The fate of national intelligence: test of a thirteen-year prediction.

1. Historical Background of the Present Research I T is a commonplace of genetics that under conditions of differential survival the mean characteristics of a population are likely to show a continuous trend in a certain direction, which may be evaluated as eugenic or dysgenic according to conditions and standards. Twenty years ago there was considerable discussion as to whether the wellknown differential birth-rate by social status, which had been strongly in evidence and well documented by Heron and others since the beginning of this century, might be producing some dysgenic trends, notably in regard to the mean level of general intelligence. For whatever questions of value may arise to complicate the question of the eugenic or dysgenic direction of such characteristics as increased stature or capacity for material productivity, there can be no question that our culture is one in which greater intelligence of the population is a boon to all and in which deteriorations of the distribution can lead to misery or catastrophe in ways predictable even from our present grasp of principles in social psychology.5 Since the attempts to infer changes in intelligence from the social status differential were fraught with excessive speculation a number of investigators-notably Burt,1 Cattell2,' 4, Roberts,20 and Sutherland and Thomson2' in England, and Lentz,"" and Conrad and Jones12 in the U.S.A. -set out to determine directly the relation between intelligence and size of family. Incidentally this theme of investigation seems to have been pursued more vigorously and to more definitive conclusions in Britain than elsewhere, despite comparative lack of research funds for social science. An earlier and less obstructed recognition of the importance, for advanced civilization of a proper eugenic regard for resources of innate capacities has perhaps been responsible for this more determined attempt at understanding these particular social processes. However, it is to be hoped that data of a comparable degree of completeness will soon become available in several other countries in order that the methods of comparative cultural anthropology8a may throw still more light on the culture pattern causes and consequences of eugenic or dysgenic trends in average innate mental capacity. The present paper is the first of a trilogy reporting new evidence on the question and suggesting some methodological advances. The first two deal with an attempt to measure directly the change of intelligence in a community over a thirteen-year interval, and with an examination of the present birthrate differential in relation to other influences,10 while the third is a summary and discussion in relation to method"' and future research. Although the present paper discusses and integrates with the recent observations of Burt,' Roberts,20 and Thomson,22 it is most organically connected with, and should be considered an extension of, the research carried out by the present writer in I936-7. That study, made possible by the generosity of the Eugenics Society, through the medium of a Leonard Darwin Research Fellowship, set out to test the ten-year-old child population of (a) a typical industrial area-the city of Leicester-and (b) a typical rural area-the more remote parts of Devonshire -using a non-verbal intelligence test based on the best psychological theory then available. The results, published in two articles in the EUGENICS REVIEW2, 3 and in a book,4 showed that size of family was negatively related to child intelligence throughout the