title: Birth weight and two possible types of maternal effects on male sexual orientation: a clinical study of children and adolescents referred to a Gender Identity Service.
abstract: This study tested predictions regarding two hypothesized maternal immune responses influencing sexual orientation: one affecting homosexual males with high fraternal birth order and another affecting firstborn homosexual individuals whose mothers experience repeated miscarriage after the birth of the first child. Low birth weight was treated as a marker of possible exposure to a maternal immune response during gestation. Birth weight was examined relative to sibship characteristics in a clinical sample of youth (N = 1,722) classified as heterosexual or homosexual based on self-reported or probable sexual orientation. No female sexual orientation differences in birth weight were found. Homosexual, compared to heterosexual, males showed lower birth weight if they had one or more older brothers--and especially two or more older brothers--or if they were an only-child. These findings support the existence of two maternal immune responses influencing male sexual orientation and possibly also cross-gender behavior and identity.
authors.affiliation: Gender Identity Service, Child, Youth and Family Services, Centre for Addiction and Mental Health, Toronto, ON, Canada.
authors.forename: Doug P
authors.initials: DP
authors.lastname: VanderLaan
authors.forename: Ray
authors.initials: R
authors.lastname: Blanchard
authors.forename: Hayley
authors.initials: H
authors.lastname: Wood
authors.forename: Luisa C
authors.initials: LC
authors.lastname: Garzon
authors.forename: Kenneth J
authors.initials: KJ
authors.lastname: Zucker
mesh_terms: D000293:Adolescent; D001722:Birth Order; D001724:Birth Weight / Q000502:physiology*; D002648:Child; D005260:Female; D005783:Gender Identity; D018451:Homosexuality, Male; D006801:Humans; D007230:Infant, Low Birth Weight / Q000502:physiology; D008297:Male; D010298:Parity; D011247:Pregnancy / Q000276:immunology; D012725:Sexual Behavior / Q000502:physiology*; D035781:Siblings
keywords: birth order; birth weight; gender dysphoria; homosexuality; maternal immune hypothesis; sexual orientation