It’s Time to Abandon the “Classical Twin Method” in Behavioral Research

Note: Many more articles on twin research and behavioral genetics can be found at 2020 analysis of reared-apart twin studies can be found HERE

(First revision 8/17/2020. Second revision 12/23/2020, based on small stylistic changes, and adding a new introductory paragraph and a paragraph about a 2018 study by Segal and colleagues)

By Jay Joseph, Psy.D.

In this article I will argue that studies using the "classical twin method" should be abandoned in the social and behavioral sciences. I will attempt to show that the main assumption these studies are based on is false
an argument I have developed in books, peer-reviewed articles, and online articles since 1998

Twin studies supply the "scientific evidence" most often cited in support of the claim that human behavioral differences are strongly influenced by heredity. Yet genetic interpretations of twin studies of behavior, including areas such as IQ, personality, criminality (antisocial behavioral), schizophrenia, and depression are based on the acceptance of highly questionable or even false assumptions.  I am compelled to keep writing about this because these studies have not gone away, despite the critics’ airtight arguments that they should have gone away a long time ago.

Some people argue that recent molecular genetic research, including studies based on the "polygenic score" (PGS) method, has rendered the twin study critique obsolete. Others argue that the "validity of twin studies" debate was settled decades ago in favor of twin studies. However, although behavioral and psychiatric gene discovery claims have been appearing since the 1960s, they rarely if ever hold up. (Problems with the PGS method are discussed HERE, HERE, HERE, HERE, and HERE.) Based on a half century of false-alarm behavioral gene discovery claims, the appropriate reaction to new claims should be extreme skepticism and caution. In part because "heritability estimates" produced by twin research guide molecular genetic research, the far-from-settled twin study debate is more important than ever.[1] 

The two main methods that use twins for behavioral research purposes have produced:

1.   Thousands of publications based on the classical twin method (usually referred to as the "twin method," and sometimes as the "classic twin design," or CTD), which uses reared-together MZ (monozygotic, identical) and reared-together same-sex DZ (dizygotic, fraternal) twins.

2.   Six published twins reared apart or "TRA" studies (also known as "separated twin studies"), which study MZ (and sometimes DZ) twins who supposedly were (but in most cases weren't) separated at birth and reared apart from each other in different family environments. Problem areas in these studies are described HERE, HERE, and HERE.

The twin method, which is the subject of the present review, compares the behavioral resemblance, concordance rates, or psychological test score correlations of MZ versus same-sex DZ pairs. MZ pairs are said to share a 100% genetic resemblance, whereas DZ pairs are said to share an average 50% genetic resemblance.[2] Twin method results usually show that MZ pairs behave more similarly, or correlate higher on psychological tests, than do same-sex DZ pairs at a statistically significant level—a finding I will designate "rMZ > rDZ," or more simply, "MZ > DZ." Genetic interpretations of MZ > DZ are based on twin researchers' acceptance of the "equal environment assumption," which is the main focus of this article.

The Twin Method’s Crucial "Equal Environment Assumption"(EEA)

As seen in Figure 1, genetic interpretations of behavioral twin studies depend on the acceptance of several key assumptions. An assumption is something taken for granted or accepted as true without proof. Whether an assumption is true or false can completely change the findings of a study.

Although Assumptions 2-4 in Figure 1 are not always fulfilled (Assumption 5 will be discussed later), and although critics have highlighted numerous twin method problems and biases relating to sampling, diagnosis, zygosity determination, tests used, the use of age-correction methods, researcher genetic bias, and so on, here I will focus on the MZ-DZ "equal environment assumption," also known as the "EEA," which is by far the most controversial twin method assumption (the red Assumption 6 in Figure 1). According to the EEA, MZ and DZ pairs grow up experiencing roughly equal environments, and the only behaviorally relevant factor distinguishing these pairs is their differing degree of genetic relationship to each other (100% versus an average 50%). As one group of twin researchers correctly observed, the EEA "is crucial to everything that follows from twin research."[3] 

From the twin method's inception in 1924 until the mid-1960s, twin researchers assumed—without adding any qualifying statements—that MZ and same-sex DZ pairs grow up experiencing roughly equal environments. To cite one example of this "traditional" EEA definition, in 1967 schizophrenia twin researcher Einar Kringlen wrote:

"The basic underlying assumption for the classical twin method is, of course, that environmental conditions of monozygotic twins do not differ from those of dizygotic twins."[4]

Critics argue that the EEA as it relates to behavioral twin studies is obviously false, since when compared with same-sex DZ pairs, MZ pairs grow up experiencing

·        Much more similar treatment by parents and others
·        Much more similar physical and social environments
·        More similar treatment by society due to their sharing a very 
 similar physical appearance
·        Identity confusion and a much stronger level of emotional attachment

I put together a table showing the results of studies that assessed levels of identity confusion and psychological attachment experienced by MZ and DZ pairs (seen HERE), and far from being "equal," levels of identify confusion and attachment are much higher among MZ pairs than among DZ pairs.   

Since the mid-1960s, research and common sense have converged on the conclusion that MZ pairs grow up experiencing much more similar environments, and are treated much more similarly, than are DZ pairs. In a 2014 article (ironically) written in defense of the twin method and the EEA, for example, criminology twin researcher J. C. Barnes and colleagues recognized, "Critics of twin research have correctly pointed out that MZ twins tend to have more environments in common relative to DZ twins, including parental treatment…closeness with one another…belonging to the same peer networks…being enrolled in the same classes…and being dressed similarly."[5]

This means that twin researchers and their critics don't have to argue anymore about whether MZ and DZ environments are different, since almost everyone now agrees that they are different. Twin researchers nevertheless continue to maintain that the EEA is valid based on four arguments, which I describe and deconstruct below.

Both sides of the debate predict that a behavioral twin study will produce a finding of MZ > DZ. The controversy centers on how we should interpret MZ > DZ. Twin researchers and the popularizers of their work argue in favor of a genetic interpretation, whereas critics often argue (1) that MZ > DZ can be explained largely or entirely by environmental influences, or (2) that MZ > DZ is uninterpretable, because the twin method is unable to disentangle the potential behavior-shaping influences of genes and environments.

The EEA critique/analysis applies to typical MZ-DZ comparisons, as well as to modern twin studies based on more sophisticated "biometrical model fitting" statistical procedures. In both cases, researchers assume that MZ and DZ environments do not differ.

Four Arguments that Fail

Faced with the reality of unequal MZ and DZ environments, since the 1960s twin researchers have defended the validity of the EEA based mainly on one or more of the following four arguments. I have called these "Argument A," "Argument B," "Argument C," and "Argument D." Respectively, these are the "twins create their environments," "trait-relevant," "assumption violations cancel each other out in favor of heritability," and the "MZ correlations are similar whether twins are reared together or apart" arguments. Let's briefly review these arguments and the main fallacies each is based on. This will be followed by a brief discussion of the "EEA-test" study literature.

Argument A: "Twins Create Their Environments" 

Twin researchers using Argument A recognize that MZ pairs grow up experiencing more similar environments than experienced by DZ pairs, but they maintain that the EEA is valid because MZ pairs "create" or "elicit" more similar environments and parental treatment for themselves because they behave more similarly for genetic reasons. Therefore, they argue, environmental influences on twins' behavioral similarity should be counted as genetic influences.

One of many examples of Argument A is found in an article published in 2000 by genetic researchers writing in support of the "validity of twin studies."[6] Although they recognized that "there is overwhelming evidence that MZ twins are treated more similarly than their DZ counterparts,"

"…the more similar parental treatment of MZ vs. DZ twins occurs in response to the greater similarity of actions initiated by MZ pairs.…It seems…likely that the increased similarity in treatment of MZ twins is a consequence of their genetic identity and the more similar responses this elicits from the environment" (italics in original).[7]

Argument A, however, is a circular one because the conclusion that genetic factors explain MZ > DZ is based on a premise that assumes the very same thing—a premise based largely on genetic interpretations of previous twin studies. Twin researchers invoking Argument A, therefore, fallaciously refer to the genetic premise in support of the genetic conclusion, and then refer back to the genetic conclusion in support of the genetic premise, in a circular loop of faulty reasoning. One observer wrote that circular reasoning is "empty reasoning in which the conclusion rests on an assumption whose validity is dependent on the conclusion."[8] The circular nature of Argument A is seen in Figure 2.

We see that Argument A uses genetic interpretations of previous twin studies as evidence supporting genetic interpretations of subsequent twin studies. The twin method validates itself!

Furthermore, Argument A makes little sense, since it portrays twins as being able to create their environments on the basis of their inherited behavioral tendencies, while simultaneously portraying parents as supposedly possessing an amazing ability to override their own behavioral tendencies by responding to their children's genetic differences. Even in this mythical parent-child "Battle of the Genes," the family environments created by the parents will still prevail because parents possess power and authority in addition to their rigid behavioral blueprints, and because they have experienced many more years of behavior-shaping events. Twins would be largely unable to "create" their family environments for the simple reason that they would be no match for the genetically driven behavior of their parents.

Argument A fails because it is a sleight-of-hand "heads I win, tails you lose" circular argument that twin researchers cannot lose, because they count both the "nature" and "nurture" sides of the behavioral coin as nature (genetic).

Argument B: "Trait-Relevant" Environmental Factors

Twin researchers using Argument B also recognize that MZ pairs experience more similar environments than DZ pairs, but claim that the EEA remains valid until critics are able to identify the "trait-relevant" aspects of the environment that cause MZ pairs to behave more similarly. An example of the Argument B definition of the EEA is found in a 1993 publication by psychiatric genetic twin researcher Kenneth Kendler and colleagues:

"The traditional twin method, as well as more recent biometrical models for twin analysis, are predicated on the equal-environment assumption (EEA)—that monozygotic (MZ) and dizygotic (DZ) twins are equally correlated for their exposure to environmental influences that are of etiologic relevance to the trait under study" (italics added).[9]

It appears that Argument B was first put forward by twin researcher Irving Gottesman (1930-2016) in a 1966 twin study of personality. Gottesman redefined the traditional definition of the EEA by inserting one italicized qualifying term into it, now writing that the twin method was based on the assumption "that the average intrapair differences in trait-relevant environmental factors are substantially the same for both MZ and DZ twins" (italics in original).[10] This subtle change in definition allowed twin researchers to continue their work, not because they had determined that MZ and DZ environments were equal, but because they could now bypass the obviously false equal environment assumption as it had been defined up to that point.[11] Theoretical sleight of hand, once again.

Supporters of Argument B attempt to shift the burden of proof from themselves onto critics for showing that MZ and DZ pairs differ in their exposure to "trait-relevant" environmental factors. According to Kendler and other leading psychiatric genetic twin researchers, "It would seem that the burden of proof rests with critics of the twin method to demonstrate that 'trait-relevant' environmental factors are more similar for identical than same-sex fraternal twins."[12]

A basic principle of science, however, is that the burden of proof falls squarely on the people making a claim, not on their critics. Therefore, given that they recognize that MZ and DZ environments are different, behavioral twin researchers using Argument B—and not their critics—are required to identify the specific and exclusive trait-relevant environmental factors involved in the behavioral characteristic or psychiatric disorder in question. After accomplishing this, they then must show (1) that MZ and DZ twins did not experience such factors, or (2) that MZ and DZ twins experienced such factors to roughly the same degree. In most cases, behavioral twin researchers have been unable to identify specific and exclusive "trait-relevant" environmental factors. Until they are able to identify such factors, and until they are able to subsequently determine that MZ and DZ pairs were similarly exposed (or not exposed) to these factors, Argument B defenses of the EEA fail completely.[13]

Argument C: Violations of the "Random Mating" and "Equal Environment" Assumptions Cancel Each Other Out in Favor of Genetics

A third Argument C defense of the EEA is that violations of the "random mating" assumption (Assumption 5 in Figure 1; sometimes called the "no assortative mating assumption"), and violations of the EEA, roughly cancel each other out in favor of genetics and "heritability." Supporters of Argument C claim that, whereas unequal MZ and DZ environments might lead to an overestimation of heritability, the existence of non-random mating patterns among the parents of twins leads to an underestimation of heritability. Non-random (assortative) mating has been defined as "the tendency for people to choose mates who are more similar (positive) or dissimilar (negative) to themselves in phenotype characteristics than would be expected by chance." Argument C was put forward by Barnes and colleagues in 2014, and was taken up by IQ hereditarian Charles Murray in his 2020 book Human Diversity. Murray "summed up" the argument as follows:

"Twin studies have come under criticism for overstating the role of genes. The reality is that violations of the random mating assumption are common and lead to modest understatement of the role of genes, whereas violations of the equal environments assumption have even more modest effects in the other direction and are uncommon. Overall, heritability as estimated by twin studies appears to be accurate, with errors tending on net to slightly underestimate heritability rather than overestimate it."[14]

To identify only one of several problems with this argument, let's suppose that the environmental null hypothesis—which states that there are no genes for behavior—is true. In this case mating patterns would have no direct genetic influence on human behavior, and MZ > DZ would be completely caused by non-genetic factors. The claim that non-random mating patterns lead to a "modest understatement of the role of genes" circularly assumes in advance that the environmental null hypothesis is false. A twin study, however, is an experiment designed to test whether the environmental null hypothesis is false. The findings of this experiment cannot be based on a built-in assumption that it is false, especially since this assumption, once again, is based largely on genetic interpretations of previous twin studies. Theoretical sleight of hand scores a hat trick.  

Argument D: "MZ Twins Correlate Similarly on Psychological Tests Regardless of Whether They Were Reared Together or Reared Apart"

The final argument is based on the results of the tiny handful of TRA (twins reared apart) studies, and the claim that MZ pairs behave similarly regardless of whether they were reared together or reared apart. People making this claim argue that this supports the EEA, because it shows that growing up in the same family does not lead twins to behave more alike. However, the argument does not take into account the numerous non-familial environmental factors that reared-together and reared-apart twins both experience. More importantly, TRA studies are greatly flawed. The massive flaws and major biases found in these studies, including the famous Minnesota study, are described in detail HERE and HERE. As seen HERE, most MZ pairs found in TRA studies were only partially reared apart. For these reasons, TRA study results cannot be used to validate genetic interpretations of MZ > DZ. 


Like the first three arguments, Argument D fails to support the EEA, and the only remaining relevant question in assessing the EEA's validity is whether—not why—MZ and DZ environments are different.

The "EEA-Test" Studies

In a series of "EEA-test" studies that have appeared since the late 1960s, researchers attempted to "test" the validity of the assumption in ways other than the only way it can be tested, which is the simple determination of whether MZ and DZ environments are roughly equal—yes or no? If no, the EEA is false. It's that simple.

Paradoxically, the starting point of most EEA-test publications is the recognition that MZ and DZ childhood environments are not equal, as researchers confirmed that MZ twins more often shared the same bedroom growing up, had common friends, were dressed alike, attended school together, and so forth. In most cases, however, these researchers concluded that MZ pairs' greater environmental similarity did not constitute a major bias in twin studies.  

Reviewing over 50 years of individual EEA-test studies would be as tedious and dull for me to write about (again) as it would be for most of you to read. For those who dare, I refer you to Chapter 9 of my 2006 book The Missing Gene, and to a 1996 article by Alvin Pam and colleagues. Barnes and colleagues published their lengthy defense of the EEA in 2014, which my colleagues and I answered in a 2015 publication

Four Major Problem Areas

I now briefly address four major problem areas in the EEA-test study literature. These problems apply to the more recent EEA-test studies by Dalton Conley and colleagues in 2013, Jacob Felson in 2014, and Nancy Segal and colleagues in 2018. In general, EEA-test researchers:

1)   Arbitrarily evaluate family studies and twin studies very differently, even though both types of studies compare groups experiencing different environments

2)   Assume that most psychiatric genetic and behavioral genetic assumptions, methods, techniques, and diagnoses are valid, when in fact they are controversial

3)   Focus narrowly on selected comparisons, while overlooking the larger picture suggesting that MZ > DZ can be explained by environmental factors

4)   Conduct studies that reproduce the major problems and biases plaguing social and behavioral science research in general

Problem #1. When evaluating family study data, behavioral geneticists drop the "trait-relevant" and "create their own environment" arguments and simply recognize that "family studies by themselves cannot disentangle genetic and environmental influences."[15] This is correct, but because MZ and DZ environments are different, the same conclusion holds true for twin studies. 

Problem #2. Most EEA-test researchers, including Conley, Felson, and Segal, accepted as valid behavioral genetic assumptions, concepts, and methods, even though most are problematic and controversial. These include "heritability estimates," variance-partitioning "model-fitting" techniques (and the many assumptions they are based upon), "general intelligence" (IQ), and "personality." Psychiatric twin studies assume that psychiatric disorders are valid discrete illnesses that can be reliably diagnosed, a position that has been repeatedly challenged by critics. Even leading behavioral geneticists such as Robert Plomin recognize that we will have to "tear up our diagnostic manuals based on symptoms," because "there are no disorders to diagnose and there are no disorders to cure."[16]

Problem #3. Although twin researchers do not recognize it as such, nine decades of twin studies have produced the greatest combined "test" of the EEA ever seen. These studies have consistently shown that twin pairs experiencing similar environments and high levels of identity confusion and attachment—MZs—behave much more alike than do pairs experiencing less similar environments and lower levels of identity confusion and attachment—DZs. The obvious conclusion in this EEA-test study is that MZ > DZ can be explained on environmental (non-genetic) grounds.   

Imagine that researchers conduct studies to test their claim that the sun never shines on New York City. They decide to perform their studies only between the hours of 11:00 pm and 3:00 am, and conclude that their findings support their claim. EEA-test studies are based on a similar folly, because researchers focus narrowly on evidence that they claim supports the EEA, while overlooking or denying the massive body of evidence that contradicts this claim. 

In Segal and colleagues' 2018 EEA-test study based on "genetically unrelated look-alike" pairs, the authors arrived at the astonishing conclusion that physical "appearance is not meaningfully related to personality similarity and social relatedness."[17] (Astonishing because, most often, common sense plus research from other fields provide a much better guide to understanding human beings than provided by dubious behavioral genetic "findings" and "laws.") The study is flawed on numerous grounds, and at best eliminates only one of over thirty environmental factors contributing to above-zero reared-together and reared-apart MZ behavioral correlations. The researchers' claim that their findings support a conclusion that "genetic influences on personality and self-esteem" does not follow from their supposed findings.     

Problem #4. Cognitive neuroscientist Chris Chambers has described several major problem areas in the research/publication process in psychology.[18] One of these is "hidden flexibility," which refers to researchers' ability to change various aspects of their study after reviewing their data, but before submitting their paper for publication. Under the current system in psychology, and presumably in other behavioral science areas as well, undetected "questionable research practices" may be common.[19]

The 2013 EEA-test study by sociologist Dalton Conley and colleagues used the "misclassified twins" method of testing the EEA.[20] Conley and colleagues concluded in favor of "the validity on the equal environment assumption."[21] Like other EEA-test studies, the hidden flexibility problem in behavioral research means that we have no way of knowing whether, after reviewing the data, they conducted and described their study as originally planned. In his 2017 book The Genome Factor, Conley defended the twin method on the basis of Argument A and Argument C.

Research findings and conclusions are often influenced by confirmation bias, which is the tendency for people to search for, interpret, favor, and publish/not publish information in ways that confirm their preexisting beliefs, theories, or professional and financial interests. Most EEA-test studies are subject to strong confirmation bias in the EEA-validating direction (though not Conley's study, according to his account in The Genome Factor). Can we really expect twin researchers to conclude in favor of the abandonment and invalidation of their life's work?

Chambers called for the establishment of research "preregistration," which would require researchers to submit an introduction, and their proposed methods, definitions, and analyses, before they collect data.[22] When in the future preregistration is a requirement in social and behavioral science research, it will become much easier to assess the soundness of this research, including the EEA-test publications  

Conclusion: It's Time to Abandon the Twin Method

MZ pairs behave much more similarly than DZ pairs behave for just about everything—almost everyone now agrees on this point.[23] The key question is what causes this to occur, and I am not aware of any valid argument in favor of a genetic interpretation of MZ > DZ. As political scientist Evan Charney concluded in 2013, "That twin studies generate results that even partisans of the methodology acknowledge as absurd is further evidence that they are to many what they have always seemed to be: an obviously confounded, unreliable methodology."[24]

The twin method is based on the assumption that reared-together MZ and same-sex DZ twin pairs grow up experiencing equal environments, even though most people—including most leading twin researchers—understand that MZ and DZ environments are very different. Instead of abandoning the twin method once unequal environments were acknowledged, as they should have, twin researchers concocted illogical arguments that allowed them to continue their work. Given the major social, political, and scientific implications of the common claim that "most behavioral traits are moderately to highly heritable," it borders on scientific scandal that authoritative social and behavioral science texts, popular books, and media outlets continue to endorse unsupportable genetic interpretations of MZ > DZ.  

Because the classical twin method's equal environment assumption is false, we must reject genetic interpretations of MZ > DZ in behavioral twin studies past, present, and future, and the social and behavioral sciences should abandon this research method without delay.    


[1]  I placed "heritability estimates" in quotation marks because, as a pair of critics wrote, "the term 'heritability,' as it is used today in human behavioral genetics, is one of the most misleading in the history of science." (More on the "heritability fallacy" HERE.)

[2]  Many previously accepted biological and genetic assumptions underlying twin research may not be true, including the assumption that MZ pairs are 100% genetically identical throughout their lives. See Charney, E., (2012), Behavior Genetics and Postgenomics, Behavioral and Brain Sciences, 35, 331-358.

[3]  Alford et al., 2005, Are Political Orientations Genetically Transmitted?, American Political Science Review, 99, 153-167, p. 155.

[4]  Kringlen, E., (1967), Heredity and Environment in the Functional Psychoses: An Epidemiological-Clinical Study, Oslo: Universitetsforlaget, p. 20.

[5]  Barnes et al., (2014), Demonstrating the Validity of Twin Research in Criminology, Criminology, 52, 588-626, p. 597.

[6]  Many more examples of Argument A can be found in Appendix C (pp. 267-272) of Joseph, J., (2015), The Trouble with Twin Studies: A Reassessment of Twin Research in the Social and Behavioral Sciences, New York: Routledge.

[7]  Evans, D. M., & Martin, N. G., (2000), The Validity of Twin Research, GeneScreen, 1, 77-79, pp. 77-78. 

[8]  Reber, A. S., (1985), The Penguin Dictionary of Psychology, London: Penguin, p. 123.

[9] Kendler et al., (1993), A Test of the Equal-Environment Assumption in Twin Studies of Psychiatric Illness, Behavior Genetics, 23, 21-27, p. 21. 

[10]  Gottesman, I. I., (1966), Genetic Variance in Adaptive Personality Traits, Journal of Child Psychology and Psychiatry, 7, 199-208, p. 200.

[11]  By inserting the term "trait-relevant" into the traditional EEA definition, Gottesman overcame the sticky problem he had faced three years earlier in a different twin study of personality. In this 1963 study, Gottesman recognized that the traditional definition of the EEA, which he described as the assumption "that the within-pair environmental variance is the same for the two types of twins," is "not necessarily true for the personality traits as measured by the tests." He nonetheless decided to "proceed only on the assumption that such variance is not too different for the two types of twins."  Although Gottesman’s definition of the EEA changed between 1963 and 1966, the differing environments experienced by MZ and DZ pairs did not. See Gottesman, I. I., (1963), Heritability of Personality: A Demonstration, Psychological Monographs, 77, (9, whole volume 572), 1-21, p. 8. 

[12]  Lyons et al., (1991), "The Genetics of Schizophrenia," in Tsuang et al., (Eds.), Genetic Issues in Psychosocial Epidemiology (pp. 119-152), New Brunswick, NJ: Rutgers University Press, p. 126.

[13]  Interestingly, Argument A potentially supersedes Argument B. This is because, even if critics show that MZ pairs experience more similar trait-relevant environments than experienced by DZ pairs, twin researchers could still argue (and have argued) that MZ pairs "created" or "elicited" these more similar trait-relevant environments.  

[14]  Murray, C., (2020), Human Diversity: The Biology of Gender, Race, and Class, New York: Twelve, p. 217.

[15]  Plomin et al., (2013), Behavioral Genetics (6th ed.), New York: Worth Publishers, p. 191.  

[16]  Plomin, R., (2018), Blueprint: How DNA Makes Us Who We Are, Cambridge, MA: MIT Press, pp. 68, 165.

[18]  Chambers, C., (2017), The Seven Deadly Sins of Psychology: A Manifesto for Reforming the Culture of Scientific Practice, Princeton, NJ: Princeton University Press.

[20]  In 1979, psychologists Sandra Scarr and Louise Carter-Saltzman described the tortuous logic of the "mistaken zygosity" EEA-test method:

"If genetic similarity were the sole determinant of behavioral likeness, then DZ twins who believe themselves to be MZs will be no more alike than other DZs, and MZs who mistake themselves for DZs will be no more different than other MZs. If, however, beliefs about zygosity determine the extent to which cotwins are behaviorally similar, then DZ twins who believe they are MZs will be as similar as true MZs. Likewise, MZs who believe they are DZs will be as different as true DZs."

[21]  Conley et al., (2013), Heritability and the Equal Environments Assumption: Evidence from Multiple Samples of Misclassified Twins, Behavior Genetics, 45, 415-426, p. 415.

[22]  See also Joseph, J., & Baldwin, S., (2000), Four Editorial Proposals to Improve Social Sciences Research and Publication, International Journal of Risk and Safety in Medicine, 13, 109-116. 

[23]  There seems to be no end to far-fetched and even comical "findings" and heritability estimates produced by twin method MZ-DZ comparisons, which use methods similar to those used in psychiatric twin research. Among these we find a twin study whose authors concluded in favor of a genetic basis for choice of news channel, being a "born again Christian," a twin study that found important genetic influences on tea and coffee drinking preferences, a twin study that "found relatively high heritabilities for entrepreneurship," a twin study that found that "drunk-driving is under significant genetic influence," a twin study that found genetic influences on "differences in humor styles," a twin study that found genetic influences on "loneliness in adults" and another than found that "subjective well-being" (happiness) is moderately heritable, a twin study that found genetic influences on "problematic masturbatory behavior," twin studies that found that the "belief in God" is substantially heritable, a twin study that found a substantial genetic component in British election voting choices, and a twin study of the "frequency of female orgasm." 

[24]  Charney, E., (2013), Nature and Nurture, [Review of the Book Man Is by Nature a
Political Animal, by P. Hatemi & Rose McDermott (Eds.)], Perspectives on Politics, 11, 558-561, p. 560.   


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