The Cognitive, Behavioral, and Personality Profiles of a Male Monozygotic Triplet Set Discordant for Sexual Orientation

Abstract

The neurohormonal theory of sexual orientation proposes that homosexual men and homosexual women are exposed prenatally to a hormonal environment that is similar to that of the other sex. Prenatal exposure to an opposite-sex hormonal environment may lead the nervous system to develop in a manner consistent with the opposite sex. If this cross-sex exposure occurs, one prediction would be that the cognitive ability profile of homosexual men would be similar to that of heterosexual women. This study examined a set of male monozygotic triplets, aged 21 years, discordant for sexual orientation: 2 of the triplets were heterosexual, 1 was homosexual. The triplets were administered measures of 23 domains of cognitive ability, as well as measures of sexual orientation and masculinity/femininity. On the measures of cognitive ability, the triplets performed similarly, yet consistent differences were found between the 2 heterosexual triplets and the 1 homosexual cotriplet. Differences having the same pattern were found for the number of Schafer homosexuality signs on the Rorschach, and on a homosexuality scale derived from items on the Minnesota Multiphasic Personality Inventory--2 (MMPI-2). Responses from the homosexual triplet were in a more feminine direction than responses from his 2 heterosexual cotriplets on measures of masculinity-femininity, which included measures derived from Rorschach responses, the MMPI-2 Masculinity-Femininity scale, the Bem Sex Role Inventory, and the Boyhood Gender Conformity Scale. Responses to the 16 Personality Factor Questionnaire also distinguished the 1 homosexual triplet from the 2 heterosexual cotriplets. These findings support the view that the prenatal hormonal environment may have enduring effects on selected behavioral traits.

Personality and Birth Order in Monozygotic Twins Adopted Apart: A Test of Sulloway's Theory; Research Reviews: Twin Births and Cancer Risk in Mothers, Male Sexual Dysfunction, Twin Study of Ultimatum Game Behavior; Human Interest: ‘The Land of Twins’, Twin-Like Reunion-I, Twin-Like Reunion-II

“…Numerous twin studies have examined genetic and environmental influences on sexual orientation (Bailey et al, 2000;Hershberger & Segal, 2004) and gender identity (Knafo et al, 2005;Segal, 2006). In contrast, fewer If the offer is rejected, both players receive nothing.…”

Neurosteroid Biosynthesis Regulates Sexually Dimorphic Fear and Aggressive Behavior in Mice

“…Prenatal exposure to high testosterone levels during embryonic development may lead to complex changes in neuroendocrine function that may impact adult cognitive function [3,4]. Studies in twins have suggested that a female twin who has been prenatally exposed to testosterone produced by her male counterpart expresses a masculinization of personality traits, including increased aggression [5,6].…”

Two Monozygotic Twin Pairs Discordant for Female-to-Male Transsexualism

“…Given an estimated frequency of female to male transsexuals of 1/100,000 and an MZ female twinning rate of 1/500, a female to male transsexual twin should occur in only 1/25,000,000 cases. However, examining such pairs can be valuable because MZ twins differing in fundamental ways constitute naturally occurring, co-twin control experiments: by holding the genotype constant, it is possible to search for prenatal and postnatal environmental influences underlying co-twins' behavioral and physical differences (Hershberger & Segal, 2004;Segal, 2000). This information can be used to address the etiology and course of behavioral development in non-twins.…”