Family Size, Birth Order, and Intelligence in a Large South American Sample

Abstract

The most popular current theory of family influence is the confluence theory, which hypothesizes that a child is helped or hindered in intellectual development according to the average absolute intelligence (mental age) in the family when that child is born. Aggregated data about such a hypothesis have been examined in previous studies from a number of nations, but not from a developing country. Here test scores, family information, and socioeconomic data are analyzed far a sample of over 36,000 college applicants in Colombia. The intellectual effects of family size were not at all the classic pattern: All family sizes smaller than six surpassed a single child family, arguing that differences were populational, rather than intrafamilial. Further analysis showed almost no family-size effect for the lower socioeconomic group among the college applicants, and birth order effects were not constant across family sizes, and not in conformity with the model. In sum, the confluence theory fared badly in these tests.