Search the Community

Herrnstein and Murray (1994) estimate that ‘the genetic component of IQ is unlikely to be smaller than 40 percent or higher than 80 percent’ (p. 105). And claim that ‘The most unambiguous direct estimates, based on identical twins reared apart, produce some of the highest estimates of heritability.’ (p. 107). Moreover, they say that: ‘The purest of the direct comparisons is based on identical (monozygotic, MZ) twins reared apart, often not knowing of each other’s existence. Identical twins share all their genes, and if they have been raised apart since birth, then the only environment they shared was that in the womb. Except for the effects on their IQs of the shared uterine environment, their IQ correlation directly estimates heritability.’ (p. 107) This ideal scenario of reared-apart twin research, put forth by Herrnstein and Murray (1994), is far from the reality of what actually occurs in studies of reared apart twins. The use of the term ‘reared apart’ is highly problematic in and of itself. Put simply, most twins ‘reared-apart’, aren’t reared apart for significant portions of their developing lives. Jay Joseph (2015) evaluated some of the most well-known twin studies and I will be relying heavily on his work to justify this claim. The first twins reared-apart (TRA) study was conducted by Newman, Freeman, and Holzinger (1937, cited in Joseph, 2015) and included 19 pairs of monozygotic twins reared-apart (MZA). Here’s Joseph’s summary of each pair: ‘the MZA age of separation ranged from 3 weeks to 6 years, and pairs often grew up in the same town or region. Rather than being “separated,” many pairs had regular and prolonged contact and, more importantly, had a relationship with each other. For example, Pair I corresponded with each other and had been living together for 1 year when studied; Pair II had lived and worked together for 5 years; Pair IV had visited each other all their lives; Pair V lived together for 1 year and had visits and were regularly in correspondence; Pair VI was in regular contact their entire adult life and were living together at age 58 when studied; Pair VII had annual visits; Pair IX lived 3 miles apart and saw each other regularly; Pair XII had seen each other often for 5 years leading up to the study; Pair XIII visited each other regularly in the years leading up to the study; Pair XIV corresponded and tried to spend a few weeks per year with each other for the 15 years leading up to the study; and Pair XIX was reared together for the first 6 years of life and studied nursing together at age 17.’ (p. 24 & 28) Shields (1962, cited in Joseph, 2015) published the second TRA study which included 44 MZA pairs. Again, Joseph summaries the supposedly ‘reared-apart’ twins: ‘Examples from Shields’ case descriptions of the 44 MZA pairs seen in Table 2.2 include, “have always been closely attached to each other,” “have been in business together for the past 8 years,” “were in cottages next door to one another and attended the same school,” “went to school together,” “came home to mother at 14,” “ were dressed alike . . . . They attended the same school,” “met about once a fortnight during adolescence,” “brought up within a few hundred yards from one another,” “met about twice a week and sometimes spent holidays together,” “met regularly,” “now correspond frequently and meet at holidays,” “until [separation at age 8] the twins had done everything together,” “formed an extremely close association,” “brought up together till the age of 7,” “were reunited most of the time from 5 to 15,” “were closely attached and went about a lot together,” “became very close . . . . they are mutually dependent,” “After reunion [at age 5] in the parental home the twins went to private schools together until [age 17],” and “lived a few roads away from each other in the same northern industrial town. They were dressed alike.”’ (p. 30-31) Juel-Nielson (1965, cited in Joseph) studied 12 MZA pairs without using a control group. Here is Joseph’s summary of the twins: ‘age at separation ranged from 1 day to almost 6 years, and 5 of the 12 pairs spent at least the first year of life together. In addition, Pair IV (“Ingegerd & Monika”) was reared together with their mother between the ages of 7 and 14. Several pairs had a close relationship and years of mutual contact. Each of the 12 case histories Juel-Nielsen presented contained a section called “The Twin Relationship,” which should not be found in a study of “reared-apart” twins where the common perception is that twins were separated at birth and had never met, and therefore had no relationship with each other. Most twins in this study grew up in impoverished rural or urban environments. This restricted range of rearing environments added an additional important similarity-producing bias to the study.’ The Minnesota Study of Twins Reared Apart (MISTRA) is the most well-known TRA study. It’s been referenced in many popular books including Pinker’s The Blank Slate (2002) and Ridley’s Nature Via Nurture (2003). One of the MISTRA’s researchers, Nancy Segal, was interviewed on FDR last year. Segal authored Born Together – Reared Apart (2012), which discusses the findings of the MISTRA. As you may by now be able to guess, the use of the term ‘reared apart’ in her book title is far from accurate. As Joseph (2015) notes, the MISTRA ‘consisted mainly of MZA pairs only partially reared apart, most of whom grew up reared together—at the same time—in similar social, political, and cultural environments.’ (p. 103). The facts about the TRA study samples enumerated above clearly conflict with reasonable conceptions of what it means to be ‘reared apart’. Joseph (2015) also evaluates other TRA studies in his book, citing similar flaws. To come back to The Bell Curve (1994) for a moment, what is concerning about Herrnstein and Murray’s work, is that they do not appear to mention any of the issues regarding the use of the term ‘reared apart’ as it relates to twin studies. In fact, their section devoted to genetics and IQ is only about 4 pages long (see p. 105-108). The sad fact is that the mislabelling of these twin studies as ‘reared apart’ is just the tip of the iceberg with regards to their flaws. Below is a partial list of TRA study issues and biases discussed by critics, summarised by Joseph (2015): Many twin pairs experienced late separation, and many pairs were reared together in the same home for several years Most twin pairs grew up in similar socioeconomic and cultural environments MZA correlations were inflated by non-genetic cohort effects, based on common age, common sex, and other factors Twins share a common pre-natal (intrauterine) environment, and the MZA pre-natal environment is more similar than the DZA pre-natal environment TRA study findings might not be (or are not) generalizable to the non-twin population In studies based on volunteer twins, a bias was introduced because pairs had to have known of each other’s existence to be able to participate in the study MZA samples were biased in favor of more similar pairs, meaning that studied MZA pairs are not representative of MZAs as a population The similar physical appearance and level of attractiveness of MZAs will elicit more similar behavior-influencing treatment by their social environments Twins sometimes had financial and other types of incentives to exaggerate or lie about their degree of separation and behavioral similarity, and their accounts are not always reliable There were several questionable or false assumptions underlying the statistical procedures used in the studies MZA pairs were not assigned to random environments There was researcher bias in favor of genetic interpretations of the data There were problems with the IQ and personality tests used The validity of concepts such as IQ, personality, and heritability are questionable In cases where evaluations and testing were performed by the same person, there was a potential for experimenter bias in favor of twin similarity A registry should be established to house raw TRA study data, which should be made available for independent inspection and analysis I encourage everyone who is interested in the topic of individual differences (including IQ) to read about the flaws in twin research. This research method has become the foundation of a lot of behavioural genetics research but it is deeply flawed. It seems that most critics of behavioural genetics are labelled as left-leaning, PC egalitarians who think that identifying differences in race, IQ, etc. is prejudicial. That may be so, but you don’t have to be a PC lefty to be sceptical of the conclusions drawn by behavioural geneticists. As Joseph (2015) has demonstrated, there is a plethora of reasons to doubt their conclusions (e.g. IQ heritability estimates). I recommend Joseph’s books The Trouble with Twin Studies (2015) and The Gene Illusion (2004), which I have referenced below. If you’d prefer not to read a whole book on the subject, Joseph also blogs at Mad in America (created by Robert Whitaker, of whom many of you may be familiar), where you can find several articles that concisely explain his arguments against the validity of twin studies. ReferencesHerrnstein, R. J., & Murray, C. (1994) The Bell Curve: Intelligence and Class Structure in American Life. New York: Simon & Schuster Joseph, J. (2015). The trouble with twin studies: A reassessment of twin research in the social and behavioral sciences. Routledge. Pinker, S. (2002). The blank slate: The modern denial of human nature. Penguin. Ridley, M. (2003). Nature via nurture: Genes, experience, and what makes us human. HarperCollins. Segal, N. L. (2012). Born together—reared apart: The landmark Minnesota twin study. Harvard University Press. Also see: Joseph, J. (2004). The Gene Illusion. New York. Algora. The Trouble with Twin Studies (Featured Blog): http://www.madinamerica.com/2013/03/the-trouble-with-twin-studies/ Studies of Reared-Apart (Separated) Twins: Facts and Fallacies (Featured Blog): http://www.madinamerica.com/2014/12/studies-reared-apart-separated-twins-facts-fallacies/ “Bewitching Science” Revisited: Tales of Reunited Twins and the Genetics of Behavior (Featured Blog): http://www.madinamerica.com/2016/03/bewitching-science-revisited-tales-of-reunited-twins-and-the-genetics-of-behavior/