Red Flag of Twin Studies

Consider this a public service announcement. The moment someone turns to twin studies as reliable and meaningful evidence, it’s a dead give away about the kind of person they are. And when someone uses this research in the belief they are proving genetic causes, it demonstrates a number of things.

First and foremost, it shows they don’t understand what is heritability. It is about population level factors and can tell us nothing about individuals, much less disentangle genetics from epigenetics and environment. Heritability does not mean genetic inheritance, although even some scientists who know better sometimes talk as if they are the same thing. The fact of the matter is, beyond basic shared traits (e.g., two eyes, instead of one or three), there is little research proving direct genetic causation, typically only seen in a few rare diseases. All that heritability can do is point to the possibility of genetic causes, but all that allows is the articulation of a hypothesis to be tested by actual genetic research which is rarely done.

And second, this gives away the ideological game being played. Either the person ideologically identifies as a eugenicist, racist, etc or has unconsciously assimilated eugenicist, racist, etc ideology without realizing it. In either case, there is next to zero chance that any worthwhile discussion will follow from it. It doesn’t matter what is the individual’s motivations or if they are even aware of them. It’s probably best to just walk away. You don’t need to call them out, much less call them a racist or whatever. You know all that you need to know at that point. Just walk away. And if you don’t walk away, go into the situation with your eyes wide open for you are entering a battlefield of ideological rhetoric.

So, keep this in mind. Twin studies are some of the worst research around, opposite of how they get portrayed by ideologues as being strong evidence. Treat them as you would the low quality epidemiological research in nutrition studies (such as the disproven Seven Countries Study and China Study). They are evidence, at best, to be considered in a larger context of information but not to be taken alone as significant and meaningful. Besides, the twin studies are so poorly designed and so sparse in number that not much can be said about them. If anything, all they are evidence for is how to do science badly. That isn’t to say that, theoretically, twin studies couldn’t be designed well, but as far as I know it hasn’t happened yet. It’s not easy research to do for obvious reasons, as humans are complex creatures part of complex conditions.

For someone to even mention twin studies, other than to criticize them, is a red flag. Scrutinize carefully anything such a person says. Or better yet, when possible, simply ignore them. The problem with weak evidence that is repeated as if true is that it never really is about the evidence in the first place. Twin studies is one of those things that, like dog whistle politics, stands in for something else. It is what I call a symbolic conflation, a distraction tactic to point away from the real issue. Few people talking about twin studies actually care about either twins or science. You aren’t going to convince a believer that their beliefs are false. If anything, they will become even more vehement in their beliefs and you’ll end up frustrated.

* * *

What Genetics Does And Doesn’t Tell Us
Heritability & Inheritance, Genetics & Epigenetics, Etc
Unseen Influences: Race, Gender, and Twins
Weak Evidence, Weak Argument: Race, IQ, Adoption
Identically Different: A Scientist Changes His Mind

Exploding the “Separated-at-Birth” Twin Study Myth
by Jay Joseph, PsyD

“The reader whose knowledge of separated twin studies comes only from the secondary accounts provided by textbooks can have little idea of what, in the eyes of the original investigators, constitutes a pair of ‘separated’ twins”—Evolutionary geneticist Richard Lewontin, neurobiologist Steven Rose, and psychologist Leon Kamin in Not in Our Genes, 19841

“The Myth of the Separated Identical Twins”—Chapter title in sociologist Howard Taylor’s The IQ Game, 19802

Supporters of the nature (genetic) side of the “nature versus nurture” debate often cite studies of “reared-apart” or “separated” MZ twin pairs (identical, monozygotic) in support of their positions.3 In this article I present evidence that, in fact, most studied pairs of this type do not qualify as reared-apart or separated twins.

Other than several single-case and small multiple-case reports that have appeared since the 1920s, there have been only six published “twins reared apart” (TRA) studies. (The IQ TRA study by British psychologist Cyril Burt was discredited in the late 1970s on suspicions of fraud, and is no longer part of the TRA study literature.) The authors of these six studies assessed twin resemblance and calculated correlations for “intelligence” (IQ), “personality,” and other aspects of human behavior. In the first three studies—by Horatio Newman and colleagues in 1937 (United States, 29 MZ pairs), James Shields in 1962 (Great Britain, 44 MZ pairs), and Niels Juel-Nielsen in 1965 (Denmark, 12 MZ pairs)—the authors provided over 500 pages of detailed case-history information for the combined 75 MZ pairs they studied.

The three subsequent TRA studies were published in the 1980s and 1990s, and included Thomas J. Bouchard, Jr. and colleagues’ widely cited “Minnesota Study of Twins Reared Apart” (MISTRA), and studies performed in Sweden and Finland. In the Swedish study, the researchers defined twin pairs as “reared apart” if they had been “separated by the age of 11.”4 In the Finnish study, the average age at separation was 4.3 years, and 12 of the 30 “reared-apart” MZ pairs were separated between the ages of 6 and 10.5 In contrast to the original three studies, the authors of these more recent studies did not provide case-history information for the pairs they investigated. (The MISTRA researchers did publish a few selected case histories, some of which, like the famous “Three Identical Strangers” triplets, had already been publicized in the media.)

The Newman et al. and Shields studies were based on twins who had volunteered to participate after responding to media or researcher appeals to do so in the interest of scientific research. As Leon Kamin and other analysts pointed out long ago, however, TRA studies based on volunteer twins are plagued by similarity biases, in part because twins had to have known of each other’s existence to be able to participate in the study. Like the famous MISTRA “Firefighter Pair,” some twins discovered each other because of their behavioral similarities. The MISTRA researchers arrived at their conclusions in favor of genetics on the basis of a similarity-biased volunteer twin sample. […]

Contrary to the common contemporary claim that twin pairs found in TRA studies were “separated at birth”—which should mean that twins did not know each other or interact with each other between their near-birth separation and the time they were reunited for the study—the information provided by the original researchers shows that few if any MZ pairs fit this description. This is even more obvious in the 1962 Shields study. As seen in the tables below and in the case descriptions:

  • Some pairs were separated well after birth
  • Some pairs grew up nearby to each other and attended school together
  • Most pairs grew up in similar cultural and socioeconomic environments
  • Many pairs were raised by different members of the same family
  • Most pairs had varying degrees of contact while growing up
  • Some pairs had a close relationship as adults
  • Some pairs were reunited and lived together for periods of time

In other words, in addition to sharing a common prenatal environment and many similar postnatal environmental influences (described here), twin pairs found in volunteer-based TRA study samples were not “separated at birth” in the way that most people understand this term. The best way to describe this sample is to say that it consisted of partially reared-apart MZ twin pairs.

The Minnesota researchers have always denied access to independent researchers who wanted to inspect the unpublished MISTRA raw data and case history information, and we can safely assume that the volunteer MISTRA MZ twin pairs were no more “reared apart” than were the MZ pairs

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