title: Same-sex twins are taller and heavier than opposite-sex twins (but only if breastfed): Possible evidence for sex bias in human breast milk.
abstract: Recent studies show that human and other mammalian breast milk may be tailored for the sex of the offspring. Such sex bias suggests that opposite-sex twins, who receive breast milk that cannot simultaneously be tailored for both sexes, may be at a disadvantage for growth compared with same-sex twins. An analysis of data from the National Longitudinal Study of Adolescent Health (Add Health) shows that, controlling for sex, age, birth weight, and zygosity, breastfed same-sex twins are, on average, about 1 inch taller and 12 pounds heavier than their opposite-sex counterparts through adolescence and early adulthood. In contrast, never-breastfed same-sex twins tend to be shorter and lighter than their opposite-sex counterparts. These results may be potential evidence for sex bias in human breast milk and its long-term effects.
authors.affiliation: Department of Management, London School of Economics and Political Science, London WC2A 2AE, UK; Department of Clinical, Educational, and Health Psychology, University College London, London WC1E 6BT, UK. Electronic address: S.Kanazawa@lse.ac.uk.
authors.forename: Satoshi
authors.initials: S
authors.lastname: Kanazawa
authors.affiliation: Department of Psychology, California State University, Fullerton, Fullerton, CA 92834, USA.
authors.forename: Nancy L
authors.initials: NL
authors.lastname: Segal
mesh_terms: D000328:Adult; D001827:Body Height / Q000502:physiology*; D001835:Body Weight / Q000502:physiology*; D005260:Female; D006801:Humans; D007407:Interviews as Topic; D008137:Longitudinal Studies; D008297:Male; D008895:Milk, Human; D036301:Qualitative Research; D063507:Sexism; D014429:Twins, Dizygotic; D014430:Twins, Monozygotic
keywords: Height; Human development; Weight