3. 1911 and 1931, over a million of the potential rural population became urbanized either on account of migration of people or enlargement of urban districts. This estimate is based on the observation that, although the birth rate is higher in the country than in the towns,[superior 9] the total rural population has not increased in due proportion to the total urban population. Irrespective of urban and rural areas, marked differences in mental capacity and in birth rate are noticeable if different social groups are compared. Some of the smallest families are found among the professional and highly skilled groups. Skilled labourers have fewer children than unskilled labourers. Larger families are found if the wages of the father are low. The Merseyside Survey,[superior 10] however, recorded a tendency for the most impoverished class to be less prolific than the classes which were slightly better off. The easily demonstrable differential fertility with respect to paternal income has often been interpreted as a differential fertility with respect to intelligence. This interpretation is supported by the work of investigators like Roberts, Norman and Griffiths [superior 11] who have shown that there is a marked negative association between a child's intelligence and the number of his brothers and sisters: that is to say, the bright children in a school are found to be members of small families and the backward children are found, on the average, to be members of large families. When the mentally defective children, outside the normal range, are studied, it is occasionally found that they come from families which are the largest of all. These large families, however, are not very common and are compensated for by the effective sterility of many defectives, due either to social, psychological or physiological causes. If defective of mild degree are left entirely to themselves some of them unite with other of similar grade and produce large numbers of defective offspring. But feeble-minded children who have been educated in special schools, and who are followed up later by after care organizations[sic], do not so usually marry their own kind and have fewer offspring than normal people.[superior 12] This relative infertility of defectives becomes very marked in the severer grades of defect: the number of children born to imbeciles is negligible and for an idiot to have any children is the greatest rarity. Within the normal range of intelligence, however, a negative association between ability of parents and between size of family is the general rule. Many causes have been postulated to account for this state of affairs. According to a theory of Fisher's,[superior 13] much depends upon the fact that fertility itself is inherited and that there is a tendency for clever people to marry those who are biologically infertile. There are other, more easily demonstrable, causes of differential fertility connected with social environment. A person who is able to obtain for his services high payment usually also has a high standard of living for himself and his family. Parents of this type develop expensive educational plans for their children[superior 14] and like to have few children. Long periods of intellectual training and of apprenticeship also lead to small families by tending to encourage late marriages for both men and women who are capable of skilled work. On the other hands, large families are found in districts where the standard of living is low: that is to say, where wage-earning begins early so that children are an asset, and where there are few alternative distractions to family life. (c ) (ii) The question as to whether national intelligence is likely to decline in the future can be provisionally answered in the light of the facts which have so far been summarized. The consequence of persistent differential fertility would appear to [end]