Human Nature

, Volume 20, Issue 3, pp 294–316 | Cite as

Birth Order Influences Reproductive Measures in Australians

  • Fritha MilneEmail author
  • Debra Judge


We examine the relationship between birth order and reproductive behaviors in a sample of Australian residents, accounting for personality, personal achievements, and family structure. Using generalized linear models and survival analyses we build predictive models for each reproductive measure and test those models on an independent data subset. Compared with functional firstborns (middle-borns more than 5 years younger than their next older sibling), male middle-borns and last-born females had younger ages of first sexual intercourse, and middle-born females had a younger age at first pregnancy. There was no difference in females’ age at first birth. Male middle-borns tended to have an older average age at first birth. Controlling for age, both male and female middle-borns had fewer children. Overall, middle-borns differ more from functional firstborns than do last-borns. Given the significant but small effect sizes demonstrated in this study, we suggest that developmental characteristics that may facilitate middle-borns’ success within the natal family carry slight fitness costs during the reproductive years.


Birth order Reproductive behaviors Personality Survival analysis Within-family analysis Fitness 



We are grateful to Marty Firth, Kevin Murray, and Prof. Lincoln Schmitt for providing consultation on statistical methods. This manuscript benefited from comments from Dr. Jennifer Bell, Dr. Silvana Gaudieri, Prof. John McGeachie, and Dr. Kathy Sanders. We thank the Life History and Ecology study group for their questions, comments, and advice. We are especially grateful to three anonymous reviewers for their careful reading and very useful critique and suggestions. The University of Western Australia Human Research Ethics Committee approved this project, and it was funded by the School of Anatomy and Human Biology at the University of Western Australia. We thank all participants for volunteering; without their candid responses, this study could not have taken place.


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Copyright information

© Springer Science + Business Media, LLC 2009

Authors and Affiliations

  1. 1.Anatomy & Human Biology, M309The University of Western AustraliaCrawleyAustralia
  2. 2.Anatomy & Human BiologyThe University of Western AustraliaCrawleyAustralia

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