Genetics, Science, Top Stories
comments 37

A Striking Similarity: The Revolutionary Findings of Twin Studies

“I have looked at the data, and I’m collecting the data, and I’m still absolutely astounded. I still haven’t settled down and absorbed this kind of a finding yet. How long is it going to take me?”

These words were uttered by Dr. Thomas J. Bouchard, research director of the Minnesota Study of Twins Reared Apart (MISTRA), during a conversation with the Danish professor of psychiatry, Niels Juel-Nielsen, in May 1981. Bouchard was trying to come to terms with the revolutionary implications of his own research into identical and fraternal twins reared apart. 16 years earlier, Juel-Nielsen had published the book Individual and Environment—a study of 12 Danish identical twin pairs reared apart. Prior to 1981, this was one of only three studies of separated twins: the others were a 1937 American study of 19 twin pairs, and a British study conducted by James Shields in 1962 of 44 twin pairs.

An archived recording of their remarkable exchange was rediscovered in 2011 by the twin researcher Dr. Nancy L. Segal. In her 2012 book Born Together—Reared Apart, Segal writes that Bouchard’s incredulity reveals the degree to which his new findings contradicted his own assumptions and the prevailing wisdom about the importance of environment in shaping a person’s traits, prospects, and outcomes. The scholars’ amazement is palpable as they discuss the exciting and groundbreaking conclusions suggested by Bouchard’s new data.

The interview with Juel-Nielsen was not recorded for transmission, but for the purposes of transparency. Shortly after his death in 1971, the British psychologist Cyril Burt had been accused of falsifying the results of twin studies and related research into the heritability of IQ. The accuracy of this allegation, and the degree to which it invalidated Burt’s findings, remain hotly contested, but the controversy significantly damaged the status of hereditarian hypotheses. Determined to avoid having his own research tainted with the same suspicions, Bouchard meticulously documented and collected his investigations on photographs and video, of which the Juel-Nielsen interview is a fascinating fragment.

The Jim Twins

Two years before this interview, in the spring of 1979, one of Bouchard’s students had drawn his attention to the story of Jim Springer and Jim Lewis. Springer and Lewis were identical twins, separated just four weeks after they were born. They had grown up in separate families, 40 kilometers apart, and only met one another for the first time at the age of 39. Astonishingly, they shared many uncanny similarities: both had married women named Linda, followed by women named Betty; both had named their son James Allen; both were diligent amateur carpenters; both were heavy smokers; their favorite beer was Miller Light; both bit theirs nails; both encountered heart problems in their 30s; and both had suffered from migraine attacks at the same age.

Bouchard immediately set about securing the funding needed to complete a detailed case study of the twins. Keenly aware of the rare opportunity identical twins reared apart offered researchers of psychology, Bouchard told a New York Times reporter that he was ready to “beg, borrow, or steal” to raise the necessary funds. The Jim twins provided an almost perfect control experiment; they were genetically identical but raised in separate environments, so any differences between the two men could be attributed to environmental influence and separated from the influence of their shared genetic inheritance.

A week after the story appeared in the Times, Bouchard had the Jim twins flown to Minnesota where they were tested for close to a week.

Virtually Identical Scores

In the interview with Niels Juel-Nielsen, Bouchard explained his initial skepticism regarding the validity of the psychometric testing used to measure IQ:

I’ve always thought that tests were interesting, useful and a valuable contribution from psychology, but I was always cautious about them. I was feeling, like there was a larger range of error ​​than was desirable. But now we have studied 21 sets of identical twins and we have a good number of cases who have virtually identical profiles on some of our psychological tests. This evidence has persuaded me that our tests in many instances are even better than we thought they were. In fact, I can’t think of any more convincing evidence than seeing two identical twins reared apart come in, take a test and get very similar scores. It’s quite striking! And although our data are just preliminary—we hope to gather many more twins—it’s pointing to the conclusion, that twins reared together in the same family are as similar as they are primarily for genetic reasons, not for reasons of their environment.

Bouchard then added, “I noticed that when I told you that a few days ago you were not a bit surprised.”

Juel-Nielsen: No, then, first of all I had a few cases demonstrating that. And James Shields found the very same thing. In his separated cases, the twins were even more similar in personality traits than those who had been brought up together.

Bouchard: This kind of finding, though, it seems to me, threatens a great deal of psychological theory. There is a lot of theorizing in psychology about the tremendous importance of the family situation, the peculiar things that happen when you are growing up. And we turn around and we look at these twins that have been reared in entirely different families and they are just as similar as twins reared in the same family!

“In What Respect Were the Twins Strikingly Similar?”

Juel-Nielsen: One could try to ask: In what respect were the twins strikingly similar, obviously similar? And where did they differ, strikingly? Then I could briefly summarize my findings. I would like to hear your comment. First of all, they looked alike!

Bouchard: Oh boy, no question. [laughs]

Juel-Nielsen: Your physical appearance, your hair and eye color, your face, the way you walk, the way you smile, the way you talk—all these mannerisms were strikingly alike. If, for instance, I talked with two twins on the phone, I could easily mistake them. If I saw them from behind, again, confusion. Many psychologists say that it’s easy to explain family resemblance. They are identifying with each other, so they consciously or unconsciously imitate each other. Or a boy develops like his father, and so on. But these reared apart twin studies demonstrate clearly that this hypothesis is not at all accurate. That is because you see the similarities between relatives, who have never met and do not know about each other or their families. This is very important! Behavior is part of your personality and will affect your environment to some degree. If you go deeper into it, as you are investigating the brain waves, you’ll find that they are as similar, as if you examined the same person twice. At one point or another you will, of course, find differences. But as you stated it: What is a difference? Is there a difference if you have an IQ-difference of say 5 points?

Bouchard: It’s still within the standard error of the instrument.

Juel-Nielsen: So, it is not a difference.

Bouchard: We have many cases where, for all practical purposes, there really is no difference. In fact, we have two sets of twins, who differ by just one point. They are remarkably similar.

Juel-Nielsen: So what’s needed are, perhaps, environments that differ more than environments do in general. I think it was Francis Galton who said something like: ‘Nature prevails enormously over nurture, provided you study twins within the same rank of society.’ What he meant was that if you have differences that are unusual from one home to another, such factors might create a difference, but what is ubiquitous in society makes no difference.

Bouchard: Yes. One of the consequences, I point out to my classes, but that does not really seem to get into the public consciousness, is that as we create more and more egalitarian and equal societies with equal opportunity, the differences between individuals become increasingly more and more genetically based. The differences don’t go away. The overall range may be reduced somewhat, but the differences don’t go away. They become more and more genetic. This is sort of a paradoxical finding, if you are an environmentalist. But if you believe that there are genetic differences, then it’s a natural consequence.

Correlations

It’s been 37 years since this conversation took place. In the intervening decades, Bouchard continued his study of identical and fraternal twins and MISTRA collected convincing data demonstrating the powerful influence of genes. In total, Bouchard studied 137 twins reared apart and what had so astonished him in 1981 is, by now, well supported. Twin studies designed to uncover the genetic influence on behavioral and mental traits have gone from ignored or derided to widely accepted.

In studies that omit anecdotal evidence of the striking similarities of twins (that laymen and researchers alike find so fascinating), heritability is found to be between 0.2–0.8 for a variety of traits and characteristics. Roughly speaking, this means that between 20 percent and 80 percent of the differences in a trait or characteristic (e.g., extraversion) can be explained by differences in genes.* When it comes to personality traits, depression, and phobias, twin studies have shown that there is more room for the environmental influence. At the low end, heritability for phobias and depression range between 0.2 and 0.4. Personality traits—specifically, ‘The Big Five’ of extraversion, openness, agreeableness, conscientiousness, and neuroticism—range from 0.4–0.6. And, at the top end, the heritability of intelligence scores about 0.75.

These findings tell us how much of the variance in a current trait is caused by variance in genetics. A finding of ‘zero’ means there is no correlation at all, while 1.0 indicates a perfect match. But psychometric tests are subject to a certain margin of error, because not even a single individual will return a perfect match if asked to sit the same test twice. But what twin studies have shown is that the general intelligence measured by IQ is significantly influenced by genetics, and that heritability increases with age—the correlation at age five is 0.22, at age seven it rises to 0.40, at age 10 to 0.54, and from 18 years of age into adulthood the heritability of intelligence approaches 0.80. In an article published in 2013, Bouchard called this phenomenon the Wilson-effect.1

The Three Laws of Behaviour Genetics

In 2000, the psychologist Eric Turkheimer concluded that the evidence from behavioral genetic data was consistent enough to summarize in three laws. The first law holds that all human traits are heritable (i.e., genetic differences account for phenotypic differences) to some degree. This assertion may not seem all that surprising today, although the word ‘all’ is still considered provocative by some. However, twin studies have produced copious data demonstrating that almost every trait is heritable to some degree or another.

The remaining two laws concern environmental influence. The second of these holds that the effect of being raised in the same family is smaller than the effect of genes. Judith Harris called attention to this theory in her 1998 book The Nurture Assumption, and was subsequently defended by Harvard psychologist Steven Pinker in The Blank Slate. Pinker encouraged parents to stop fretting about what they had or had not done in order to turn their offspring into wonderful individuals. Parents, he argued, do not hold their children’s future in their hands, only their present. Pinker emphasized that parenthood remains an awesome ethical responsibility, and that it is important to give one’s child a childhood worth remembering. But parents cannot shape a child’s personalities and IQ as a sculptor fashions clay. As Dr. Nancy Segal has put it, homes do matter, but they do not make people alike.

The third law holds that a substantial portion of the variation in complex human behavioral traits is not accounted for by either the effects of the genes or families. In other words, while about 50 percent of the variation is due to the environment, this environmental effect does not come from the family. Instead, it may be produced by the wider culture, society, the neighborhood, school, peer-groups and friends, but also simply chance: random encounters or openings in the social hierarchy, cosmic rays that damage a piece of DNA, neurons that go zig instead of zag, and so on.

Twin studies have uncovered the enormous importance of genetics. They have laid to rest the notion that parents are omnipotent sculptors, and a child is a piece of clay. They have hammered another nail into the coffin of the Freudian guilt complex, where everything that goes wrong in an individual’s life may be attributable to poor parenting.

Nevertheless, questions remain—twin studies have not been able to identify conclusively the environmental factors that make up the remaining ‘nonshared’ variation. The answers may require us to capsize even more articles of faith and received wisdom in our pursuit of knowledge and understanding. Thomas Bouchard had difficulty coming to terms with his preliminary results in 1981, but he had the integrity to follow the data wherever they led him. It remains to be seen if others will have the courage to follow his example.

*This sentence has been amended.

 

Henrik B. Dynesen is a writer and teacher at Sorø Akademis Skole in Denmark. You can follow him on Twitter @DynesenB

Reference:

1 Bouchard, T. (2013). The Wilson Effect: The Increase in Heritability of IQ With Age. Twin Research and Human Genetics, 16(5), 923-930. doi:10.1017/thg.2013.54

If you liked this article please consider becoming a patron of Quillette

37 Comments

  1. Jack B Nimble says

    From the Journal of Politics 80[3]:1023-1027

    Genetic Attributions: Sign of Intolerance or Acceptance? by S.P. Schneider et al.

    Abstract–Many scholars argue that people who attribute human characteristics to genetic causes also tend to hold politically and socially problematic attitudes. More specifically, public acceptance of genetic influences is believed to be associated with intolerance, prejudice, and the legitimation of social inequities and laissez-faire policies. We test these expectations with original data from two nationally representative samples that allow us to identify the American public’s attributional patterns across 18 diverse traits. Key findings are (1) genetic attributions are actually more likely to be made by liberals, not conservatives; (2) genetic attributions are associated with higher, not lower, levels of tolerance of vulnerable individuals; and (3) genetic attributions do not correlate with unseemly racial attitudes.

    I’m a liberal Democrat [in the U.S. sense] and also a genetic determinist. That is, I believe that genetic variation plays a significant role in determining human behavioral phenotypic variation. See, eg, https://www.the-scientist.com/daily-news/gene-expression-overlaps-among-psychiatric-disorders-30302

    However, I am NOT a genetic pessimist. That is, I don’t think that genetically-determined phenotypic variation in humans is fixed and unchangeable. I also don’t think that genetic determination of variation within a family or social group can be generalized to between-group differences without controlling for environmental effects, and so far that hasn’t been done. Like all scientists, psychologists work with the data they have, not with the data they might wish to have at some future time. But that doesn’t change the fact that the models they use are really inadequate compared to those used by animal and plant breeders–you know, the people who do genetics for a living. And the ‘genomics revolution’ by itself doesn’t address the issue of environmental effects.

    • Nick Ender says

      I have one question… why is the “legitimation” of “laissez-faire policies” include along side prejudice and bigotry? I noticed this in the Sam Harris vs. Ezra Klein debate. Just stating that Murray desires to roll back government interventions like welfare was proof to Klein (and to some degree Harris because he never challenged this argument even though it was made repeatedly) of Murray’s racism. How is being libertarian racist? It’s a very strange tactic that seems to slide by almost unquestioned.

      • Jack B Nimble says

        @Nick Ender — The wording is from Schneider et al., not from me, and their point was that ‘many scholars’ hold these beliefs [about genetics and politics], not that these are universal truths.

        FWIW, I think that ‘laissez-faire’ and economic inequality CAN be defended on philosophical principles without recourse to genetics, systemic racism, etc., but I don’t know the libertarian literature well enough to have a strongly-held opinion either way.

      • Cerastes says

        The argument, as I understand it (not that I agree with it) is one of disparate impacts – if the policy adversely affects minorities more severely or more commonly, it counts as racist even if there was zero racial animus in the passage of the policy (or even if the disparate impact is an unintended side effect). Even the legal application of this is not without controversy, let alone assessing individual people’s policy preferences as racist/not.

        Like I said, this isn’t my view, just what I’ve heard used as a rationale, FYI

    • I have no reason to read this full article (Schneider) when the author clearly articulates unscientific and biased views right out of the gate. Any argument damning ‘social inequities and laissez-faire policies’ is a philosophical or political one, not a scientific one. Everyone is ‘intolerant’ of some ideas or groups of people (I am intolerant, for example, of true racists and Islamists who want all nonbelievers dead. Schneider is clearly intolerant of meritocracy). The real point should be that almost no one alive believes purely in either nature or nurture; virtually everyone I’ve ever met clearly were somewhere near the middle, and it looks like the research supports this view generally.

      • Just Me says

        General Tsu-

        “I have no reason to read this full article (Schneider) when the author clearly articulates unscientific and biased views right out of the gate. ”

        Please reread the abstract. Schneider is not articulating his own beliefs, he is testing the beliefs widely held by the Left, and finding them wrong.

  2. Pingback: A Striking Similarity: The Revolutionary Findings of Twin Studies – Foggytown's Micro Blog

  3. “In studies that omit anecdotal evidence of the striking similarities of twins (that laymen and researchers alike find so fascinating), heritability is found to be between 0.2–0.8 for a variety of traits and characteristics. Roughly speaking, this means that between 4 percent and 64 percent of the differences in a trait or characteristic (e.g., extraversion) can be explained by differences in genes.”

    This is a wrong interpretation of heritability. Heritability is the ratio of genetic variance to total variance. Equivalently, it’s the square of the correlation between genotype and phenotype. It’s a variance component, a squared quantity, so you don’t square it to get the variance explained–if you do that, you’re squaring a square! If heritability is between 0.2 and 0.8, it means that genetic differences explain between 20-80% of phenotypic differences.

  4. “…heritability is found to be between 0.2–0.8 for a variety of traits and characteristics. Roughly speaking, this means that between 4 percent and 64 percent of the differences in a trait or characteristic (e.g., extraversion) can be explained by differences in genes” I believe that these coefficients should be directly interpreted as an estimate of the variability explained by differences in genes. Thus, the range is 20 – 80%.

  5. prince says

    “the general intelligence measured by IQ is significantly influenced by genetics, and that heritability increases with age—the correlation at age five is 0.22, at age seven it rises to 0.40, at age 10 to 0.54, and from 18 years of age into adulthood the heritability of intelligence approaches 0.80. ”

    If the environment (“Nurture”) had any significant role in influencing the human IQ, we should have seen the reverse trend: Natural IQ should have been mostly pronounced in young children and we should have observed a growing divergence of the twins as the different environments make deeper and deeper imprints on their traits.

    The fact we are seeing the opposite trend, where the correlation is actually growing over time show the immense robustness of the heritable IQ that is so resilient to 18 years of environmental influence.

    • Samir Younan says

      “If the environment (“Nurture”) had any significant role in influencing the human IQ, we should have seen the reverse trend: Natural IQ should have been mostly pronounced in young children and we should have observed a growing divergence of the twins as the different environments make deeper and deeper imprints on their traits.”

      That’s not necessarily true. Replace IQ with height, and a framework in which one trait “lags” behind the other, I can see how something like the results would make perfectly reasonable sense. Height may be stunted by the environment but will eventually catch up. The same principles may apply with IQ. Taking into account the fact that: children brains develop rapidly during infanthood (where one would thus expect a higher variation in IQ as “nurture” would be more effective) then the results fit the evidence we have now.

  6. Peuri says

    This is all quite interesting, but even if psychological traits are highly heritable, that doesn’t yet tell us how those traits relate to action in a social setting. To be a succesfull drug dealer for example requires probably as much IQ as being a succesfull business owner, but the paths of people becoming either a drug dealer or a business owner are still more about the nurture and social setting the person is grown in, right?

  7. As the evidence (especially from genomics research) supporting the importance of genes and heredity continues to accumulate and further threaten the left’s foundational “blank slate” model, we can expect the left to become more desperate and intolerant, even less rational, and more intent than ever on putting cultural and political pressure on those advancing our knowledge in these areas. It’s going to get ugly.

    • Yep. We’re already seeing evolutionary psychology denounced as an illegitimate field. I can see genomics in general being attacked by dogmatic progressives. Labs vandalized. Researchers vilified.

      What’s interesting is I suspect they’ll find allies in the religious right, another group for whom evolution and genetics has never sat well. But then, it’s no secret that adherents of the far left and far right share many traits.

  8. Zarley Zalapski says

    When we’re talking about parents fretting about raising their kids the wrong way, are we talking about tiger moms putting their kids in fencing instead of tennis or the very good local public school instead of private school? Or are we talking about parents being absent and hands-off versus involved and encouraging? Permissive versus disciplinarian?

  9. David of Kirkland says

    As a parent, I can attest to the fact that our kid was his own person and very different from both his parents in many ways (such as not being into sports and music). But we’re happy to take genetic credit for his being smart!

  10. augustine says

    Data supporting heritability of IQ and other traits is fine and well. Unfortunately, this will have zero impact on the thinking of those engaged in social improvement ideologies where the ostensible goal is to help those who are marginalized to the point of visible dysfunction and suffering. Such help entails not only aiding and supporting them directly (however misguided or beneficial) but also identifying and vanquishing their alleged oppressors, even if the latter can only be called out as such because they are materially more successful. Hence today’s open intolerance and hostility toward whites and their traditions, Christianity, capitalism, etc. The premise of these ideologues is focused solely on the attitudes and status of those who are held to be responsible for creating and maintaining social inequality. This view precludes any argument that genetic limitations, in individuals or groups, in part or in toto, are a factor to be considered. Articles like this one, while informative, are preaching to the choir.

    • ga gamba says

      Likely mixed on that. Remember, Bouchard himself was astounded. Will the true believers change? Unlikely. Does that matter? They still have to be hired and apply for research grants. At some point the public may view them like they do others pushing crackpot ideas and funding will disappear. I acknowledge they have some powerful allies presently, so it won’t happen overnight. If the voters demand their legislators scruntinise public funding of research with the intent to forbid funding the unrepentant denialists then that community either adapts or it withers away.

  11. Stevec says

    “..twin studies have not been able to identify conclusively the environmental factors that make up the remaining ‘nonshared’ variation”

    Conclusively? My understanding, as someone not in the field but I’ve read lots of books and academic papers on the field, is it’s actually not understood at all. The standard description is the heritable portion (typically 50%), the shared environment (0-10%), and the “non-shared”, aka “unique”, environment at 40-50%. But this last item has a description unsupported by evidence. It could equally be called the unknown factor. It could be random genetics, it might be nothing to do with the environment at all.

    If someone could clear up this point for me, and highlight the evidence for the 40-50% actually being a result of the environment that would be awesome.

    • You’re right. I think the correct name for the “unshared environment” would be “noise” or “stochasticity.” Its magnitude is simple to estimate: it’s the average difference between MZ twins raised together. Because MZ twins raised in the same family represent the reasonable upper limit of how similar two individuals can ever be, the prospect of identifying the elements of the “nonshared environment”, let alone manipulating or changing them is slim. Even genetically identical lab animals raised in the same controlled environment will not be phenotypically identical–some slack or noise in the expression of phenotypes is an inevitable part of biological development.

      • Surge says

        Measurement error is also in that piece of the pie. For example, if you measure the height of MZ twins that are the same height, and the measurement has a +/- 1 inch error, the. You get 1-2% non-specific env. variance.

    • Everything that is not heritability is environmental by definition. So “random genetics” is not included, depending on what you mean.

      But it does include unknown factors. It could include random thing that happen to you which would affect someone else the same way. It could also include when you experience something but it changes you differently than when someone else does.

      So while it’s a dumping box for anything and everything, these will be environmental explanations. Just quite possibly ones we can never make use of.

      • Jack B Nimble says

        @Ed Morrison et al.

        See Wikipedia articles on:

        Developmental noise
        Canalization
        Epigenetics
        Gene regulatory network
        Phenotypic plasticity

        for an entrance into a rather vast literature on developmental genetics and factors affecting gene expression.

      • “Everything that is not heritability is environmental by definition”

        I don’t think you can just do 1.0 – Genetics = Environment.

        It is entirely possible that there is further genetic influence but it just can’t be measured yet because the science and testing hasn’t got there. Imagine that IQ testing wasn’t be done well due to inadequate tests, the measured genetic effect would be lower.

        I think most people’s interpretation of environment is not “everything that isn’t genetic” but instead “things that can be changed by the environment”. Another way to put this is to reverse it and test that validity.

        1.0 – Environment = Genetics.

        • AlexNesta13 says

          @tds – I understand the nuance you are trying to convey with your way of expressing the equation, but I hope we can all agree that the two equations are equivalent.

  12. Max Hillaert says

    Between 0.58 and 0.80 as intelligence heritability coefficient still leaves a *lot* of room for environmental control in raising kids. Surely even a small 10% edge is enough over time for your kid to accumulate advantages and get opportunities than the other kids who doesn’t have they 10% edge. This parent will still spend money to give his kids the edge over others.

    • Andrew Mcguiness says

      Similarly, even given exactly the same inherited intelligence, actual outcomes in terms of academic achievement could be very different, depending on environmental factors. Presumably, the child who is well fed, well slept, cared for, who goes to school and whose parents take an interest in their academic performance and support it with resources and attention at home, will grow up with very different capabilities from the person whose childhood was characterized by physical and emotional abuse, deprivation, and lack of support at home.
      Heritability of intelligence doesn’t mean that we don’t need schools, nor that the home environment makes no difference to how a child grows up.

    • Charlie says

      When it comes to the edge what I think may be important is the Mother up to the age if 7 years or so. Children tend to be curious. If the Mother answers the child’s questions , particularly why then it develops an open mind. Intelligence is learning knowledge and applying it.
      A Mother who is abrupt or critical or unable to answer a child instills an attitude an attitude whereby it is best not to ask questions

    • This is what Murray refers to when he says the future of society depends on how we educate the academically gifted. Why would anyone (other than leftist ideologues and the SPLC) assume he’s referring only to privileged, wealthy white kids?

  13. As Pinker points out in The Blank Slate, any parent who has two or more children recognizes that they have innate capabilities and dispositions. You can nudge children this way or that, but they’re nowhere close to being blank slates.

    We have non-identical twins, and despite being raised in as close to an identical environment and upbringing as humanly possible, they were demonstrating dramatically different temperaments from an early age. By kindergarten, their teacher remarked that they were the two most different children in her class.

  14. Liam Gaul says

    It seems the author has an agenda by presenting the most extreme case study they could find as typical of MZ twins reared apart. Bouchard himself was clear that these twins were extremely and atypically similar:

    “There probably are genetic influences on almost all facets of human behavior, but the emphasis on the idiosyncratic characteristics is misleading. On average, identical twins raised separately are about 50 percent similar — and that defeats the widespread belief that identical twins are carbon copies. Obviously, they are not. Each is a unique individual in his or her own right.”

    Current data support the theory of phenotype being the product of gene-environment interaction. For any given gene or trait, the contribution of environment will be vastly different. This article seems to me to be quite ideological, much like the dogma it is trying to dispel.

    • ga gamba says

      It seems the author has an agenda by presenting the most extreme case study they could find as typical of MZ twins reared apart.

      OK, it may seem that way to some, but I don’t find where he states or even implies it’s typical, representative, etc. It this something you inferred? Do you have a sentence or two you can quote? If he wanted what you find objectionably to be inferred why would he add this, “In studies that omit anecdotal evidence of the striking similarities of twins (that laymen and researchers alike find so fascinating)… ” and he covers several coefficients.

      The “most extreme case study” is addressed by the omission of those MZ twins. He admits that those twins are fascinating to both researchers and laymen alike, which, I assume, is why they get the press.

      I think it’s a good idea to scrutinise any article’s framing. For example, the story of a teen who is arrested and jailed for possession of a trifling amount of cannabis is used as an introduction to an article complaining about the large number of people incarcerated for drugs offences. One digs into the data (not presented in the article) and finds the offences are trafficking, dealing, and production. Those banged up on minor cannabis possession are rare – fewer than 0.7% of prisoners – and often it’s because either they violated a condition of their probation/parole, were arrested in a drugs zero-tolerance zone such as a school area, or committed other crimes and accepted a plea deal for possession because it doesn’t look as serious on one’s record as burglary or grievous bodily harm. That those articles don’t state the example of our unfortunate youth is an “extreme” one, or is not representative of almost all others imprisoned for drugs, implies that his case is typical. For me, that’s when it’s misleading.

  15. Fran says

    “..twin studies have not been able to identify conclusively the environmental factors that make up the remaining ‘nonshared’ variation”

    One source of variation that is much greater in identical twin than dizygotic twins is intrauterine environment. Dizygotic twins are usually genetically different enough that each has its own separate placental attachment. Monozygotic twins have genetically identical placentas, and the consequence of this that is permits the development of perfusion from placenta to twin 1 and on to twin 2. This results in one twin receiving more nutrients than the other. In extreme cases, one twin develops an enlarged heart from having to pump blood to the other, and the other is starved. The consequence of twin to twin perfusion syndrome is that monozygotic twins are more different in birth weight than dizygotic twins. I looked this all up when I was pregnant with monozygotic twins 25 years ago, and lost them due to severe twin to twin perfusion – I am not going to give references here to something anyone can get on Google Scholar.

    The differences in birthweight of monozygotic twins have not been addressed in any of the twin studies I have read. However, I believe this is a potential source of ENVIRONMENTAL differences that may contribute to the ‘nonshared variation’.

  16. It has been shown that environment can affect expression of genetics so that MZ twins with the “same” DNA can have effectively different DNA. This is the field of study of epigenetics and the presence or absence of this phenomenon was was not noted or accounted for in Bouchard’s studies. So there is a definable sense in which these studies are unreliable.

    They are also unreliable in that the important twin studies, including this work were done by members of eugenic societies who promoted each others work without mentioning this nexus e.g., Bouchard himself, James Shields, Cyril Burt and Francis Galton who are all mentioned in the above article were all eugenic society members taught by eugenic society members and teaching eugenic society members and publishing with eugenic society members. Furthermore, most twin studies of the post World War II era including those of Bouchard and his associates were sponsored by the Pioneer Fund. Many others were funded by the Tobacco Research Institute. The fact that one belongs to a school of thought does not invalidate work but concealing significant intellectual and institutional connections is problematic. For instance, from 1945 to the end of the Human Genome Project the measure of heritability in all studies was very similar as it is now but the two numbers from the two eras are quite different. That is why the article says the range is 20 to 60%. If it were acknowledged that we are talking about two schools of thought within the one field of eugenics, then all would be clear. Is it right to muffle up these facts and these differences and hide them from the public in articles presumed to be informative?

Leave a Reply