2. the same as the average intelligence level of their parents. The expression "average intelligence", here, is not intended to suggest that the children of one family are all alike in grade. It implies that approximately equal numbers of children will have abiities which surpass or fall short of the mean parental ability. The rule holds good only if the environment for the children and for the parents is constant. Outside the normal range, the general rule is different: diseases which cause gross mental defect are often recessive hereditary characters, and, in such cases, the parents are usually unaffected and most brothers and sisters are also unaffected. Though the fundamental nature of the contribution of heredity in mental development is rightly stressed, the importance of environment is not to be underrated. Adverse environment can certainly reduce intelligence and it is almost as certain that favourable environment can increase intelligence. In extreme instances of certain forms of idiocy associated with physical maldevelopment, the prenatal environment has been proved to be of significance. Young women are much less likely to give birth to children who suffer from mongolism[superior 5] and certain other congenital malformations which involve the nervous system than are older women[superior 6]. This is a biological fact which has its parallels in animal genetics. Again, the first born child is more liable than children born subsequently to specific accidents at birth which cause serious mental impairment, though these cases are, fortunately, not common. A few cases of mental defect owe their origin to cerebral injury in postnatal life. Furthermore, there are certain infectious diseases which damage the nervous system and may cause mental impairment and these can take effect at any time in a child's history. How far the nutritional factor is important in the development of mental abilities is not yet fully known. Existing evidence suggests that the effect is slight but until satisfactory criteria of malnutrition are available the point cannot be properly investigated. Finally, as I have already pointed out, the educational facilities available must always be taken into account in assessing any person's intelligence. (c) (i) There are two kinds of investigations which may lead to results useful in determining the constitution of the population with respect to genetic factors which determine intelligence. We have to study the levels of ability in different social groups and we also have to examine the sizes of the families of people in these groups. Surveys of intelligence of children in different localities have indicated fairly consistently that mental ability is, on the average, higher in urban than in rural districts. In the towns, for example, the mean Binet intelligence quotient has been estimated to be 100 or slightly above: in rural districts, the average intelligence quotient is only about 95[superior 7]. Since the rural population is only one-fifth of the total, the average intelligence quotient of the whole community at the present time cannot be far from 100 but, according to some estimates, it is a little below this figure. It does not follow conclusively, from such observations, that the townspeople are more intelligent than the country people because the rural inhabitants may be more intelligent about the particular matters which concern them. In Lewis's survey of 1925-1929, however, a higher incidence of cases of mental defect was found in rural than in urban areas. The explanation of these phenomena, given at the time of the publication of the Wood Report,[superior 8] was that industrialization had tended to attract the more intelligent members of agricultural populations into the towns where economic prospects for them were better. The gradual ubanization of the rural population and the decay of rural life is a fact and the process has been continuing for a long time. It can be estimated that, in the twenty years between [end]