Does Birth Order Affect Intelligence?
Originally prepared by: Amber Esping (fall 2003)
The answer to the question is probably yes, if you base your conclusions on cross-sectional data. The answer is most likely no if you base your conclusions on longitudinal data.
Scholarly interest in the relationship between birth order and extraordinary achievement can be traced to 1874 when Francis Galton published English Men of Science: Their Nature and Nurture. This book chronicled the lives of 180 eminent men from various scientific fields. Galton was able to collect birth order data from 99 of his subjects, revealing that 48% of them were firstborn sons or only sons. (Note: Galton did not count female children when reporting his results. Theoretically, a subject could be counted as a “first born” even if he was the 10th child, providing that his 9 older siblings were female.)
Interest in birth order and eminence has continued unabated, and countless studies have confirmed Galton’s conclusion: Firstborn children are overrepresented among Nobel Prize winners (Clark & Rice, 1982) classical music composers (Schubert, Wagner & Schubert, 1977) and prominent psychologists (Terry, 1989). Indeed, a study of 314 eminent 20th century personalities found that 46% of them were firstborn children (Goertzel, Goertzel, & Goertzel, 1978; See Simonton, 1984/1999 p. 26-27 and Simonton, 1999, p. 133 for reviews).
It must be noted that the correlation between firstborn status and eminence
is probably limited to certain types of scientific achievement. Laterborn
children are more likely to become revolutionary leaders and scientists,
and they may in fact be more creative than their firstborn siblings (Sulloway,
1996; 1999; Simonton, 1984/1999, 1999).
In 1973 Lillian Belmont and Francis Marolla published family size, birth order and intelligence test (Dutch version of the Raven Progressive Matrices) data from nearly the entire population of 19 year-old Dutch men (386, 114 subjects). Their study design was complex, so interested readers should consult the primary source. However, a general overview of the results will be presented here. Belmont and Marolla found:
Cross-sectional studies (like the Belmont and Marolla study described above) generally find that the higher the birth order, the lower the IQ (See Zajonc, 1976 for a review; See De Lint, 1966 for an exception to this pattern). Longitudinal studies, which track individual families over time, usually demonstrate that there is no relationship between birth order and IQ (Berbaum & Moreland, 1980; Retherford & Sewell, 1991; Rodgers, et al., 2000; Schooler, 1972). However, the tendency for large families to produce lower IQ children holds regardless of the research approach (Rodgers, et al., 2000).
The Admixture Hypothesis
In 1874 Francis Galton offered several reasons why birth order might affect eminence. Among them, he listed:
The Resource Dilution Model
The Confluence Model
It is possible that firstborn children are more intelligent than their siblings. However, there are several reasons why this finding, if true, may not be very important. First, growing body of research suggests that intelligence is not the most important factor in the achievement of eminence (Simonton, 1984/1999, 1999; Sulloway, 1996). Several studies have demonstrated that specific personality traits such as conscientiousness and openness to experience are up to 10 times more important than IQ (Sulloway, 1996). Second, in studies showing a statistically significant advantage for firstborns, birth order accounts for only one percent of the variance in IQ scores (Sulloway, p. 473). Third, the firstborn advantage is tiny-about 1 IQ point higher than the second sibling, 2 points higher than the third sibling, and so on. (Sulloway, p. 74). This minute difference is not likely to matter in the pursuit of eminence:
Even within science, IQ is only weakly related to achievement among people who are smart enough to become scientists. Research has shown, for example, that a scientist who has an IQ of 130 is just as likely to win a Nobel Prize as a scientist whose IQ is 180 (Hudson, 1966, p. 104, cited in Sulloway, p. 357).
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