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.

    47/128 (page 41)
    It is clear that if there is a negative correlation between size of family and intelligence, and if the explanation is that intelligence is largely an inherited quality and that intelligent people tend to produce smaller families, then the average intelligence will sink generation by generation. There is indeed some direct evidence that this is so.7¢) Various calculations have been made based on correlations such as have been discussed in this memorandum, and on the correlation of parental intelligence with that of a son or daughter. The drop in average intelligence quotient has thus been estimated by Cattell() to be 3 points per generation, but I think he exaggerates. Fraser Roberts thinks 14 points. Perhaps even this is an overestimate, but I find it difficult to avoid the fear that the fall may be as much as one point per generation: and this is a serious matter. Much of course depends on the relative influences of nature and nurture in determining intelligence. There has been a great deal of quite good research done on this, but to go into it would make this memorandum too long. No one thinks that heredity has no influence: doubts arise only when one asks to what extent education can develop potential intelligence or even “ create ” intelligence. No doubt the Commission has had reports and bibliographies on this from others. I would only like to say that in my opinion the work of the University of Iowa Group in recent years, extolling the power of environmental factors to change intelligence, is invalidated by statistical errors in drawing their conclusions. See for example the criticisms of Dr. McNemar (a competent statistician) in the Psychological Bulletin for February, 1940. I think myself that at least half the scatter of intelligence in a community like the United Kingdom would remain even if all environmental handicaps were removed. This statement is, of course, not inconsistent with a belief that there are at present environmental handicaps which ought to be lightened and if possible removed, and that we have by no means reached anything like the ideal of equal educational opportunity at which we ought to aim. Equal opportunity, however, if it removed the danger of a continued fall in intelli- gence, would not do so by a direct effect, but by lessening the differential incentives: for if intelligent and ambitious parents were assured that their children even if numerous would certainly have every chance to reach what they consider a desirable position in life, they might, and I think they would, have larger families. 2. COMMENT BY PROFESSOR R. A. FISHER I am in very substantial agreement [with Professor Sir Godfrey Thomson’s memorandum], save for one, perhaps small, point. I think Dr. Thomson exaggerates the difference between Cattell’s and Fraser Roberts’ estimate of the rate of fall per generation in the average intelligent quotient. It should, of course, be made clear that the estimates mentioned by Thomson are on the Binet IQ. scale, not on the Otis IB. scale, which is rather more than twice as wide. Fraser Roberts’ data for mean intelligence of children belonging to families of different sizes (Table IV, p. 186, Annals of Eugenics, Volume VIII) are on an Index of Brightness scale, and give a mean for all children of 99.768, compared with a mean for all families of at least one child of 103.906, difference 4.138, representing the average fall in intelligence in one generation, ignoring all of the last genera- tion who have no children. If to translate this rate of change into the 1.Q. scale we use the regression of Binet I.Q. on Otis IB., namely, b = 0.3906640 (p. 191), the corresponding I.Q. value is 1.6167. If the (1) For example, E. O. Lewis’s investigation. See Report of the Mental Deficiency Com- mittee (A. H. Wood, Chairman) H.M. Stationery Office, 1929, especially Vol. IV, and therein page 82. Z (?) Cattell, R. B., The Fight for our National Intelligence. London, 1937.