The Trouble with Twin Studies


As most readers are aware, it is widely believed that both within and without of psychiatry genetic factors play an important role in causing major psychiatric disorders such as schizophrenia, bipolar disorder, major depression, ADHD, autism, anxiety, and even post-traumatic stress disorder (PTSD). Twin studies provide the main pillar of support for this belief which is often, though mistakenly, presented as a scientific fact.

The mainstay of twin research is the “classical twin method,” developed in the 1920s, in which researchers compare the trait resemblance of reared together identical twin pairs (MZ, monozygotic) versus reared together same-sex fraternal (DZ, dizygotic) twin pairs. If identical pairs (who share a 100% genetic similarity) resemble each other more than fraternal pairs (who share an average 50% genetic similarity) for the disorder or trait in question, twin researchers conclude that the disorder or trait has an underlying genetic component. They arrive at this conclusion on the basis of several theoretical assumptions about twins, the most important and controversial of which is the assumption that identical and same-sex fraternal twin pairs grow up experiencing roughly equal environments. This is known as the “equal-environment assumption” (EEA) of the twin method. Psychiatric twin studies are based on twin pairs reared together in the same home, and in most cases identical pairs resemble each other more for psychiatric disorders and behavioral traits than do fraternal pairs.

Although the authors of psychiatry and psychology textbooks and other mainstream publications usually endorse genetic interpretations of psychiatric twin studies uncritically, there is a fatal flaw underlying these studies: identical twin pairs grow up experiencing much more similar environments than experienced by same-sex fraternal pairs, meaning that the equal environment assumption — upon which all conclusions in favor of genetics are based — is false. Therefore, many critics have argued that it is likely that identical-fraternal comparisons capture nothing more than identical pairs’ more similar treatment, greater environmental similarity, stronger attachment and emotional bond, and greater levels of identity confusion (feeling like they are two halves of the same whole).

Remarkably, since the 1960s most leading twin researchers have conceded the point that identicals experience more similar environments than fraternals. However, instead of relegating the twin method to a place alongside other discarded pseudosciences, twin researchers have attempted to validate the twin method by subtly redefining the equal environment assumption (EEA). The main way they have done this has been to argue that although identicals do indeed experience more similar environments than fraternals, identical pairs “create” or “elicit” more similar environments for themselves because they are more similar genetically.1 However, this “twins create their own environment” argument is a circular one, because twin researchers’ conclusion that identical pairs behave more similarly because they are more similar genetically is based on the assumption that identical pairs behave more similarly because they are more similar genetically. This means that twin researchers’ position that genetic factors explain the greater behavioral resemblance of identical twin pairs is, illogically, both a conclusion and a premise of the twin method. In defending the validity of the twin method, modern twin researchers refer to the premise in support of the conclusion, and then refer back to the conclusion in support of the premise, in a continuously circular loop of faulty reasoning.

Another argument twin researchers put forward in support of the equal environment assumption is that, while again conceding that identicals grow up experiencing more similar environments than fraternals, in order to invalidate the EEA it must be shown that identical and fraternal environments differ in aspects that are relevant to the trait in question. This is known as the “trait-relevant” definition of the equal environment assumption. (For example, witnessing trauma is a trait-relevant environmental factor contributing to PTSD.) However, even if it is shown that identical pairs experience more similar trait-relevant environments than fraternal pairs, twin researchers could still argue (and have argued) in favor of the validity of the twin method and the equal environment assumption by reverting to their previous circular argument that identical pairs “create” or “elicit” more similar “trait-relevant” environments for themselves because they are more similar genetically.2

In addition, there are many other methodological problems and unsupported assumptions associated with the twin method, which critics have described since its development nearly 100 years ago.3

The question of whether the twin method is a valid instrument for the detection of genetic influences comes down to two basic positions, which have been put forward by twin researchers and their critics respectively: (1) based on the acceptance of the equal environment assumption, the greater behavioral similarity of identical versus fraternal twin pairs demonstrates that important genetic influences underlie variation in most human behavioral traits; or (2) based on the rejection of the equal environment assumption, the greater behavioral similarity of identical versus fraternal twin pairs is caused by non-genetic influences.

The bottom line is this: despite being cited in countless textbooks, scholarly journal publications, and popular books and articles, the little-disputed finding that identical pairs experience much more similar environments than fraternal pairs means that non-genetic factors plausibly explain twin method results. The fact that psychiatric twin studies continue to be cited in support of genetics, largely uncritically, speaks volumes about the scientific status of psychiatry in the 21rst century. Psychiatry’s acceptance of twin studies is even more remarkable in the context of the decades-long failure of molecular genetic research to uncover genes that investigators believe cause psychiatric disorders (see my February 15th MIA posting)—research that is based largely on genetic interpretations of the results of psychiatric twin studies.

In 2011, journalist Brian Palmer wrote an article in an edition of the U.S.-based online magazine Slate, entitled “Double Inanity: Twin Studies are Pretty Much Useless.”4 This article, which as its title suggests was critical of twin research, produced over one hundred comments, mainly in opposition to what Palmer wrote. Although Palmer’s conclusion that the assumptions underlying the twin method are “deeply flawed” is accurate, his article had some glaring holes in it, which opened the field for the critics.

Twin researcher Nancy L. Segal and her colleagues, including behavioral geneticist Eric Turkheimer and longtime twin researchers Irving Gottesman and Nicholas Martin, provided their own response to Palmer in an online comment entitled “The Value of Twin Studies: A Response to Slate Magazine.” Segal and colleagues wrote that Palmer’s article was “one of the most inaccurate, misleading and uninformative essays on twin research to come along in a while.” Regardless of their opinion of the article, it doesn’t necessarily follow that Palmer’s conclusion that twin studies are “useless” for detecting genetic influences is wrong. Segal and colleagues referred to themselves as “scientists who have been involved in twin research for decades,” and signed their response using the title “Dr.” before their names, invoking the authority of Ph.D. level experts in twin research and implying that they deserve to have the authoritative last word on the validity of the twin method. “The Value of Twin Studies” was subsequently reposted in Segal’s Psychology Today “Twofold” blog, and was published in the journal Twin Research and Human Genetics with a some additional comments by Segal.5

Segal and colleagues attempted to answer some of Palmer’s main points while presenting twin researchers’ typically weak arguments in support of the equal environment assumption and the twin method, based on how the EEA supposedly has been tested and validated by other twin researchers. The flaws of Palmer’s article provided Segal and colleagues with an opportunity to successfully counter some of his points, but in general their argument was largely based on the implied authority of these eminent twin researcher co-authors. However, appeals to authority of this type do not validate twin research any more than the existence of heaven and hell would be validated by an open letter authored by the College of Cardinals proclaiming that they exist.

After reprinting “The Value of Twin Studies” in her Twin Research and Human Genetics publication, Segal mentioned two articles by a pair of critics “unsympathetic to twin research.”6 She wrote that “such work should not go unanswered by our membership,” referring to the International Society for Twin Studies (ISTS). Despite calling on her colleagues to answer the critics, in practice Segal and colleagues grabbed the low hanging fruit of a journalist’s weak attempt to critique twin research, while continuing to largely ignore or dismiss detailed scholarly critical assessments of twin research by others. Although Segal and other twin researchers at times discuss selected arguments by “critics,” they often fail to answer detailed arguments by specific critical experts.7

During the past 15 years, in two books and several peer reviewed articles and chapters, I have described the methodological errors and false theoretical assumptions underlying twin research.8 Segal is well aware of my work, having referred to me by name in her most recent book as a “harsh critic of behavior genetics” and citing some of my publications.9 And in a different publication Turkheimer referred to a textbook chapter I wrote in 2010 that included a detailed critique of the twin method and its assumptions.10

Even in the context of the responses to Palmer’s Slate article, Segal failed to answer serious criticism of the twin method and the equal environment assumption. Following Palmer’s article, I posted several online comments outlining the main problems with twin studies, as did psychologist Ken Richardson. Following this, Segal posted “The Value of Twin Studies,” which failed to specifically address these points. Segal and colleagues took offense to Palmer’s (overstated) claim that “One of the main messages of science over the last couple of decades is that genes are destiny.” They angrily asked, “Who exactly is supposed to have said this?” In response, I quoted four leading genetic researchers who did make published statements implying that genes are all-important, including Nobel Laureate James D. Watson’s 2003 forward to a behavioral genetics textbook published by the American Psychological Association, where he wrote, “The older one gets…the more most of us conclude that children come into the world with fixed personalities that are hard to ascribe to specific home or school environmental influences. Particularly happy children almost seem to be born that way.” There were no responses to my postings by Segal or her colleagues. This prompted commenter (and MIA blogger) Jonathan Leo to write,

“This is a great discussion, but I think it is only fair that once a researcher like Dr. Segal decides to enter a discussion like this, especially when they have very harsh words for those who disagree with her, that it is only proper that they also reply to those, like Jay Joseph, who point out the problems in their logic. Take Dr. Segal’s first point that the idea that genes are destiny is ‘a grotesque caricature of genetic research in general and twin studies in particular.’ Joseph pretty much shows that this is not at all a caricature and that indeed there are prominent researchers who do have this view. If researchers [such as Segal, Turkheimer and colleagues] are going to come down from the ivory tower and take pot shots, then they should also be willing to take the heat and defend their ideas when someone as knowledgeable as Joseph also enters the discussion.”

Yet Segal and the others did not respond, even after Jonathan Leo publically challenged them to do so.

Following Segal’s reposting of “The Value of Twin Studies” in her Psychology Today blog, on September 7th, 2011 Ken Richardson and I posted a response where we highlighted the main arguments against the equal environment assumption, including the position that the twin method is based on a circular argument. Neither Segal nor any other twin researcher responded to our posting, which as I mentioned appeared in connection to her own Psychology Today blog. This failure to respond flies in the face of Segal’s position that such criticism “should not go unanswered” by twin researchers, and suggests that she has few convincing arguments with which to respond. In fact, I am unaware of any twin researcher attempting to argue against the published position that the twin method is based on circular reasoning. Nevertheless, Segal and her colleagues continue to uphold the validity of the twin method.

It is clear that the twin method is no more able to disentangle the potential roles of genetic and environmental influences on behavioral traits and disorders than is the finding from family studies that such traits and disorders tend to “run in the family.” Given that both research methods are confounded by environmental factors, the results of family studies and studies using the twin method should be evaluated in exactly the same way, leading to the conclusion that neither provides scientifically acceptable evidence in support of genetic influences on psychiatric disorders and behavioral traits. The acceptance of this conclusion does not mean, as twin researchers sometimes write, that the twin method might “overestimate heritability,” or that the equal environment assumption should be tested “trait-by-trait,” but rather that genetic interpretations of all twin method data produced by psychiatry and psychology, and social science fields such as political science and economics, must be rejected outright. It follows that the claim that genes play a significant role in explaining behavioral differences in these fields must undergo a thorough public reexamination.

In the spirit of her stated desire to answer the works of twin research critics, I encourage Nancy Segal and other twin researchers to respond to each and every major point that I and other critics have raised, and to engage in an ongoing public discussion of whether genetic interpretations of twin method findings should be accepted or rejected.


1 For example, see Kendler, K. S. (1983). Overview: A Current Perspective on Twin Studies of Schizophrenia. American Journal of Psychiatry, 140, 1413-1425; see also Joseph, J. (2012). The “Missing Heritability” of Psychiatric Disorders: Elusive Genes or Non-Existent Genes? Applied Developmental Science, 16, 65-83

2 Examples of twin researchers arguing in support of the equal environment assumption and the twin method by claiming that identicals’ more similar “trait relevant” environments are elicited by their greater genetic resemblance include Derks et al. (2006). A Test of the Equal Environment Assumption (EEA) in Multivariate Twin Studies. Twin Research and Human Genetics, 9, 403-411; Smith et al. (2012). Biology, Ideology, and Epistemology: How Do We Know Political Attitudes are Inherited and Why Should We Care? American Journal of Political Science, 56, 17-33; True et al. (1993). A Twin Study of Genetic and Environmental Contributions to Liability for Posttraumatic Stress Symptoms. Archives of General Psychiatry, 50, 257-264.

3 Political scientist Evan Charney argues that many previously accepted biological and genetic assumptions underlying twin research may not be true, which is “necessitating a rethinking of every one of the assumptions of the classical twin study methodology.” See Charney, E. (2012). Behavior Genetics and Postgenomics. Behavioral and Brain Sciences, 35, 331-358; quotation from Charney, E., & English, W. (2012). Candidate Genes and Political Behavior. American Political Science Review, 106, 1-34.

4 Palmer, B. (2011, August 24th). Double Inanity: Twin Studies are Pretty Much Useless. Slate. Retrieved from

5 Segal, N. L. (2011). The Value of Twin Studies: A Response to Slate Magazine / Research Reviews / Twin News Worth Noting. Twin Research and Human Genetics, 14, 593-597.

6 The twin research critics Segal mentioned were W. Joseph Wyatt and Donna M. Midkiff. See Wyatt, W. J., & Midkiff, D. M. (2006). Biological Psychiatry: A Practice in Search of a Science. Behavior and Social Issues, 15, 132–151; Wyatt, W. J., & Midkiff, D. M. (2007). Psychiatry’s Thirty-Five Year, Non-Empirical Reach for Biological Explanations. Behavior and Social Issues, 16, 197–213.

7 Segal, like most leading twin researchers, has put forward the “twins create their own environment” defense of the twin method. For example, in 2009 she wrote, “It is important to note that if MZ twins are treated more alike than DZ twins, it is most likely associated with their genetically based behavioral similarities.” See Segal, N. L., & Johnson, W. (2009). Twin Studies of General Mental Ability. In Y. Kim (Ed.), Handbook of Behavior Genetics (pp. 81-99). New York: Springer.

8 Critical analyses of twin research that I have published since 1998 include: Joseph, J. (1998). The Equal Environment Assumption of the Classical Twin Method: A Critical Analysis. Journal of Mind and Behavior, 19, 325-358; Joseph, J. (2002). Twin Studies in Psychiatry and Psychology: Science or Pseudoscience? Psychiatric Quarterly, 73, 71-82; Joseph, J. (2004). The Gene Illusion: Genetic Research in Psychiatry and Psychology under the Microscope. New York: Algora. (2003 United Kingdom Edition by PCCS Books); Joseph, J. (2006). The Missing Gene: Psychiatry, Heredity, and the Fruitless Search for Genes. New York: Algora;  Joseph, J. (2010). Genetic Research in Psychiatry and Psychology: A Critical Overview. In K. Hood et al. (Eds.), Handbook of Developmental Science, Behavior, and Genetics (pp. 557-625). Malden, MA: Wiley-Blackwell; Joseph, J. (2012), “Missing Heritability” (see Footnote 1); Joseph, J., & Ratner, C. (2013). The Fruitless Search for Genes in Psychiatry and Psychology: Time to Re-examine a Paradigm. In S. Krimsky & J. Gruber (Eds.), Genetic Explanations: Sense and Nonsense (pp. 94-106). Cambridge, MA: Harvard University Press; Joseph, J. (forthcoming). The Use of the Classical Twin Method in the Behavioral Sciences: The Fallacy Continues. Journal of Mind and Behavior.

9 Segal, N. L. (2012). Born Together—Reared Apart: The Landmark Minnesota Twin Study. Cambridge, MA: Harvard University Press.

10 Joseph, J. (2010), “Genetic Research in Psychiatry and Psychology” (see Footnote 8); Turkheimer, E. (2011). Still missing. Research in Human Development, 8, 227-241.

11 Watson, J. D. (2003). A Molecular Genetics Perspective. In R. Plomin et al. (Eds.), Behavioral Genetics in the Postgenomic Era (pp. xxi-xxii). Washington, DC: American Psychological Association Press.

12 In psychology, so-called studies of “twins reared apart” are frequently cited in support of major genetic influences on traits such as IQ and personality. Over the years, however, critics have pointed to many potentially invalidating problems and biases found in these studies. See Joseph, J. (2001). Separated Twins and the Genetics of Personality Differences: A Critique. American Journal of Psychology, 114, 1-30; Joseph, J. (2004), The Gene Illusion (referenced in Footnote 8); Joseph, J. (2010), “Genetic Research in Psychiatry and Psychology” (referenced in Footnote 8).


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.


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  1. Thanks for this very informative article.

    A lot of heritability talk is based on simple causality reasoning (and the assumption that genetics and environment contributions are overall independent/additive) that seems useless in discussing outcomes as complex as psychiatric disorders.

    I like the following thought experiment: let say in XVIII century america, an anonymous “black” newborn and an anonymous “white” newborn are discovered the same day at the door of the main house of a traditional plantation. One baby become a slave, the other become a plantation owner. Is that outcome a result of the genes determining their skin color, or is it the result of an environment that discriminates based on skin color? Would anyone suggest we dismiss the differences in the environment they got confronted with based on the fact those environmental differences were “elicited” by their genes?

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  2. Thank you for this article Jay Joseph

    The same cicular logic still rules psychiatry, where if the patient is not displaying any bad behaviour then the patient is in remission, but the reason the patient is not displaying any bad behaviour is because the patients brain has been stopped from functioning by the psychiatric medicines.
    Dr Nancy Anderson in the New York Post tells of the self fulfilling prophesy of brain shrinkage.

    Why is the patient behaving badly? Maybe because you messed with their normal brain chemistry doctor? Shrunk the patients brain, turned down their ability to BE intelligent.

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  3. Genes – one factor.
    Gene *expression* – a much larger *collection* of factors.

    At the end of the day, we’re talking about thoughts, feelings, relationships, spiritual matters.

    So, all of this becomes very complex.
    In fact, perhaps *impossible* to address medically or even scientifically…

    Which leads to the topic of *recovery* – probably best left as an *individualized* experience – unique for each person who experiences it.


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  4. This a really great essay Jay, highlighting how each of us will tend towards false assumptions when we fail to pause and feel the impulse to our rationalizations? The need for self-empowerment and its manifestation as critical thinking in our 1st world cultures and the fine line between critique and criticism.

    Just how much do we all fail to feel the power of now and autonomically function with an “expectation,” that produces its predictable observation bias? A biased product of remaining trapped in the mind-body split of critical thinking, and our false, “educated” assumptions about our own level of self-awareness?

    Do we act with a “look how much I remember from what I’ve been taught,” in a schooled sense of who I am? From an education system which never helped us find an inner sense of how we actually function? In the so-called developed world have we produced an over developed, “left-brained” sense of self? An “I think, therefore, I know myself?” Please consider;

    “Offering data at the neuropsychological, cultural, and historical levels, McGilchrist (2009) echoes this principle:

    “If what one means by consciousness is the part of the mind that brings the world into focus, makes it explicit, allows it to be formulated in language, and is aware of its own awareness, it is reasonable to link the conscious mind to activity almost all of which lies ultimately in the left hemisphere” (p. 188).

    He adds, however, “The world of the left hemisphere, dependent on denotative language and abstraction, yields clarity and power to manipulate things that are known, fixed, static, isolated, decontextualized, explicit, disembodied, general in nature, but ultimately lifeless” (p. 174). In contrast, “the right hemisphere … yields a world of individual, changing, evolving, interconnected, implicit, incarnate, living beings within the context of the lived world, but in the nature of things never fully graspable, always imperfectly known— and to this world it exists in a relationship of care” (p. 174). Indeed, the “emotional” right hemisphere “has the most sophisticated and extensive, and quite possibly most lately evolved, representation in the prefrontal cortex, the most highly evolved part of the brain” (p. 437).”

    Excerpts from “The Science of the Art of Psychotherapy” by Allan N. Schore.

    Another researcher, writer who takes a more “holistic” approach to the human condition within and try’s to see beyond the false assumptions of specialization, is Jakk Panksepp. Who points us towards our “observation bias” from the inside-out, so to speak. Please consider what he has to say about our tendency to rationalize our internal needs and ignore discoveries which challenge our left-brained love affair with our intellectual rationalizations, or our “cognitive constructs” as others call them;

    “SEEKING: Systems & Anticipatory States of the Nervous System:


    The Seeking System: Like other emotional systems, arousal of the seeking system has a characteristic feeling tone– a psychic energization that is difficult to describe but is akin to that invigorated feeling of anticipation we experience when actively seeking thrills and other rewards. Clearly this type of feeling contributes to many distinct aspects of our active engagement with the world.

    This harmoniously operating neurochemical system drives and energizes many mental complexities that humans experience as persistent feelings of interest, curiosity, sensation seeking, and in the presence of a sufficiently complex cortex, the search for higher meaning. Although this brain state, like all other basic emotional states, is initially without intrinsic cognitive content, it gradually helps cement the perception of causal connections in the world and thereby creates ideas. It appears to translate correlations in environmental events into perceptions of causality, and it may be a major source of “confirmation bias,” the tendency to selectively seek evidence for our hypotheses.

    When this seeking system is manipulated by electrical impulse in other mammals, they will eagerly continue to “Self-Stimulate” for extended periods, until physical exhaustion and collapse set in. There are powerful descending components, probably glutametergic in part, that remain to be functionally characterized, but they may be important for the generation of self-stimulating behaviors. When these descending systems are fully characterized, they may have powerful implications for understanding such psychiatric disorders as schizophrenia.

    1, The underlying circuits are genetically pre-wired and designed to respond unconditionally to stimuli arising from major life-challenging circumstances. 2, The circuits organize behavior by activating or inhibiting motor sub-routines (and concurrent autonomic-hormonal changes) that have proved adaptive in the face of life-challenging circumstances during the evolutionary history of our species. 3, Emotive circuits change the sensitivities of sensory systems relevant for the behavior sequences that have been aroused. 4, Neural activity of emotive systems outlasts the precipitating circumstances. 5, Emotive circuits come under the control of neutral environmental stimuli. 6, Emotional circuits have reciprocal interactions with brain mechanisms that elaborate higher decision-making processes and consciousness.

    It is remarkable how long it has taken psycho-biologists to begin to properly conceptualize the function of the self-stimulation system, in the governance of behavior. The history of this field highlights how an environmental-behavioral bias (world out there), with no conception of internal brain functions, has impeded the development of compelling psycho-behavioral conceptions of self-stimulation. One of the most fascinating phenomena ever discovered, yet still largely ignored by mainstream psychology.

    The prevailing intellectual zeitgeist is not conducive to conceptualizing this process in psychological terms. This would involve discussion of the inner neurodynamic aspects of the “mind” and the nature of intentionality and subjective experience. A neurophysiological understanding of such brain systems can explain how we spontaneously generate solutions to environmental challenges. And how this type of spontaneous associative ability characterizes normal human thinking, as well as the delusional excesses of schizophrenic thinking.

    Arousal of the seeking system spontaneously constructs causal “insights” from the perception of correlated events. Some of the relationships may be true, but others are delusional. Indeed, all forms of inductive thought, including that which energizes scientific pursuits, proceed by this type of logically flawed thinking. An intrinsic tendency for “confirmation bias” appears to be a natural function of the human mind.

    The seeking system can promote many distinct motivational behaviors, and the underlying neural system is prepared to jump to the conclusion that related environmental events reflect causal relationships. It is easy to appreciate how this may yield a consensual understanding of the world when the underlying memory reinforcement processes are operating normally ( i.e, yielding a reality that most of the social group accepts). It is also easy to understand how it might yield delusional conclusions about the world. If this self-stimulating system is chronically overactive, it may be less constrained by rational modes of reality testing.

    The fact that the system is especially responsive to stress could explain why paranoid thinking emerges more easily during stressful periods, and why stress may promote schizophrenic thinking patterns. If the normal function of this system is to mobilize the organism for seeking out resources in the world, then we can begin to appreciate how the seeking system might also generate delusional thoughts. Apparently when this emotional system is over-taxed and becomes free-running (self-stimulation), it can generate arbitrary ideas about how world events relate to internal events.

    Is delusional thinking truly related to the unconstrained operation of spontaneously active associative networks of a self-stimulating, seeking system? If so, we may have a great deal more to learn about schizophrenia from a study of the SEEKING circuits that mediate self-stimulating behavior? Through a study of this system, we can also begin to understand the natural eagerness that makes us the emotionally vibrant creatures we are.

    One might also predict that there is an intimate relationship between self-stimulation and dreaming. REM deprivation leads to increased “sensitivity” in the self-stimulation system It is noteworthy that schizophrenics fail to exhibit compensatory elevations of REM sleep following imposed periods of REM deprivation. There appears to be a fundamental relationship between the schizophrenic process and the emotional discharge that occurs during both REM sleep and the seeking system discharge of self-stimulation. These findings suggest that there may yet be considerable substance to psychodynamic theories that relate dreaming mechanisms to symbol-&-reality-creating mechanisms of the brain.”

    Excerpts from, “Affective Neuroscience: The Foundations of Human and Animal Emotions.” by Jaak Panksepp.

    Panksepp suggests that if we can accept this stress sensitized self-stimulation system as fundamentally a SEEKING system, which requires discharge, new ways of alleviating delusional thinking may be created to provide discharge, while stimulating reality testing, perhaps via computer games for example?

    Its interesting how we love to study the world “out there” and of coarse, other people, yet fiercely avoid studying our own sense-of-self? Perhaps we’re afraid of discovering what we really are, having never been taught how to sense our internal stimulation, in our 1st world sense of education?

    As people sight these neuroscience references, will they pause to feel the “impulsed” reaction within, which stimulates our self-empowerment needs, and our mind’s love affair with its self-stimulating rationalizations?

    Best wishes to all,

    David Bates.

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  5. Jay,

    Another great post; thank you for the scientific truth!

    I hope you have a chance to provide an overview of some of the more famous twin studies for this website; their poor scientific methodology and confirmation bias are palpable.

    Speaking of a confirmation bias, I think that the affect of a belief in behavioral genetics shared by most identical twins is vastly underrated.

    Best wishes, Steve

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    • Thanks, Steve. Which twin studies did you have in mind? If they are studies of twins reared together (the twin method), then the main thing is that they all rest on the unsupported (false) assumption that identical and fraternal twins grow up experiencing equal environments. Although most studies using the twin method have many other methodological problems, I focus mainly on the equal environment issue. If you are thinking of so-called studies of “reared apart” twins, such as the Bouchard Minnesota studies, that is another matter with another set of issues. I plan to address problems and biases related to those studies in a future posting.

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      • Can you please link me to a post/book/article where you discuss the issue with reared apart twins? This type of design is certainly not subject to the MZ-DZ equal environment assumption. What is this other set of issues that you talk about?

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        • Hi. My first article on reared-apart twin studies appeared in 2001. It can be downloaded from the Publications page at my website. I also covered the topic in Chapter 4 of my 2004 book The Gene Illusion.

          But by far the most in-depth analysis of the many problems and biases in reared-apart twin research is found in my new book, The Trouble with Twin Studies: A Reassessment of Twin Research in the Social and Behavioral Sciences. For more information on this book, including a link to chapter summaries, please see

          I am thinking of posting on this topic in the near future.

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      • Hi Jay,

        Is your argument on the flaw of twin studies based on the EEA? I am under the assumption that twin researchers assess environment not just as shared but as non-shared environment. In actuality, non-shared environment has been well-established even within previously assumed ‘shared’ environmental factors such as parental negativity. The EEA surely suggests that the effect of twinning (which may elicit similar environments such as an evocative genotype-environment correlation) does not account for variance between MZ and DZ twins? So differences between MZ twins are equated to non-shared effects, within the family and without, and similarities are genetic and shared environmental factors. Twins reared separately shows us that the importance of genetics and the lack of importance of shared environment in that it matters little. Or is the EEA here still infringed?

        Thanks, Fred.

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  6. Tell me if I’m wrong, but with the evidence they present, and the logic with which they present it; one could simply replace “schizophrenia” with “poverty” and you could supposedly conclusively prove that “poverty” is a result of bad genes. And if it were considered politically correct, it would be accepted.

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    • That is the implicit message of a lot of genetic research, except that instead of poverty, they study “intelligence,””personality traits,” “antisocial behavior,” and so on. The false implication of this research is that poor and working class people have fewer genes for high intelligence, and more genes for bad personality traits, and so “science” seems to show that people are poor for hereditary reasons. Such pseudoscientific claims have a long history. A good starting point would be Allan Chase’s 1977 “The Legacy of Malthus.”

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      • Yes thanks, I’ve taken the trouble to get the book and the video in my hands.

        I had another question or maybe statement. These studies have always been plagued by such fabrications about environment. I also seem to remember reading that Ernst Rudin or his students, in order to get the numbers that proclaim that identical twins get “schizophrenia” … they actually counted the twins twice. That is, an identical pair of twin’s and their twin were each counted as having schizophrenia. Thus A had twin B and B had twin A; which they added up as A plus twin adds up to two and B plus twin adds up to two more. And they counted this as four…

        I’m sorry I can’t find a reference to that anymore. I think I read that somewhere, as well.

        There also was a very popular video circulating around (a few years ago or so) pointing out that when you increase someone’s salary it can promote better work if it’s manual labor, but this doesn’t promote better work or a better a product if it’s mental work.

        I wish I could find that video again, but I can’t today.

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        • Hi, the method you mentioned to calculate twin concordance is still widely used. It is called the “probandwise” concordance rate method. The probandwise method of double counting does not create concordance where there is none, but it serves to inflate concordance rates calculated as a simple percentage, which is known as the “pairwise” rate.

          Since you mentioned the German psychiatric geneticist Ernst Rüdin, you may be interested in an article about him that I co-wrote that has recently been published. The URL is

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          • Jay,

            Thanks for this great informative article as usual.

            I am also thrilled about the article you wrote about Ernest Rudin and his evil contributions to psychiatry’s vile eugenics/euthanasia agenda before and after Hitler rose to power. This is the same agenda of the current 1% global robber barrons with its bogus DSM eugenics lies and fraud to push poison drugs, ECT and other deadly torture treatments on a brainwashed public to cull the planet.

            I’m thrilled to see that you are pressuring psychiatry and other Rudin eugenics supporters or deniers to admit his sordid history as the real instigator of the gassing to death of those stigmatized by psychiatry as mentally ill in Germany used as practice for the NAZI Holocaust.

            With your extensive background, it would be great if you like Robert Whitaker could make the connection between psychiatry’s bogus, deadly eugenics theories that led to most if not all human holocausts or so called ethnic cleansings and the equally bogus, deadly biopsychiatry practiced today in the guise of genetic advances that employs the same eugenics pseudoscience and euthanasia goals that those in power judge inferior.

            Dr. Peter Breggin wrote an excellent article about these issues and makes a clear connection of biopsychiatry’s past and current eugenics predation on humanity I’ve cited elsewhere that you may have read.

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          • I finally looked this up, The “probandwise” concordance method. What this does is actually count both twins who have a concordance with schizophrenia as two pairs, but the one where only one ends up being schizophrenia (although both have a pair that they are not concordant with), this is then only counted as one. Although it remains the same. Both pairs are either concordant with each other (both have the disease) or aren’t concordant (don’t both have the disease).

            I don’t know how that can pass as science, actually, except it’s convoluted, and sounds complicated enough that people will believe it’s amazing. In reality, I think that this might point out that genetic research is quite flawed, white often.

            And then there’s theories like Bruce Lipton’s, which point out we aren’t controlled by are genetics the way we are told, and that we are rewriting our genes all the time, depending on how we respond to our environment. Whether we respond with fear and shut down our system using the flight or run response, or whether we respond with empathy, forgiveness, understanding, perspective; something beyond what we have been taught form is.

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  7. Another great post and a very accurate one at that. I remember sitting in a psychology lecture on twin stuides years ago and questioning this raised together, means everything is genetic, to be told, it has to, it can’t possibly be anything else, as the environment is always the same.

    It simply did not add up for me, as I had two friends who were identical twins, and while they were raised together, there were way too many differences in there environments to say they were the same. They were in different classes at school for one. Even if in the same class, one would be sitting in one seat, another in another seat. They lived with a severly disabled sibling. The one with the bedroom right near that sibling had more impact from that sibling. And while the parents did at one stage change the rooms over regularly for that reason, they felt it was the wrong thing to do long term. And these are just the most logical ones. One could go on and on about all the small tiny differences that occured in there lives, even though they were reared in the same house by the same parents.

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  8. It’s great to criticise studies and paradigms (more criticism is needed in science!) but if you’re keen to stimulate a true discussion (with no presumptions on what’s right and wrong – after all this is science, there is no simple right and wrong) then I suggest more evidence-base and proportion (on both sides of the argument). It’s sad to read such an undifferentiated and dogmatic article, especially by someone who has been working in academia and should be aware that science is no black-or-white business. Of course being differentiated doesn’t sell as well. If a method has flaws (which I would argue is the case for all methods ever used in science, even in the supposedly “exact” sciences!), that doesn’t mean it has to be abandoned completely. Flaws should be attended to; where possible, assessed and an effort should be made to improve the method. If a whole method would be ditched because it has flaws, there simply wouldn’t be any science. I doubt that’s what you’re after.

    A contribution to the discussion:

    “The equal environment assumption across zygosities assumes that environmentally caused similarity is roughly the same for both types of twin pairs reared in the same family. This assumption is the most basic assumption of the twin method and has been the subject of great debate over the years. It is generally agreed that MZ and DZ twins do share their environment to the same extent in many respects: they
    share the womb at the same time, are exposed to the same environmental factors, are raised in the same family and are the same age. However, there is also some evidence that MZ twins are treated more similarly by their parents and have more frequent contact as adults than DZ twin pairs.(13)

    • Implications: more similar treatment of MZ twins will increase their correlations relative to DZ correlations, which can result in an overestimation of the genetic effect and an underestimation of the shared environmental effect. (Note: there are also factors that can have the opposite
    effect and increase variability between MZ twins. One example is when MZ twin pairs are forced to attend different classes at school, while DZ twins are allowed to remain in the same class. This could lead to an underestimation of the genetic effect.)

    • How do we detect this effect?If parental treatment is more similar for MZ twins, than DZ twins who are mislabelled as MZ twins should be more alike than correctly labelled DZ twins and conversely, MZ twins mislabelled as DZ should be less alike than correctly labelled MZ twins. Little or no effect of mislabelling was found. The effect of degree of contact among twins showed that more frequent contact does not lead to behavioural similarity in same-sex DZ or MZ twins. While in some cases MZ twins in frequent contact were more similar than those with less contact, these correlations tended to be small (16). Another argument in defence of the equal environments assumption is the fact that studies of MZ twins reared apart have provided correlations for personality variables that are almost the same as those for MZ twins reared together (17)

    (13) Plomin R., DeFries, J. C., McClearn, G. E. and McGuffin, P. (2001), ‘Behavioral Genetics’, 4th edn, Worth Publishers, New York

    (16): Kendler, K. S; Heath, A., Martin, N. G. and Eaves, L.J. (1986), ‘Symptoms of anxiety and depression in a volunteer twin population: The etiologic role of genetic and environmental factors’,Arch. Gen. Psych., Vol. 43, pp. 213–221.

    (17) Bouchard, T. J. and McGue, M. (1990), ‘Sources of human psychological differences: the Minnesota study of twins reared apart’, Science, Vol. 268, pp. 223–228

    All taken from “Analytic approaches to twin data using structural equation models” by Rijsdijk et al. (2002).

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    • “However, there is also some evidence that MZ twins are treated more similarly by their parents and have more frequent contact as adults than DZ twin pairs.(13)”

      This is a rather blanket reference, any particular data from this book in regards to sharing similar environments? Jay Joseph mentions that a 1967 schizophrenia twin study by Kringlen showed that over 90% of MZ experience “identity confusion” during childhood (only 10% for DZ), 72% of MZ were “brought up as a unit” vs 19% of DZ, and 65% of MZ had an “extremely strong” level of closeness vs only 19% for DZ. Jay Joseph also mentions that “I’ve never seen these results discussed by any twin researcher other than Kringlen.” (1)

      “Note: there are also factors that can have the opposite
      effect and increase variability between MZ twins. One example is when MZ twin pairs are forced to attend different classes at school, while DZ twins are allowed to remain in the same class. This could lead to an underestimation of the genetic effect.”

      The magnitude of effect this example is paltry compared to the marked effects of similarity noted in the Kringlen reference.

      “How do we detect this effect?If parental treatment is more similar for MZ twins, than DZ twins who are mislabelled as MZ twins should be more alike than correctly labelled DZ twins and conversely, MZ twins mislabelled as DZ should be less alike than correctly labelled MZ twins. Little or no effect of mislabelling was found.”

      This confuses me, when would we “mislabel” MZ twins? It’s pretty apparent they’re identical. Have you read the reference so that you can expound on the above statement?

      “The effect of degree of contact among twins showed that more frequent contact does not lead to behavioural similarity in same-sex DZ or MZ twins. While in some cases MZ twins in frequent contact were more similar than those with less contact, these correlations tended to be small (16). Another argument in defence of the equal environments assumption is the fact that studies of MZ twins reared apart have provided correlations for personality variables that are almost the same as those for MZ twins reared together (17)”

      Jay Joseph has thoroughly gone over the many limitations of MZ twins reared apart research and has an extensive critique of the Bouchard study cited above at reference (2)

      1) Joseph, J. Twin Studies in Psychiatry and Psychology: Science or Pseudoscience. 2002.

      2) Joseph, J. Separated Twins and the Genetics of Personality Differences: A Critique.
      American Journal of Psychology, 114, 1-30. 2001.

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  9. Hello Jay, congratulations for the post!

    I would like to ask you something. When you say undoubtely “there is a fatal flaw underlying these studies: identical twin pairs grow up experiencing much more similar environments than experienced by same-sex fraternal pairs, meaning that the equal environment assumption — upon which all conclusions in favor of genetics are based — is false”, could you give us some key references of this?
    I am writting an article about ADHD and genetic links and I would appreciate very much your advice on this question.

    Luis Carlos

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  10. Dear Luis,

    I discussed the equal environment assumption of the twin method in both of my books, and more in-depth in an article that came out this year in The Journal of Mind and Behavior (2013, Vol.34, pp. 1-40). I also have some publications on ADHD and genetics, including a 2009 chapter and a 2000 article. All of these can be downloaded from my website at

    Thanks for your interest.

    Jay Joseph

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  11. Wouldn’t you think this is the hugest flaw, for similar phenotypes to be, it takes both similar genes AND similar environment, so that proves that separating twins into different households does not necessarily change the environment in a meaningful way? The amazing match of these separated phenotypes proves it, a simple truth table, no? All adoptable households have the same environment for kids.

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