title: Twinning in Norway Following the Oslo Massacre: Evidence of a 'Bruce Effect' in Humans.
abstract: 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.
authors.affiliation: School of Public Health,University of California,Berkeley,California,USA.
authors.forename: Ralph A
authors.initials: RA
authors.lastname: Catalano
authors.affiliation: Department of Biology,Santa Clara University,Santa Clara,California,USA.
authors.forename: Katherine B
authors.initials: KB
authors.lastname: Saxton
authors.affiliation: Department of Demography,University of California,Berkeley,California,USA.
authors.forename: Alison
authors.initials: A
authors.lastname: Gemmill
authors.affiliation: Institute for Housing and Urban Research,Uppsala University,Uppsala,Sweden.
authors.forename: Terry
authors.initials: T
authors.lastname: Hartig
mesh_terms: D000022:Abortion, Spontaneous / Q000453:epidemiology / Q000235:genetics*; D000293:Adolescent; D000328:Adult; D005260:Female; D059647:Gene-Environment Interaction; D006708:Homicide; D006801:Humans; D008297:Male; D009664:Norway; D011247:Pregnancy; D014427:Twins / Q000235:genetics*
keywords: Bruce Effect; evolution; selection in utero; twins