3 when parents are both of slightly subnormal mentality, for example, when they have I.Q. between 90 and 95. The effect of a declining birth rate on intelligence can be considered separately. Certain cases of mental defect are due to birth injury: this is a rare accident which is likely to occur to first born children. Other special types of mental retardation are more likely to be born to mothers older than 35 then to those younger than 35. A tendency to late marriage, or to long intervals between births, increases the likelihood of the births of such children. After the maternal age of 25 years, the probability of mongolism is more than doubled every five years. In England, during the last fifty years, the mean size of families has been halved, twice the number of children are now subjected to the hazards of primogeniture. The number of only children, with the special problems which they present, is also much greater now than formerly. On the basis of some experimental work, Cattell estimated that our national intelligence was diminished at least 1 per cent. each decade : he predicted a precipitate decline. My own view [end]