Examining the effects of birth order on personality

Edited by Christopher F. Chabris, Union College, Schenectady, NY, and accepted by the Editorial Board September 17, 2015 (received for review April 1, 2015)
October 19, 2015
112 (46) 14224-14229
Commentary
Settling the debate on birth order and personality
Rodica Ioana Damian, Brent W. Roberts

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.

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Data Availability

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.

Acknowledgments

The National Child Development Study (NCDS) data were made available by the Centre for Longitudinal Studies at the Institute of Education, University of London, and the UK Data Service. The National Longitudinal Survey of Youth (NLSY) data were made available by the U.S. Bureau of Labor Statistics. The Socio-Economic Panel (SOEP) data were made available by the German Institute for Economic Research (DIW). We thank these institutions for providing these datasets.

Supporting Information

Supporting Information (PDF)
Supporting Information

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Information & Authors

Information

Published in

Go to Proceedings of the National Academy of Sciences
Go to Proceedings of the National Academy of Sciences
Proceedings of the National Academy of Sciences
Vol. 112 | No. 46
November 17, 2015
PubMed: 26483461

Classifications

Data Availability

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.

Submission history

Published online: October 19, 2015
Published in issue: November 17, 2015

Keywords

  1. birth order
  2. personality
  3. Big Five
  4. within-family analyses
  5. siblings

Acknowledgments

The National Child Development Study (NCDS) data were made available by the Centre for Longitudinal Studies at the Institute of Education, University of London, and the UK Data Service. The National Longitudinal Survey of Youth (NLSY) data were made available by the U.S. Bureau of Labor Statistics. The Socio-Economic Panel (SOEP) data were made available by the German Institute for Economic Research (DIW). We thank these institutions for providing these datasets.

Notes

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.
See Commentary on page 14119.

Authors

Affiliations

Julia M. Rohrer
Department of Psychology, University of Leipzig, 04109 Leipzig, Germany;
Department of Psychology, Johannes Gutenberg University of Mainz, 55099 Mainz, Germany
Stefan C. Schmukle1 [email protected]
Department of Psychology, University of Leipzig, 04109 Leipzig, Germany;

Notes

1
To whom correspondence should be addressed. Email: [email protected].
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.

Competing Interests

The authors declare no conflict of interest.

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    Examining the effects of birth order on personality
    Proceedings of the National Academy of Sciences
    • Vol. 112
    • No. 46
    • pp. 14103-E6410

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