Terman’s belief in high heritability of intelligence: unwarranted confidence?

Russell Warne has a new piece out reviewing the life and criticism of Lewis Terman. While Warne generally defends Terman against criticism, he thinks that Terman went beyond his evidence regarding his strong belief in the heritability of traits. He writes:

For all the previously mentioned flaws, one criticism of Terman seems absent from the previous literature: Terman’s willingness to form a forceful opinion when the data were not strong enough to support this degree of confidence. For example, Terman stated, “All the available facts that science has to offer support the Galtonian theory that mental abilities are chiefly a matter of original endowment” (Terman, 1922e, p. 659). In response to sentiments like this, Minton (1988) stated, “Terman never provided unequivocal evidence that IQs reflected native ability. Based on his weddedness to Hall and Galton’s biological determinism, he simply assumed that IQs were genetically determined” (p. 200). Indeed, no psychologist at the time had data that could separate the influences of heredity and environment on intelligence test scores, so anyone who had a strong opinion about the topic—including Terman— lacked the empirical evidence to support their views. The first behavioral genetics studies that could estimate the impacts of genetics and environment would be published a few years later (Burks, 1928/1973; Hildreth, 1925; Tallman, 1928; Wingfield, 1928). 3 In 1922, there were cor – relational data that supported Galton’s and Terman’s views, but these same data just as easily supported theories of purely environmental causes of interindividual differences for intelligence. Terman seems to have downplayed this possibility in the first half of his career when discussing the relative importance of nature and nurture (e.g., Terman, 1906, p. 372, 1922e, 1928a; Terman et al., 1915). He had a penchant for interpreting correlational data as being causal in nature—a tendency that extended beyond his research on intelligence (Hollingworth, 1939). [my bolding]

Warne is referring to the lack of modern style twin and adoption studies at the time Terman was writing. However, the absence of these do not imply that evidence was generally unavailable. It was however more indirectly. I can think of several lines of evidence:

  1. Family studies of physical features and various animal breeding studies show similar and strong relationships, and these are unlikely to be significantly environmental in origin. This increases the prior that the same will be found for other human traits. This argument was made by Galton in 1865 (Hereditary Character and Talent):

    As no experiment of this description has ever been made, I cannot appeal to its success. I can only say that the general resemblances in mental qualities between parents and offspring, in man and brute, are every whit as near as the resemblance of their physical features; and I must leave the existence of actual laws in the former case to be a matter of inference from the analogy of the latter.

  2. Evolution cannot select for traits that are not heritable, so if it has evolved — as one could know at the time from the study of ancient crania — it must have been or still be heritable. From a variety of data one can infer that intelligence is not likely to have reached fixation at some maximum value, hence must still be heritable.
  3. Animal breeding, e.g. in dogs, was known to be able to breed for any desired trait, including intelligence and other psychological traits. This is only possible if traits are heritable, and thus, by generalization to humans, so are they expected to be as well.
  4. The main alternative hypothesis, that social class or parenting causes children’s intelligence, was already known at the time to be unsatisfactory on its own. Children of very rich people did not show extreme rates of giftedness as one would expect on a wealth model. Terman’s student Catherine Cox found (in 1926) that illustrious people were usually very gifted already in early childhood, before environmental effects could have had major cumulative effect. Furthermore, children within a family were known to vary widely, which cannot be plausibly explained by any parenting effects, but genetics is the obvious hypothesis (it could also be random).
  5. Galton had in fact carried out the first crude adoption study by investigating the relative eminence of popes’ biological children vs. adopted sons, often distant relatives. This argument is described in Hereditary Genius (1869):

    It is difficult to specify two large classes of men, with equal social advantages, in one of which they have high hereditary gifts, while in the other they have not. I must not compare the sons of eminent men with those of non–eminent, because much which I should ascribe to breed, others might ascribe to parental encouragement and example. Therefore, I will compare the sons of eminent men with the adopted sons of Popes and other dignitaries of the Roman Catholic Church. The practice of nepotism among ecclesiastics is universal. It consists in their giving those social helps to a nephew, or other more distant relative, that ordinary people give to their children. Now, I shall show abundantly in the course of this book, that the nephew of an eminent man has far less chance of becoming eminent than a son, and that a more remote kinsman has far less chance than a nephew. We may therefore make a very fair comparison, for the purposes of my argument, between the success of the sons of eminent men and that of the nephews or more distant relatives, who stand in the place of sons to the high unmarried ecclesiastics of the Romish Church. If social help is really of the highest importance, the nephews of the Popes will attain eminence as frequently, or nearly so, as the sons of other eminent men; otherwise, they will not.
    Are, then, the nephews, &c. of the Popes, on the whole, as highly distinguished as are the sons of other equally eminent men? I answer, decidedly not. There have been a few Popes who were offshoots of illustrious races, such as that of the Medici, but in the enormous majority of cases the Pope is the ablest member of his family. I do not profess to have worked up the kinships of the Italians with any especial care, but I have seen amply enough of them, to justify me in saying that the individuals whose advancement has been due to nepotism, are curiously undistinguished. The very common combination of an able son and an eminent parent, is not matched, in the case of high Romish ecclesiastics, by an eminent nephew and an eminent uncle. The social helps are the same, but hereditary gifts are wanting in the latter case.

I submit that the above lines of argument were available to Terman early in 1900s, as some of them were to Galton and Darwin in the late second half of 1800s, and a reasonable person would take these into account and adjust the prior for heritability of human intelligence upwards to a high value. In fact, his beliefs have been mostly vindicated by later studies, which is evidence that he either didn’t base his beliefs on as thin air as supposed, or that the common perceptions of psychology of that time were remarkably accurate. I’ll let the reader pick their favorite interpretation!

Between group heritability

Aside from the within group heritability, Warne faults Terman for confidently saying that between group gaps in USA were also due to genetics. He quotes Terman as:

How much of this inferiority [in intelligence test scores of Hispanic groups] is due to the language handicap and to other environmental factors it is impossible to say, but the relatively good showing made by certain other immigrant groups similarly handicapped would suggest that the true causes lie deeper than environment. (Terman, 1926, p. 57)

The quote choice is curious considering the parts I highlighted. Warne also quotes another chunk:

. . . represent the level of intelligence which is very, very common among Spanish-Indian and Mexican families of the Southwest and also among negroes. Their dullness seems to be racial, or at least inherent in the family stocks from which they come. The fact that one meets this type with such extraordinary frequency among Indians, Mexicans, and negroes suggests quite forcibly that the whole question of racial differences in mental traits will have to be taken up anew and by experimental methods. The writer predicts that when this is done there will be discovered enormously significant racial differences in general intelligence, differences which cannot be wiped out by any scheme of mental culture. (Terman, 1916, pp. 91-92

But it has similar problems. Predicting something is not the same as holding a confident view about it. Writing that something “suggests quite forcibly” is hardly being extremely confident in a belief.

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