Debunking the Minnesota Study of Twins Reared Apart

The Minnesota Study of Twins Reared Apart informs our current nature vs. nurture debate on intelligence, but the results are now in question.

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The 1990 Minnesota Study of Twins Reared Apart (MISTRA) is an influential piece of heritability research. Published in Science, it is still heavily cited as one of five essential studies that examined monozygotic (MZA, or identical) twins who were considered to have been raised separately from each other—which is supposed to account for the difference between genetic “nature” and parenting/environment (“nurture”).

The MISTRA focused on intelligence (measured as IQ), and the researchers concluded that intelligence is highly heritable—that very little of it is due to upbringing or environment.

The MISTRA, and the other studies like it, form the basis for a layperson’s understanding of heritability in which almost all outcomes are reducible to genetics—environment seems to matter little, if at all. For instance, just this weekend, an article in The Atlantic cites this research to argue that parenting does not matter except, confusingly, the specific community in which you live. The argument is that you can do anything with your kids as long as you live in one of the top communities the author identifies. And conversely, if you don’t live in one of these spaces, it won’t matter how good a parent you are—nothing you do will ever make up for it.

In a new paper published in the journal Human Development, researcher Jay Joseph explains that the MISTRA publications left out critical data. When this data is included, the MISTRA results actually demonstrate that IQ was not genetically based. Joseph writes:

“I conclude that the MISTRA IQ study failed to discover genetic influences on IQ scores and cognitive ability across the studied population and that the study should be evaluated in the context of psychology’s replication problem.”

(By the way, the MISTRA study received $1.42 million in funding from the Pioneer Fund, which Joseph writes is “an organization with a history of promoting racial differences research and eugenics.”)

Joseph goes on to explain that the MISTRA IQ study is the paradigmatic example of this type of research but that the same issues are pervasive throughout all the twins-reared-apart studies—and this research should be considered part of psychology’s misleading history of drawing startlingly firm-sounding conclusions from flawed and poorly conducted research. Articles like that Atlantic piece are ultimately based on a layperson’s misinterpretations of research that was bad to begin with.

One of the main problems with the MISTRA, according to Joseph, is that the control group—reared-apart dizygotic (DZA, or fraternal) twins—was omitted from the publication. If identical twins (who share all the same genetic material) had a similar IQ, but fraternal twins (who have different genetic material, like ordinary siblings) had different IQs, that would lend credence to the notion that IQ was hereditary. As the researchers themselves wrote in the 1990 Science publication:

“MZA and DZA twin pairs are a fascinating experiment of nature. They also provide the simplest and most powerful method for disentangling the influence of environmental and genetic factors on human characteristics.”

The researchers emphasized the strengths of comparing the MZA twins with the DZA twins—even suggesting elsewhere that this was a significant improvement over previous studies that focused only on MZA twins with no control group. So why did they not include this data in their published article? According to the authors of the MISTRA publication, it was due to space limitations and the small sample size. But, according to Joseph, this rationale is at odds with their statements that this was the “simplest and most powerful” part of their study and the one aspect of their research that made it superior to previous attempts.

Perhaps the honest answer, Joseph writes, is that when the data from both sets of twins are included, there was no difference between the groups:

“Near-full-sample correlations published after the study’s 2000 endpoint show that the reared-apart monozygotic twin (MZA) and DZA group IQ correlations did not differ at a statistically significant level, suggesting that the study failed the first step in determining that IQ scores are influenced by heredity.”

Thus, the actual results from the MISTRA study—although misleadingly omitted from the seminal publication—demonstrate that IQ was not hereditary.

This comparison between the MZA and DZA pairs is called the “important first question” in one chapter of a leading genetics textbook. In another chapter, Robert Plomin—a leading geneticist and first author of the textbook—writes, “If the MZ correlation does not exceed the DZ correlation for a particular trait, there is no genetic influence.” Nancy Segal, a MISTRA researcher who has written extensively about the study and promoted the high heritability estimates, also calls this comparison the “important first step.”

Yet this first step was violated by the MISTRA when the comparison was left out of the publication. And now—long after these writers accepted the MISTRA conclusions despite that omission—Joseph demonstrates that it was left out because the comparison failed. There was no difference between the DZA and MZA groups, so, to quote Plomin’s textbook again, “there is no genetic influence.”

Later publications from the MISTRA group even found that the fraternal twins were, amazingly, more similar than the identical twins but dismissed this finding in a footnote, calling it “sampling variability”:

The MISTRA researchers wrote, “For four tests, DZ correlations actually exceeded MZ correlations, a situation we attribute to sampling variability.”

Worse, the researchers prevented critics from reviewing their data, ensuring that no one would be able to test whether their conclusions were adequate or not.

There are many other important limitations of twins reared apart studies, including:

  • Twins aren’t actually separated at birth. In these studies, 33% were separated after a year or more spent growing up together;
  • 75% of the pairs of twins still had contact with each other while growing up;
  • More than half (56%) were raised by a close family member;
  • In 23% of cases, the twins ended up being raised together again at some point or lived next door to each other.

These problems aren’t unique to the MISTRA—these specific numbers come from the other seminal studies of twins reared apart.

One of the most severe limitations of all these studies is that the twins were not randomly selected or followed from birth. Instead, the participants were adults who had already reconnected with each other, noticed similarities, and decided to participate in a study demonstrating heritability. In many cases, these twins ended up in the study after already being promoted in the media as being remarkably similar.

This means that the participants were a self-selected group of people who had already found themselves similar, who had been in contact with each other, and were usually not fully raised apart, as the researchers claimed.

 

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Jay Joseph has published numerous more detailed critiques of twins-reared-apart studies on Mad in America. A sampling of these blogs can be reviewed here:

 

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Joseph, J. (2022). A reevaluation of the 1990 “Minnesota Study of Twins Reared Apart” IQ study. Human Development, 66, 48-65. DOI: 10.1159/000521922 (Link)

14 COMMENTS

  1. I read the book on the MISTRA study a number of years ago, not the whole thing; I couldn’t get through it because it was so poorly written. I was very suspicious of the study at the time and criticisms I’ve read of it since then, including the one here, have just added to my skepticism.

    My original objections included that the methodology was slipshod, with the author insisting, for example that the twins didn’t exchange information because they understood the importance of not doing so and fully complied.

    Also, the book examined dozens of the twins’ traits. When that many comparisons are made at least some of the positive heritability correlations are going to be due to chance. This was never considered in the study.

    The study was also homophobic: homosexuality was discussed under the heading of psychopathology.

    That a study like this is still given credence indicates, once again, how misguided the field of psychology is.

  2. I just know what the culture was among researchers in the 1990’s. America has been a crass country that is one of the only countries that refuses to enact family leave laws, arguing that the “free market” should decide everything.

    This has been politically rigged by the “community” of large corporations who donate excessively to academia. And part of that rigging, I believe, may involve using the power of “strings attached” donations to academia to promote the exact same corruption of economic research and behavioral science relating to parenting and development that we all acknowledge Big Pharma does to academia with respect to psychiatry.

    And it is also true that these corporations try to corrupt or influence the media and other parts of our culture as well. And we might also want to acknowledge or look back through history to understand it better. For instance, some researchers have argued that the debilitating foot binding of women in China which only stopped during the 20th century was tied to economics as well. Helping to create a “captive” pool of female workers in textiles, where the disability caused by foot binding made them less able to bargain effectively for better wages since they couldn’t do anything else, and they were also more reliable and steady workers since they couldn’t do anything else as well. If it hurts to walk, you will prefer to just sit down and sew.

    However, elites in China never actually openly proclaimed “we want you to do foot binding because it will make you better workers.” Instead, elites got women to do that by promoting it as a fad and glamorizing it. Pretending that men wanted it. (As a man, I can say I don’t think most men care about feet, and when they do, they prefer them natural.)

    The question I’d ask is, might researchers for this twin study have WANTED to say parenting doesn’t matter? So as to minimize family leave and maximize female labor hours?

    I should also explain, people should try to bone up on the economics laws of supply and demand. A policy like family leave will take (female) workers out of the job market, and this increases competition among companies for workers, forcing them to pay the remaining workers higher wages in order to retain them. The more guilt mothers have regarding whether parenting matters or not, the more of them will drop out of the work force to spend more time with their kids (or at least cut back their hours) and a trend like this really can force companies to try extra hard to keep workers in the workforce, inevitably leading to higher wages and lower profits.

    So, per economics, the laws of supply and demand mean that rigging a study to say that “parenting doesn’t matter and it’s all genetic” is going to keep more women voluntarily in the workforce, and help companies keep women’s wages lower. And if you publish a study that says parenting DOES matter, and it triggers feelings of guilt in working mothers who work long hours nationwide — you really could cause a trend among women cutting back work hours, dropping out of the workforce entirely, this lowers the pool of female workers, forcing those wages up, and that will lead to lower profits and higher wages.

    Look at the corruption Big Pharma has been able to achieve with psychiatry, simply by donating to academia? I believe the rest of the business world, all the other corporations who donate to academia, are doing a lot more distorting of the output of academic researchers than MadInAmerica focuses on.

    And this distorting of academic research may be part of why wages for Americans have systematically dropped in the last few decades. Academic fraud is what helps manipulate the public or mislead the public just right so politicians getting money from the same corporations can “pretend to help” or “pretend to be trying to do the right thing” and it all just doesn’t work out for anyone but the corporations.

    We have a democracy, but if corporations can infiltrate academia and distort academic science and economics and everything else just right, they can run the show like dictators.

    • “might researchers for this twin study have WANTED to say parenting doesn’t matter? So as to minimize family leave and maximize female labor hours?”

      I think you are right, and I know this has been a goal of American psychiatry for decades. Since one of my psychiatrists didn’t even bother to ask me what I did; she must have illegally nosed into my private income statement or something, since she called me “unemployed” in her medical records.

      In reality, I was a stay at home mom, very active volunteer, as well as an artist working on my portfolio, in the hopes of future profits.

      But, if you watch this video, Aaron Russo explains this is the stated goal of the “elite.”

      https://www.youtube.com/watch?v=6rWeRL_0aEs

      He speaks about this topic @ 5:25.

      “We have a democracy,” actually America is a democratic republic, not a democracy. Or at least it’s not supposed to be a “democracy,” according to the Constitution.

      “but if corporations can infiltrate academia and distort academic science and economics and everything else just right, they can run the show like dictators.”

      And that’s pretty much what’s going on.

      As to the nature vs. nurture debate, it seems the truth is likely some combination of nature and nurture – not one or the other. I do believe there may be a genetic component to IQ, given my father and son both graduated from university Phi Beta Kappa (within the top 10%, within the top 10% of universities).

      But I do know quickly getting my child away from satanic child molesters, and eventually leaving my systemic child abuse covering up former religion – some within whom have freely confessed to being “partners” with the scientific fraud based, systemic child abuse covering up, “mental health professionals.”

      https://books.google.com/books?id=xI01AlxH1uAC&printsec=frontcover&source=gbs_ge_summary_r&cad=0#v=onepage&q&f=false
      https://www.indybay.org/newsitems/2019/01/23/18820633.php?fbclid=IwAR2-cgZPcEvbz7yFqMuUwneIuaqGleGiOzackY4N2sPeVXolwmEga5iKxdo
      https://www.madinamerica.com/2016/04/heal-for-life/

      I guess I say, thank you Peter, for consistency speaking truth regarding the scientific fraud of the unrepentant, systemic, child abuse and rape covering up, “mental health system.”

      • “As to the nature vs. nurture debate, it seems the truth is likely some combination of nature and nurture – not one or the other. I do believe there may be a genetic component to IQ, given my father and son both graduated from university Phi Beta Kappa (within the top 10%, within the top 10% of universities).”

        ——

        Actually, I majored in math at Harvard and do have a very good memory and very good intuition in math as well. And physics too.

        The whole nature versus nurture debate could be a VERY interesting math problem. Because, well, think about it. Imagine the brain growing and developing like how a tree or shrub grows, and then nurture is like outside influences exerting influence at times, but the brain is still genetically programmed to grow in only certain ways in response to those outside influences, so you get a combination of nature versus nurture.

        Maybe certain parts of the brain are stubborn, though, and will not bend to nurture too much. Like the heart, for instance, has to be within a certain range regarding geometric shape and other stuff, and cannot stray too far from the norm.

        Other things like survival instinct and how you respond to danger – like when you get PTSD and it’s stubborn and irrational. You can try to reason yourself out of PTSD but, no, you can’t.

        You know you are safe. It doesn’t matter. Another part of your brain over rules that and can only be treated the way it can be treated.

        It’s stupid to have a debate where some academics say it’s all 100 percent nurture, some say it’s all nature, when in fact you could try to develop a whole science that starts out with the assumption that it is a combination of both, one where it’s going to be very hard to sort things out, but at least you can try.

  3. This study definitely had its share of flaws which makes it an essentially useless study. However, we can easily bring attention to these useless studies by also making arguments that completely miss the point. In my opinion, it seems that no matter the study our natural emotionalism seems to get in the way, and we miss the light. Until we see the light and consider SPIRIT which also includes mind, brain, and body, all “studies” will be useless whether the flaws of the study or of society are illuminated or not. It is also time that in addition to the SPIRIT, we must acknowledge the natural frailty of all humans and the unique individuality of each human. Perhaps, we need to spend less time on study experimentation and its flaws and take the time to learn about each person on an individual basis. Thank you.

  4. First, I again, was just thinking of this study and how cruel it is to rear twins or any siblings apart. Even if the parenting of the children was untenable and the children needed to be in foster care or adopted, siblings, including twins should not be reared apart and to do it for “scientific” reasons is just child abuse. Second, all these so-called studies are destined for failure because they concentrate on the “appearances” whether than what’s inside a person. But then we do seem to have the fault of only viewing what we see or think we see, rather than what is true. When we finally begin to see what the appearances of people, events and everything in this world, we will have made progress. Thank you.

  5. I was largely unaware of these studies, except to know that they existed, and was surprised to learn that inherited IQ was still being pushed by anybody as “science.”

    If anything, the studies that include fraternal twins probably point to the conclusion that how the children are raised is more important than their genetics, at least for traits like IQ.

    I have been taught that IQ can be changed (improved) by various intellectual drills. I don’t know if this has been studied by academics, but my group is quite certain this is true. In fact, there are mental drills that can alter all sorts of personality traits. It’s an interesting subject that has been largely ignored in psychology. We can only assume that psychologists are being used to find ways to control people better, not to make them more intellectually and emotionally free and happy.

    • What you say is interesting and I have given this much thought even before this. In my opinion, it seems things about us can be changed and some things can’t be change. And I would say that trying to change those things that can’t be changed is one of the reasons for the great stress and distress that many go through in life and in fact can lead to great trauma. Also, in my opinion, that one of the purposes of the psychiatric drugs was to change the personality of the patient. In fact, I saw a blurb in a magazine back at the time that “Prozac” came out and the blurb said, “pills for personality.” I guess I saw that as important to me as it still sticks in my mind. I also think of the old book and movie, “Flowers for Algernon” where the main character was given drugs that made him “smarter.” But, in the end, the drugs failed miserably, and the main character basically went back to who he was. In my personal experience, the psych drugs did something similar to me. After all those years being drugged and then going through the years of withdrawal, I have basically returned to whom I was meant to be. Maybe some of these drills you mention can change some things about a person, but I must confess I am skeptical. Still, I am beyond thinking of this as a nature/nurture issue; although no one can be who he or she is without input from the “wiring” of the brain. In a manner of speaking, I think of each one of us is being “wired” within like a radio or something similarly electrical. And we do each of us operate on both unique and shared frequencies. When these frequencies clash, that’s when can experience relationship issues. I, personally, don’t think these “frequencies” can be changed without great agony such as what the psych drugs and ECT does. They may be modified or adapted somewhat to suit the environment. But we really need to be careful with what we do to our brains, even when our brains and sometimes our bodies seem to “fail” us. Thank you.

      • Short term personality changes are common. A person is happy in life, then hears that a parent died and becomes sad. Then the question becomes: How long before they return to their “normal” selves?

        What happened there? The person had his attention out on the various activities of life, and was happily handling those activities, then his attention got pulled onto a tragedy in his life. So it is a matter of attention and control of attention.

        Drills that could put the being more in control of his own attention would help him feel more impervious to the bumps and twists that are bound to show up in life.

        Similarly, if a person loves dogs and carpentry, but feels sad all the time, the ideal “therapy” results in a person who still loves dogs and carpentry, but is less sad more often.

        The idea that a person has a depressed “personality” if he is sad or apathetic all the time is not very workable. The idea that a person who is depressed just has his attention stuck on something that is very depressing is more workable. It’s not a matter of changing his whole personality. It’s just a matter of unfixing his stuck attention.

        I see IQ as more of an acquired skill. Of course there are people who seem to be smarter than others, but a general aptitude like intelligence should not be confused with personality traits like what a being wants to achieve in life or how they like to dress or things like that.

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