The role of biological factors in gender identity development
Professor Dianna Kenny, The University of Sydney
Biological factors also make a significant contribution to gender identity development. Recent research in both humans and primates shows that sexually dimorphic toy preferences reflect basic neurobiological differences between boys and girls that precede social or cognitive influences (Williams & Pleil, 2008). Children’s gendered selections of playmates, toys, and activities can be traced to the influence of biological factors, in particular, prenatal exposure to testosterone that occurs via testicular development, which in turn is determined by the presence of the Y chromosome (Hines, 2010).
These sex differences in children’s play commence very early, before gender development and sexual orientation (Berenbaum, Martin, Hanish, Briggs, & Fabes, 2008). Studies with nonhuman primates, who have not been affected by gendered socialization processes or gender identification, confirm sex differences in novel toy selection (e.g., males prefer wheeled toys, females prefer plush toys, dolls) observed in young children (Hassett, Siebert, & Wallen, 2008). In both monkeys and children, males are more rigid in their toy selections than girls. However, girls who have been exposed to abnormally high levels of testosterone, such as those with congenital adrenal hyperplasia (CAH) (Pasterski et al., 2011), and those whose mothers took androgenic progestins during pregnancy show increased male-typical play and toy selection (Hines, 2003). Further, levels of testosterone in the blood or amniotic fluid of pregnant women are associated with the degree of male-typical behaviour demonstrated by their children (van de Beek, van Goozen, Buitelaar, & Cohen-Kettenis, 2009). As well as sex-nontypical behaviour in childhood, women with CAH evince lower heterosexual orientation, diminished identification with female gender, and higher preference to live as a man in adulthood, despite having been raised as girls. CAH women are 600 times more likely than women in the general population to experience severe gender dysphoria (Hines, 2010).
Some personality characteristics such as empathy (higher in females) and aggression (higher in males) are also associated with pre-natal testosterone exposure (Hines, 2008). Hormonal surges in sex hormones (testosterone for boys, and oestrogen for girls) soon after birth may also affect gender development. Those children who have absent or reduced hormonal surges such as boys with hypogonadism and girls with Turner’s syndrome show differential deficits in developments normally associated with their assigned sex (Alexander, Wilcox, & Farmer, 2009).
Biological factors other than testosterone can also affect gender development. For example, newborns can generally be assessed along nine dimensions of infant temperament - activity level, distractibility, intensity, regularity, sensory threshold, approach/withdrawal, adaptability, persistence and mood (Gartstein & Rothbart, 2003). Zucker and colleagues (Zucker, Wood, Singh, & Bradley, 2012) offer a highly cogent example of how infant temperament can interact with gender development processes to cause confusion about gender identity in young people. Activity level, a sex-dimorphic trait that tends to be higher in boys, is associated with higher physical energy expenditure, one form of which is rough-and-tumble play. Activity level is lower in boys and higher in girls with gender identity disorder (GID) – an inversion of levels that occur in children without GID. Boys low in activity level might find the behaviour of girls more compatible with their temperament, leading them to affiliate with girls, which may direct their toy and play interests towards those typical of girls. This process, if continued, may lead to the development of female gender identity and later to gender confusion and/or dysphoria. This process may be interrupted if boys with low activity level meet and befriend other boys with similar activity levels (e.g., boys who prefer reading and playing chess rather than playing body contact sports). Meeting like, same-gendered minds may make it possible for such boys to expand their previously-held, somewhat rigid views about the nature of boys, thus allowing them to view themselves as a subset of all males, rather than as female “trapped in the wrong body.”
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Hassett, J. M., Siebert, E. R., & Wallen, K. (2008). Sex differences in rhesus monkey toy preferences parallel those of children. Hormones and Behavior, 54(3), 359-364.
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Pasterski, V. L., Geffner, M. E., Brain, C., Hindmarsh, P., Brook, C., & Hines, M. (2011). Prenatal hormones and childhood sex segregation: Playmate and play style preferences in girls with congenital adrenal hyperplasia. Hormones and Behavior, 59(4), 549-555. doi: https://doi.org/10.1016/j.yhbeh.2011.02.007
van de Beek, C., van Goozen, S. H., Buitelaar, J. K., & Cohen-Kettenis, P. T. (2009). Prenatal sex hormones (maternal and amniotic fluid) and gender-related play behavior in 13-month-old infants. Archives of Sexual Behavior, 38(1), 6-15.
Williams, C. L., & Pleil, K. E. (2008). Toy story: Why do monkey and human males prefer trucks? Comment on “Sex differences in rhesus monkey toy preferences parallel those of children” by Hassett, Siebert and Wallen. Hormones and Behavior, 54(3), 355-358. doi: https://doi.org/10.1016/j.yhbeh.2008.05.003
Zucker, K. J., Wood, H., Singh, D., & Bradley, S. J. (2012). A developmental, biopsychosocial model for the treatment of children with gender identity disorder. Journal of Homosexuality, 59(3), 369-397.