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Edited by Christopher F. Chabris, Union College, Schenectady, NY, and accepted by the Editorial Board September 17, 2015 (received for review April 1, 2015)
Significance
The question of whether a person’s position among siblings has a lasting impact on that person’s life course has fascinated both the scientific community and the general public for >100 years. By combining large datasets from three national panels, we confirmed the effect that firstborns score higher on objectively measured intelligence and additionally found a similar effect on self-reported intellect. However, we found no birth-order effects on extraversion, emotional stability, agreeableness, conscientiousness, or imagination. This finding contradicts lay beliefs and prominent scientific theories alike and indicates that the development of personality is less determined by the role within the family of origin than previously thought.
Abstract
This study examined the long-standing question of whether a person’s position among siblings has a lasting impact on that person’s life course. Empirical research on the relation between birth order and intelligence has convincingly documented that performances on psychometric intelligence tests decline slightly from firstborns to later-borns. By contrast, the search for birth-order effects on personality has not yet resulted in conclusive findings. We used data from three large national panels from the United States (n = 5,240), Great Britain (n = 4,489), and Germany (n = 10,457) to resolve this open research question. This database allowed us to identify even very small effects of birth order on personality with sufficiently high statistical power and to investigate whether effects emerge across different samples. We furthermore used two different analytical strategies by comparing siblings with different birth-order positions (i) within the same family (within-family design) and (ii) between different families (between-family design). In our analyses, we confirmed the expected birth-order effect on intelligence. We also observed a significant decline of a 10th of a SD in self-reported intellect with increasing birth-order position, and this effect persisted after controlling for objectively measured intelligence. Most important, however, we consistently found no birth-order effects on extraversion, emotional stability, agreeableness, conscientiousness, or imagination. On the basis of the high statistical power and the consistent results across samples and analytical designs, we must conclude that birth order does not have a lasting effect on broad personality traits outside of the intellectual domain.
Footnotes
- ↵1To whom correspondence should be addressed. Email: schmukle{at}uni-leipzig.de.
Author contributions: B.E. and S.C.S. designed research; J.M.R. and S.C.S. analyzed data; and J.M.R., B.E., and S.C.S. wrote the paper.
The authors declare no conflict of interest.
Data deposition: All data used in this study are publicly available to scientific researchers. The NCDS data are available from the UK Data Service, ukdataservice.ac.uk; the NLSY97 data from the US Bureau of Labor Statistics, www.bls.gov/nls; and the SOEP data from the German Institute for Economic Research (DIW), www.diw.de/en/soep. To achieve full transparency and to allow the reproducibility of our analyses, the scripts for replicating our data analysis are archived in the Open Science Framework, https://osf.io/m2r3a.
This article is a PNAS Direct Submission. C.F.C. is a guest editor invited by the Editorial Board.
*Sulloway postulated an opposing effect of birth order on dominance as a facet of extraversion (15). However, because the extraversion items in the questionnaires used in this study did not include the dominance facet, we were unable to test this additional hypothesis.
†Parental age might be a potential confounding variable that is causing the effects on intelligence and intellect. For example, a higher paternal age at conception carries the risk of a higher number of new genetic mutations that might lower intelligence in later-borns. Assuming this kind of process, one would expect that spurious birth-order effects caused by differences in parental age would become larger with increasing age gaps between siblings. We tested this possibility by including the difference in age between the target person and firstborn as an additional predictor in our between- and within-family analyses of intelligence and intellect. Age differences did not significantly explain any variance above and beyond birth-order position in any of these four analyses (all P > 0.52). This result suggests that parental age is not the driving force behind the effects on intelligence and intellect.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1506451112/-/DCSupplemental.
Birth-order effects on personality
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