Intelligence and family size.

The writer reviews the literature in psychology on testing involving the relationship between family size and intelligence level. A complicating factor is that fertility differentials among educated groups have decreased recently. There has always been a negative correlation between intelligence test scores of children and the number of their siblings. The question is found to be more complex that originally thought. Number of years between siblings is a complicating factor. The exact effects of heredity and environment parental motivation and contact with parents are other factors to consider. On the basis of early data several writers had predicted a drop in the intellectual level of the population. This conclusion is doubtful and needs more corroboration. Testing the relationship between intelligence and family size presents certain methodological problems: 1) incomplete families are included in the sampling; 2) selective factors such as several children from one family may skew the test data; and 3) mean family size can be defined in different ways. There is growing interest and a need for a carefully designed test to measure this relationship. A battery of aptitude tests not one single test should be used. Subjects should be measured until their families are complete. Ages of parents at birth of first and last child should be recorded. Information on occupation income and education levels is important.