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Collider bias (aka sample selection bias) in observational studies: why the effects of hyperandrogenism in elite women’s sport are likely underestimated
  1. Nicolai Topstad Borgen
  1. Department of Sociology and Human Geography, University of Oslo, PO Box 1096 Blindern, Oslo 0317, Norway
  1. Correspondence to Dr Nicolai Topstad Borgen, Department of Sociology and Human Geography, University of Oslo, PO Box 1096 Blindern, Oslo 0317, Norway; n.t.borgen{at}

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The medical condition of hyperandrogenism has entered common parlance because the International Association of Athletics Federation (IAAF) decided to ban women with the condition from competing against women who have ‘normal’ levels of testosterone (see box 1). This paper argues that the two articles substantiating the IAAF’s regulation1 2 fail to prove that elevated levels of natural testosterone are causally linked to better sporting performance. By studying elite athletes both papers induce a collider stratification bias that attenuates the competitive advantage of testosterone—a little-known bias that often plagues studies of highly selective samples.

Box 1


  • In April 2018, International Association of Athletics Federation (IAAF) introduced a new eligibility regulation for female athletes, requiring women with differences of sexual development to reduce their blood testosterone levels to <5 nmol/L.

  • IAAF builds its case on two articles in BJSM from 2017, by authors with declared connections to IAAF, suggesting that testosterone levels in female athletes are positively correlated with athletic performance.1 2 Several studies have criticised this research,4–7 highlighting the lack of correlation analysis,5 the focus on free testosterone,5 and the problem of type I error in statistical tests,4 all of which the authors of the original articles have attempted to address.8 There has also been calls to retract the study because of serious flaws in the data.9

  • The regulation was challenged in the Court of Arbitration for Sport by Caster Semenya, 800 m Olympic and world champion. The ruling upheld the regulation, amidst further disagreement between medical professional bodies.


Do high levels of natural testosterone cause women to run faster or throw further? And if so—how much faster or further? The size of the competitive advantage of …

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  • Contributors NTB has written the manuscript.

  • Funding This study was funded by the Norwegian Research Council (grant #238050).

  • Competing interests None declared.

  • Patient consent for publication Not required.

  • Provenance and peer review Not commissioned; externally peer reviewed.