,. At the time of the 1990 Science article, the MISTRA MZA sample from 56 MZA -pairs and 30 DZA pairs exist According to the MISTRA genetic "general model", the DZA correlation should be half the MZA correlation (McGue & Bouchard, 1989, p. 24).
15 The more similar prenatal environments of MZA versus DZA pairs did not contribute to greater behavioral similarity of the former. As we saw, Plomin and colleagues wrote this. the twin design estimates genetic influence based on the difference between MZ and DZ correlations. Bouchard, 2008: “Correlations for monozygotic twins reared apart directly estimate the heritability of the trait” (Bouchard, 2008, p. 12).
Nevertheless, there is little doubt that the MISTRA MZA sample was biased in favor of more similar pairs for many of the same reasons that the classic TRA studies were so biased. When twins told similar stories to the MISTRA team, they were (and still are) held up by journalists, textbook writers and the researchers themselves as "frightening" examples of the powerful influence of genetics. Until proven otherwise by examination of the raw data by independent analysis, we must assume that the split status of the MISTRA MZAs was similar to the 44 partially split Shields MZA pairs seen in Table 2.2.
According to Bouchard and colleagues, "The study of IQ is paradigmatic for genetic research in human behavior" (Bouchard, Lykken, McGue, Segal, & Tellegen, 1990, p. 224). According to Bouchard and colleagues, "The Raven Progressive Matrices (Standard Set) is a widely used nonverbal measure of problem-solving ability" (Bouchard, Lykken, et al., 1990, p. 224). We saw in previous chapters that, as Segal put it, "the long-awaited [MISTRA] IQ data" was published in the 1990 Science article.
As Segal described it, “the ScienceIQ paper was an important moment in the history of the study” (Segal, 2012, p. 104). In this Actapaper's concluding section, Bouchard, Segal, and Lykken argued that "the relatively large DZA correlations were accounted for by statistical variability." Although they were willing to "explore". While Bouchard acknowledged that the DZA correlation "is reported as a control variable" in the MISTRA, he wrote that "the MISTRA [DZA] correlations have not yet been fully analyzed," and that he and his colleagues.
These sample correlations do not differ significantly at the 0.05 level, which means that from a statistical point of view they are the same.4 The researchers did not discuss or evaluate these Raven IQ correlations. However, the twins chose not to base the study's findings on some questionable key assumptions, not to publish the MISTRA DZA IQ correlations, and to deny access to the raw data—the researchers did. In the context of the TRA study, we have seen Segal write that "genetic effects on a trait are demonstrated when" the intraclass correlation of MZA exceeds the intraclass correlation of DZA (Segal, 2012, p. 59), which is consistent with the one previously stated. The behavior genetics position that "quantitative genetic theory states that the degree of similarity between family members should vary as a function of their genetic relatedness" (Segal, 1993, p. 944).
STUDIES OF REARED- TOGETHER TWINS
THE MZT–DZT EQUAL ENVIRONMENT
ASSUMPTION
The perceived validity of ART studies is often cited in support of the twin method and its underlying assumptions. During the first 40 years of the twin method (roughly 1924-1964), twin researchers in psychology and psychiatry defined EEA—without qualification—as the assumption that MZT and DZT pairs grow up experiencing roughly equal environments. An assumption of the twin method is] that the effects of nutrition are the same for both types of twins.
The analysis I present here, including an examination of the 'EER test' study by Kevin Smith and his colleagues, applies broadly to the use of the twin method in psychology (including IQ and personality). Since the 1960s, most twin researchers have moved away from the traditional KEE definition and have defended the validity of the twin method on the basis of two main arguments, sometimes making both arguments simultaneously. Many other leading behavioral genetic researchers have defended the validity of the twin method based on argument A, examples of which can be found in Appendix C.
Interestingly, the argument A position that twins create their own environments because they are genetically more similar makes argument B possibly irrelevant, because even if critics or researchers show that MZT pairs experience more similar "trait-relevant" environments than DZT -pairs, twins. researchers can still argue in favor of the validity of the twin method and the EEA based on MZT pairs that "created". In other words, in the process of developing a questionnaire designed to help future twin researchers reliably distinguish between MZT and DZT pairs, the researchers noticed that their data, which represented largely dissimilar MZT–DZT childhoods -environments have shown, cast doubt on the validity of the twin method itself. Hibbing was right, but one year earlier he and his colleagues had lumped together potential biological and genetic influences in their defense of the EEA and the twin method.
Kupfer and the APA have admitted that they have no genes for psychiatric disorders, only the inference of genes based largely on genetic interpretations of the results of psychiatric twin studies. It is a telling point that American psychiatry, which has created more than 30 "task forces" since 1970 to investigate various issues, has never created a task force charged with an in-depth critical evaluation of the assumptions of psychiatric not undertake twin studies.4. This means that the validity (or non-validity) of the MZT–DZT EEA is the decisive factor in judging the results of psychiatric twin studies, although many people unfamiliar with twin research in psychiatry assume that twins are raised apart.
We saw in Chapter 7 that until the mid-1960s twin researchers used the traditional definition of the EEA, which states without qualification that MZT and DZT childhood and adult environments are more or less equal. Critics usually do not dispute this, but instead have focused on the validity of the EEA and thus disagree with psychiatric genetic and mainstream psychiatric interpretations of this pervasive finding. Psychiatric genetic researchers usually cite the body of "EEA test" studies in support of the validity of the EEA, concluding that the assumptions and genetic interpretations of psychiatric twin studies are valid or at least "reasonable." Although he acknowledges that MZTs experience more similar environments than DZTs, Kenneth Kendler of the Virginia Institute for Psychiatric and Behavioral Genetics has been the strongest and most widely published supporter of this position in psychiatry (see, for example, Kendler Kendler & Prescott, 2006) .
Kendler has defended genetic interpretations of psychiatric twin studies based on argument A, writing in 1983 that “studies have shown that the similarity of the social environments of monozygotic twins is the result and not the cause of their behavioral similarity” (Kendler, 1983) . , p. 1422). In this passage, Kendler acknowledged that MZT couples experience more similar environments than DZT couples, but again defended the EMA's position based on Argument A's position that MZTs create more similar environments for themselves because they are genetically more similar – an untenable argument, as seen in Chapter 7. It seems that even the leading defender of the EEA had at least some doubts about this assumption, and this conclusion, which reappeared in a leading psychiatric journal (Archives of General Psychiatry, now JAMA Psychiatry), is hardly a convincing statement of support. of the double method and the EER.
