Memoranda presented to the Royal Commission.

  • Great Britain. Royal Commission on Population.
Date:
1950
Catalogue details

Licence: Public Domain Mark

Credit: Memoranda presented to the Royal Commission. Source: Wellcome Collection.

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    (almost as high as the correlation between two successive testings of the same individual). Even when the twins have been reared separately from early years, the correlation is still 0.77—far higher than that between brothers or sisters brought up in the same home. In these and other respects the inheritability of tested intelligence is thus remarkably like that of stature. The measurable height of a child may be reduced by illness or malnutrition, particularly during infancy; but that does not obscure the fact that the main determining influence, especially when all are brought up in favourable condi- tions, is still the individual’s inborn constitution. And the same holds good of the measurements obtained with intelligence-tests. Definition of Intelligence. Accordingly, I think it safe to say that the vast majority of psychologists would now agree that, of the variance exhibited by measurements of the hypothetical factor underlying all cognitive tests, one half at the very least is attributable to “aature”’ as distinct from “nurture”. Hence, in view of its evident importance, we need a special term to describe the “native” part of the “common factor”. Since this native component corresponds precisely with what the testers of intelligence originally set out to measure, it seems appropriate in psychological discus- sions to keep their term intelligence for this particular component, and so to define intelligence as innate, general, cognitive ability. As thus defined, intelligence is merely a distinguishable influence, not something that can be separated in actual fact, or measured in isolation. Our knowledge of it is there- fore bound to be imperfect. But I would add that, even if we adopt the view of the more cautious writers, and insist that intelligence may be no more than a statistical construct, the foregoing arguments about the decline of the average level of this common factor would still hold good. However, I personally believe that there is further evidence of a non-statistical kind which enables us to give a more concrete interpreta- tion to the term. A Physiological Interpretation, The mental processes essentially required by intelli- gence tests involve the integration of a variety of perceptual and motor activities into a systematic whole: the more the processes tested depend on this integration, the closer they correlate with intelligence. \Now the essential characteristic of the central nervous system is, in Sherrington’s phrase, its “integrative action”. (Hence it is natural to suggest that the individual differences in integrative capacity revealed by the tests may be primarily due to differences in the neuronic architecture of the central nervous system. The histological study of the cortex shows that in feeble-minded persons the neurons are (i) comparatively few:in number, (ii) have comparatively few branches, (iii) are less systematically arranged. These are precisely the kind of sturctural differ- ences that we should expect if the foregoing suggestions were true. We may plausibly suppose that, in normal members of the population, somewhat similar if less conspicuous variations in the cortical architecture are also present. Further, since any given kind of bodily tissue tends to be of much the same general quality in the same individual wherever it is found (e.g. muscle, hair, skin, etc.), we may expect that the nervous tissue would also have the same structural qualities throughout the individual’s nervous system, or at any rate throughout his cerebral cortex, though, of course, with minor regional differences. Finally, being structural, these differences may readily be con- ceived as being largely, if not mainly, dependent on genetic factors, i.e. in popular language, as being inherited or at least innate.(’) Alternative Definitions. (1) The best-known statement of the case against the heredi- tarian view is that of Bagley, who represents the self-styled school of “rational equalitarians’’. His first paper,(°) which virtually started the psychological controversy, left his critics with the impression that, in his opinion, “ human mentality alone among all biological traits is not subject to biological variation”; and that differences alike in everyday efficiency and in efficiency at intelligence tests were due mainly, if not solely, to the physical, social, and educational advantages or disadvantages of the individual tested. Similar contentions are still frequently brought forward by medical and social writers who distrust the evidence of intelligence tests. (1) I believe this expresses the tacit assumption of most psychologists, though it is seldom stated explicitly. The nearest approach is to be found in Thorndike’s recent ‘‘ quantitative hypothesis ’’, viz. the hypothesis that “‘ quality of intellect depends on quantity of neural connections ’”’ (The Measurement of Intelligence, Chap. xv). As an adherent of the old associationist school, Thorndike identifies the qualitative superiority of intelligence with a superiority in ‘‘ the number of possible associations the individual mind can make’’, and attributes this to the “* number of possible contacts in the associative neurones’. He refers in passing to the view that higher forms of intellectual activity may be dependent on “‘ integration, that is, capacity for having the neurones act with reference to one another ’’;but considers it an open question whether such a capacity (if it exists at all) may not be independent of intelligence as he conceives it. (2) Bagley, W. C., ‘““Democracy and the I.Q.”, incorporated as Chapter I in the volume cited below.