Relation of birth order, family size and social class to psychological functions.

Findings are reported on birth-order and family-size effects for five psychological functions measured by the military preinduction test battery which was administered to a total population of 19-yr.-old Dutch men. These men were born between 1944 and 1946, were members of 1- to 6-child families and were from the two major social classes. A clear birth-order gradient was present on all test measures for both social classes. Family-size effects, however, differed by social class. The results extend to a range of psychological functions the findings previously reported for a single test of non-verbal intelligence, suggesting that family-structure variables play a role in understanding intellectual development.