Birth order as a quantitative expression of date of birth.

Abstract

Publications on birth order mostly differ in their findings. In 1874 Galton1 stated that firstborn dominated among eminent men. Thirty years later, Pearson2 claimed that society was corrupted through the defects of firstborn. Surveys39 covering hundreds of books and articles give a remarkably inconsistent picture of the influence of sibling position on physical and psychological development. In 1913 Weinberg10 showed that methodological slips are an important source of confusing results. One such slip is the idea that the number of older siblings (earlyborn) and the number of younger siblings (lateborn) should balance in the population. If a total sibship is considered as a unit, beginning at 0 with the oldest sibling and ending at 1 with the youngest one, it is obvious that 05 is the exact mean position of its members. The same statement is true for any sample of total sibships but not for a sample consisting of members of different sibships. In fact, Cobb,11 as early as 1914 pointed to circumstances that might fundamentally distort birth order balance in the general population, but his warnings were not taken seriously. The consequences have been devastating. Those who have studied the influence of sibling position on disease, character, performance, or fate have mostly used methods based on the Greenwood-Yule model,12 with its erroneous assumption of a balanced birth order distribution in the general population. If a researcher found that a group of trait-carriers contained more lateborn than earlyborn, or the reverse, he asked whether the ratio deviated significantly from a 50 : 50 distribution. If so he felt entitled to state that the trait and the skewed distribution (overrepresentation) had something to do with each other.