The biology of mental defect / by Lionel S. Penrose ; with a preface by J.B.S. Haldane.

  • Penrose, L. S. (Lionel Sharples)
Date:
[1954]
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Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: The biology of mental defect / by Lionel S. Penrose ; with a preface by J.B.S. Haldane. Source: Wellcome Collection.

    45/342 (page 27)
    INCroENCE, DEFINITION AND MEASUREMENT 27 less than 70 or are beyond — 2 <?, we shall actually remove rather more than exactly the 2-27 per cent, whose measurements would be included under a Gaussian distribution. The tail of the actual distribution, based partly on estimates given by Lewis and partly upon I.Q. ratings for this age group, obtained by the Massachusetts Department of Health, extends to zero. A total population of 100,290 would include about 2560 cases below the 70 line. The Gaussian distribution in the table covers 99-7 per cent, of the total population, leaving 290 outside its confines. Also the Gaussian population includes 2270 of the group who are, on the basis of test measurements, mentally defective. The curve shown in the figure is, of course, only one theoretical explanation. A single skewed curve might be found to cover the whole distribution accurately (see also page 45). MALE AND FEMALE DISTRIBUTIONS There is general agreement among different observers that the distribution of intelligence test scores for males is more widely scattered than it is for females. Terman (1925) showed that, on the Binet test, there were more boys than girls in the highest scoring groups of children. There appear also to be more males who have very low scores. The effect can be ex¬ pressed succinctly by saying that the standard deviation of intelligence scores is usually found to be greater for boys than for girls. In the Scottish Survey (1933) the difference was as 17 points to 16. Similar findings were reported by Duff and Thomson (1923), and Roberts (1945) stated that boys were 13 per cent, more variable than girls on intelligence tests. Thus, if males and females are to be judged on the same scale, we shall expect to fimd a preponderance of male defectives in the extreme, or low grade, groups. In fact, most large surveys do show such a pecuHarity. Lewis (1929) reported a high incidence in males, especially in the low grades. The excess of male idiots and imbeciles is liable to be counter¬ balanced by a converse excess of females of feeble-minded, or moron, grade, as in Pollock's (1926) data. It is difficult to be certain whether or not this effect represents a genuine biological difference between the sexes. The recognition of an excess of male institutional cases might be due to some form of social selection, e.g. if male cases are more difficult to look after at