Origins, Publications, and Scandal
When we come to the separated [twin] cases, a somewhat dif- ferent situation exists. It seems possible that our group is more heavily weighted with extremely similar pairs than with identi- cal twins of less striking similarity.
— TRA researchers Horatio Newman, Frank Freeman, & Karl Holzinger, 1937 (Newman, Freeman, & Holzinger, 1937, p. 31) In all 12 pairs there were marked intra-pair differences in that part of the personality governing immediate psycho- logical interaction and ordinary human intercourse. . . . The twins behaved, on the whole, very differently, especially in their cooperation, and in their form of and need for contact.
Corresponding with these observations, the twins gave, as a rule, expression to very different attitudes to life, and very divergent views on general culture, religion and social prob- lems. Their fields of interest, too, were very different. . . . Those twins who had children treated, on the whole, their children differently, and their ideas on upbringing were, as often as not, diametrically opposed. Characterologically, the twins presented differences in their ambitions and in their employ- ment of an aggressive behavior . . . . Various traits of person- ality found their expression in differences in taste, mode of dress, hair style, use of cosmetics, [and] the wearing of beard or of glasses.
— TRA researcher Niels Juel-Nielsen, 1980 (Juel-Nielsen, 1965/1980, Part I, p. 75, italics in original) Without exception [MZAs] were brought up in different homes for at least five years during childhood.
— TRA researcher James Shields, 1962 (Shields, 1962, p. 27)
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The most accurate description of this sample is “MZ twins par- tially reared apart.”
— Susan Farber, author of Identical Twins Reared Apart: A Reanalysis, 1981, writing of all pairs studied through 1980 (Farber, 1981, p. 273)
The First Studied Reared-Apart Monozygotic Twin Pair
The first single-case MZA study published in the context of the nature–nurture question has gone down in history as the “Popenoe–
Muller pair,” based on the original 1922 report by Paul Popenoe, fol- lowed by a subsequent 1925 analysis of the same pair by future Noble Laureate H. J. Muller. Popenoe and Muller were prominent members of the American eugenics movement, and both were supporters of com- pulsory eugenic sterilization (e.g., Muller, 1933; Popenoe & Johnson, 1933), although Muller represented the left wing of the American eugenics movement, whereas Popenoe was a champion of the hard- line eugenics positions put forward by Charles Davenport and others (Davenport, 1911).
In his original report, Popenoe discussed “Jessie and Bess,” two American girls in their late 20s separated at 8 months and raised “by two families who lived on ranches” (Popenoe, 1922, p. 142). The twins had been reunited at age 18. Popenoe did not meet or study the twins personally, and relied on correspondence from Jesse. In her letters, Jesse wrote about herself and her twin sister, and their subsequent close rela- tionship. For example, she wrote, “It is almost uncanny, the way we are always doing identical things at the same time,” “We are both high strung,” “An intelligence test would find our capacities very similar,”
and “We have never had a disagreement between ourselves, and while I am fond of my older sister and two brothers, yet [sic] they have never seemed as close to me as Bess” (p. 144). In the last passage Jessie spoke of her close relationship with Bess, a critical factor contributing to the similarity of supposedly “separated” MZ twin pairs. Thus, from the very first pair studied we see that, far from being “separated” with no subse- quent contact, these twins had a close emotional bond with each other for nearly a decade.
On the basis of some differences in their upbringing, Popenoe con- cluded that, due to their “mental similarities, it is impossible to resist the conclusion that the psychical make-up of the individual is very largely settled by the time he is born” (1922, p. 144). It is indeed “impossible”
to resist this conclusion when one already believes strongly that peo- ple are born largely with fixed mental properties and tendencies (see Popenoe & Johnson, 1918), and Popenoe is one of the first of a long line of twin researchers (reared together or reared apart) who interpreted
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twin data in ways that fit their strong preexisting beliefs in the impor- tance of heredity.
Three years later, Muller reported on an examination and testing of Jessie and Bess. Muller (correctly) noted that pedigree charts showing that traits run in families could be explained by the common environments expe- rienced by family members, leading to a “false appearance of inheritance”
(Muller, 1925, p. 433). In the case of twins, however, he believed that “we are presented with material of the type sought for, namely, genetically iden- tical individuals, in the cases of identical twins” (p. 433). He believed that the environments MZT pairs experience when growing up were not very different, and that instances in which twins are reared apart could supply important information about hereditary influences on psychological char- acteristics. Jessie and Bess were given psychological tests in the summer of 1923 while they were living in different states. They registered very similar IQ scores. On personality tests, however, they scored very differently, about as differently as two randomly selected unrelated persons would score.
Muller concluded,
The responses of the twins to all these tests—except the intel- ligence tests—are so decisively different almost throughout, that this one case is enough to show that the scores obtained in such tests indicate little or nothing of the genetic basis of the psychic make-up. And yet the results of such tests have, it is claimed, been shown to be correlated distinctly with characteristics of importance.
(p. 442) Muller believed that, in the future, additional MZA pairs should be identified and tested in order to assess the relative importance of heredity and environment, although like most subsequent TRA investigators he already believed that “genetic differences . . . undoubtedly do underlie much human psychological variation” (p. 444). Soon after, University of Chicago biologist Horatio Newman and his colleagues accepted this challenge.
The Classical Reared-Apart Twin Studies
By the early 1970s there were four recognized “classical” TRA studies, which included an IQ study by the British psychologist Cyril Burt that was subsequently discredited by the late 1970s (see below). The three remaining classical TRA studies were published by Horatio Newman (1875–1957) and colleagues in 1937, James Shields in 1962, and Niels Juel-Nielsen in 1965 (republished with an update in 1980). Each of the three studies was described in a carefully detailed book, which included case histories and test scores for most pairs.
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TRA studies are sometimes referred to as “adopted twin studies,”
which reminds us that many of these twins/adoptees were abandoned children (Cassou et al., 1980), or “throwaway kids” (Pam, 1995), and most grew up in working-class, rural, or impoverished families. Their adopted family lives were often chaotic and nurtureless, and many twins undoubtedly suffered deeply by being kept apart from their co-twin. As Juel-Nielsen described his Danish sample, “If using a concept of ‘bro- ken homes’ . . . the homes of every twin pair in the present investiga- tion must be said to belong to this category” (Juel-Nielsen, 1965/1980, Part I, p. 126).1
Newman, Freeman, and Holzinger, 1937
The first TRA study was performed by Newman, Freeman, and Holzinger in the United States, and was published in 1937 as Twins: A Study of Heredity and Environment. The researchers studied 19 MZA pairs, and used MZTs and DZTs as controls (Newman et al., 1937).
The study was divided into two parts. The first was a twin method comparison of 50 MZT pairs and 50 DZT pairs. The second part con- sisted of a study of 19 MZA pairs, whose results were compared mainly with the MZTs. The main areas of focus were IQ as measured by the Stanford-Binet test, and personality as assessed by the Woodworth- Mathews questionnaire.
In addition to assessing MZA correlations and behavioral similarities, a major goal of the study was to compare MZA correlations or mean dif- ference scores to the same measures among MZTs:
One obvious way to treat the data is to compare the average dif- ferences for the [MZA] group as a whole with the average differ- ences in the case of the identical twins reared together [MZTs].
(Newman et al., 1937, p. 356) Newman and colleagues reported Stanford-Binet IQ correlations of 0.91 MZT, and 0.67 MZA. For the Woodworth-Mathews personality test, they reported an MZT correlation of 0.56, and an MZA correlation of 0.58. They also reported correlations for their DZT group.
According to Eysenck, writing 30 years later about the personality tests used by Newman and colleagues, “It is doubtful whether any psycholo- gist would nowadays wish to make very strong claims for these meas- ures . . . . the measures used would not now be regarded as either reliable or valid” (Eysenck, 1967, p. 192). Furthermore, according to Eysenck, these “were essentially tests for adults,” and “it is quite inadmissible to use tests of this kind on children” (Eysenck, 1967, p. 193). Five of the 19 Newman and colleagues MZA pairs were under the age of 18
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(Table 2.1). Newman and colleagues themselves recognized, in relation to the Woodworth-Mathews test, the “unreliability of the measure,” casting further doubt on the validity of the reported correlations (Newman et al., 1937, p. 348).
The researchers came from different fields: Newman was a biologist, Freeman a psychologist, and Holzinger was a statistician. In their final section they recognized the differing viewpoints and interpretations each brought to the study, and, while concluding that hereditary factors play a role, they concluded that environmental factors also play a major role.
Indeed, three years later Newman would write, “A fairly common criti- cism of our book on twins is that it plays up the environmental factors and tends to minimize the hereditary factors” (Gardner & Newman, 1940, p. 126). For example, they wrote:
Differences in the environment which actually sometimes occur, as exemplified in our separated pairs, are sufficient to produce differences in weight, ability, and behavior large enough to over- shadow the genetic differences which occur between siblings.
(Newman et al., 1937, p. 359) There are a number of instances in which a rather large difference exists between the personalities of twins as shown by the tests or by observation, or by both, in which a large difference also exists between their environments, and in which it seems plausible to infer a relation between the two sets of differences.
(p. 360) They also found “consistent and significant positive correlations”
between “educational ratings” and IQ scores (p. 341), and found that
“the advantages of several years of schooling of one twin over the other produce marked differences in the broad educational test” (p. 341).
Overall, their “correlations indicate that differences in education and social environment produce undeniable differences in intelligence and scholastic achievement as measured by our tests” (p. 341).
Newman and colleagues concluded that, if at the beginning of their project they had hopes of “reaching a definitive solution of the general nature–nurture problem . . . they were destined to be rather disillusioned”
(p. 362). They believed that they had not “provided a comprehensive or final solution of the problem within our field of study,” and invited others to evaluate their data and to arrive at their own interpretations (p. 363).
Sampling bias
In TRA studies it must be assumed that the MZA sample is representa- tive of the total population of MZAs (which of course is not large), and is
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not systematically biased in favor of MZAs who are more similar to each other than are MZAs in general. Newman and colleagues, who recruited their MZA pairs through newspaper and radio appeals, were well aware of the likelihood that their MZA group was not representative of the pop- ulation of MZAs, and therefore was biased in favor of more similar pairs:
When we come to the separated [MZA] cases, a somewhat dif- ferent situation exists. It seems possible that our group is more heavily weighted with extremely similar pairs than with identical twins of less striking similarity.
(p. 31) And later they wrote that “the separated group may very likely be a somewhat biased sampling” (p. 356). As most critics (and some twin researchers) have pointed out, MZA samples recruited through media appeals are biased because the twins had to have known of each other’s existence to be able to respond to these appeals, and may have come forward because they believed themselves to be similar. MZA pairs who did not know of each other’s existence and had no contact, and who therefore may have differed behaviorally and intellectually to a far greater degree, are missed in TRA studies based on media appeals.
Some examples of bias in the methods used to recruit twins are taken from pages 132–136 of Newman and colleagues’ book. The investigators were describing the responses they received to their newspaper and radio appeals for twins to come forward in the interests of science. Following each passage, I will comment on the potential for bias, as well as the motivation for twins to exaggerate or even lie about themselves and their degree of separation. It is even possible that some pairs were not reared apart at all, but simply invented their separation for monetary gain or for publicity, especially under the severe hardship conditions of the Great Depression. I want to emphasize that I am speaking of the possible exag- geration and deception of the twins themselves as one of many potential pitfalls of TRA studies, not of Newman, Freeman, and Holzinger, who undoubtedly were honest and thorough investigators. Nevertheless, the financial limitations of this Depression-era study led to a biased sample in part because the investigators decided not to study dizygotic twins reared apart (DZAs), and therefore intentionally sought more similar MZA pairs in order to decrease the likelihood that they would incur the expense of bringing pairs to Chicago who would turn out to be DZAs.
• “The first case studied by us of identical twins reared apart gained considerable publicity through no fault of ours, for the twins themselves gave their photograph and their life-stories to an enterprising local reporter, who sent his news story to an American newspaper, from
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which it was copied far and wide.” Comment: These twins appeared to be seeking publicity and notoriety. They may have exaggerated their similarities and degree of separation for this reason.
• “The second pair of separated twins studied by us desired publicity in the hope that it might be the means of bringing them information about their unknown parents.” Comment: This pair might have exaggerated their similarities and degree of separation for the purpose of locating their parents.
• “In one of the cases a man wrote that his twin brother, from whom he had been separated at four years, was now in Denmark and does not speak English. Apart from the prohibitive expense of bringing this man from Denmark, it would have been impossible to give him any of the mental tests we have been using.” Comment: Twins who lived far apart and grew up in different cultures—and who may have greater IQ and personality differences because of these differing environments—were excluded from the study for financial and language reasons.
• “In another case one twin lived in Alaska and the sister in California.
This case might have been managed by a Californian, but the distances were too great for us.” Comment: The researchers were unable to study this pair because of their geographical distance both from each other and from the researchers’ home base of Chicago. It is likely that this and other excluded pairs living far apart from each other were culturally and environmentally separated as well (Taylor, 1980).
• “In still another case the twins were separated by the whole width of the American continent, and, moreover, neither twin could leave her job long enough to come to Chicago unless we would pay their salaries during their absence.” Comment: See the above comment.
• “Pair after pair, who had previously been unmoved by appeals to the effect that they owed it to science and to society to permit us to study them, could not resist the offer of a free, all-expense-paid trip to the Chicago Fair.” Comment: This offer of an all-expense-paid trip to Chicago in the midst of the Great Depression may have induced twins to exaggerate their similarities and degree of separation (Kamin, 1974).
• “Because of the great expense involved in bringing these separated twins to Chicago, no chances were taken that any of them might prove to be fraternal twins. In every case an affirmative answer to the following questions was required before twins were asked to come to us for study:
1 Are you or have you been at some time so strikingly similar that even your friends or relatives have confused you?
2 Do you yourselves believe that you are far more alike than any pair of brothers or sisters you know of?
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3 Can you send us a good photograph of yourselves, taken together in about the same positions?”
Comment: This meant that twins reporting dissimilarity, who nevertheless may have been dissimilar MZAs, were excluded on the grounds that they might be DZAs. The behavioral “alike- ness” and “striking similarity” of twin pairs were requirements for participation in the study.
• “One case was excluded because the twins wrote: ‘A good many people think we are identical twins, but we ourselves do not think we are so very much alike.’ Another case failed to meet our requirements because one of the twins wrote that, while they look very much alike so that they were sometimes mistaken for each other, they were ‘as different as can be in disposition, and I am almost as much like my older sister as I am like my twin.’ These two cases may have been monozygotic, but the uncertainty was too great for us to advance the rather large sum of money required for their transportation.”
Comment: Once again, the researchers described how they excluded dissimilar twins and recruited only twins reporting that they were similar. The study was therefore heavily biased by this fact alone, in that the researchers recruited only pairs who reported that they shared many similarities.
As Newman and colleagues recognized, their MZA sample was biased by more similar pairs. We will see in Chapter 3 that subsequent critics elaborated further on this point, and questioned the twins’ accounts of their similarities and degree of separation.
Were these really “reared-apart” MZ twins?
A major issue in TRA studies is the question of whether the pairs under study were truly “reared-apart twins.” Table 2.1 contains information provided by Newman and colleagues for all 19 MZA pairs they studied, including twins’ degree of contact and association. The names, ages, and placements of each pair are listed, along with some passages from the case descriptions.
As seen in Table 2.1, 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