Twinning in Norway following the oslo massacre: Evidence of a 'bruce effect' in Humans

Ralph A. Catalano, Katherine B. Saxton, Alison Gemmill, Terry Hartig

Research output: Contribution to journalArticlepeer-review

14 Scopus citations


Emerging theory and empirical work suggest that the 'Bruce Effect', or the increase in spontaneous abortion observed in non-human species when environments become threatening to offspring survival, may also appear in humans. We argue that, if it does, the effect would appear in the odds of twins among male and female live births. We test the hypothesis, implied by our argument, that the odds of a twin among male infants in Norway fell below, while those among females rose above, expected levels among birth cohorts in gestation in July 2011 when a deranged man murdered 77 Norwegians, including many youths. Results support the hypothesis and imply that the Bruce Effect operates in women to autonomically raise the standard of fetal fitness necessary to extend the gestation of twins. This circumstance has implications for using twins to estimate the relative contributions of genes and environment to human responses to exogenous stimuli.

Original languageEnglish (US)
Pages (from-to)485-491
Number of pages7
JournalTwin Research and Human Genetics
Issue number5
StatePublished - Oct 1 2016
Externally publishedYes


  • Bruce Effect
  • evolution
  • selection in utero
  • twins

ASJC Scopus subject areas

  • Pediatrics, Perinatology, and Child Health
  • Obstetrics and Gynecology
  • Genetics(clinical)


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