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.

    51/128 page 45
    The figure of 14 points of Binet I.Q. per generation is quoted from a semi-popular paper in which I said: “a little more than 14 points.” My workings gave 1.6. Professor R. A. Fisher has shown me, however, that in converting units of Otis I.B. into units of Binet 1.Q. I had not utilized all the information available regarding the interrelationship of the two tests. The best estimate is actually 2.0 points per generation. The calculation can be made from a table showing mean intelligence test scores for each size of family. The difference between the mean of all children and the mean of all families provides the required figure. Cattell’s tables give only the reverse information, i.e. the mean family size corresponding to different intelligence test scores. This allows a somewhat rough and inaccurate estimate. The inaccuracy, however, is likely to be much the same for the two sets of data. Applying this approximate method to the Bath data, the estimated fall per generation is 2.2 points of Binet I.Q. (instead of the correct figure of 2.0); Cattell’s material also yields an estimate of 2.2 points. Investigations on the relationship of intelligence and fertility depend upon testing schoolchildren and counting the number of their brothers and sisters. All authors mention one factor which must tend to lower the association as measured—the fact that some of the families are incomplete. Further information is available about the Bath group which permits an approximate allowance to be made for this factor. The families of the cleverest 4 per cent., the median 4 per cent. and the dullest 8 per cent. of children were studied in some detail and it has been shown (Roberts, Eugenics Review xxx: 237, 1939) that the extra children who would be born by the time that all the mothers passed the age of 50 was respectively 0.25, 0.76 and 1.31. Using the same approximate method as above and allowing for its inaccuracy as far as possible the figure for the decline per generation in Binet I.Q. is raised from 2.0 points to 2.6 points. A number of other factors which cannot be allowed for must affect the estimate ; most of them would increase it, while some would lower it. The most important is the omission of the childless from the record. There can be no doubt that their inclusion would raise the figure for the decline. Altogether, taking all these factors into consideration the most likely figure for the decline in intelligence per generation is about 3 points of Binet LQ. This is the figure given by Cattell, though, as he explains, it was largely a guess, as he did not deduce it in any rigid way. The estimate of the fall is not based on any assumptions as to whether differences in intelligence are mainly determined by heredity or by environ- ment—in other words, whether the missing children in the second generation would have owed their superiority over those actually born to superior heredity or to superior environment. Professor Thomson states: “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 is conservative. A review of the literature points to a figure of more like three-quarters of the variability as being due to heredity. The association between intelligence and infertility could be ascribed to an indirect effect depending on the differences in fertility between different social classes or to a direct association independent of social class. Actually, of course, both are true. (Professor Thomson quotes some excellent evidence which indicates that the direct association, independent of any differences in socio-economic status, is the more important. This conclusion is reinforced by the findings of J. C. Chapman and D. M. Wiggins (Pedagogical Seminary, xxxii: 414, 1925) and by results given in my own paper quoted
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