Brain Sex Differences Related to Gender Identity Development: Genes or Hormones?

Abstract

The complex process of sexual differentiation is known to be influenced by biological and environmental determinants. The present review has the aim of summarizing the most relevant studies on the biological basis of sexual development, and in particular, it focuses on the impact of sex hormones and genetic background on the development of sexual differentiation and gender identity. The authors conducted a search of published studies on Medline (from January 1948 to December 2019). The evidence suggests that the sexual dimorphic brain could be the anatomical substrate of psychosexual development, on which gonadal hormones may have a shaping role during prenatal and pubertal periods. Additionally, according to several heritability studies, genetic components may have a role, but a promising candidate gene has not been identified. Even though growing evidence underlines the primary role of biological factors on psychosexual development, further studies are necessary to better explain their complex interactions.

Disorders of Sex Development: Classification, Review, and Impact on Fertility

“…Indeed, hormonal influences not only affect internal and external genital development and differentiation; the embryo's brain has also been shown to differentiate sexually [12][13][14], perhaps through control mechanisms similar to those developed by the external genitalia [15]. On the other hand, it is also possible that the induction of the central nervous system (CNS) by hormones affects the patterns of hormonal secretion and, consequently, sexual behavior in adults [16][17][18][19][20]. Thus, the sex of an individual and their sexual expression, i.e., homoor heterosexual, must be considered the result of all influences that the individual receives, both before and after birth [8].…”

Neural Correlates of Gender Face Perception in Transgender People

“…The potential impact of biologic and social/cultural factors in the etiology of GI/GD remains under debate. However, biology seems to play a major role [ 3 , 4 ]. In the last years, the idea of the sexual dimorphic brain as the anatomic substrate of psychosexual development has been more widely pursued and research has focused on the influence and shaping role of genes and gonadal hormones on sexual differentiation of the brain [ 5 ].…”

Sex vs Gender: Are We there Yet? A Biological Location for Gender, not Sex.

“…Differences in the hippocampus (Ristori et al, 2020;Jones & Lopez, 2014;Sundermann et al, 2010), hypothalamus (Denley et al, 2018Mhaouty-Kodja et al, 2018;Xu et al, 2012;Sandremann et al, 2010), and amygdala (Mhaouty-Kodja et al, 2018;Jones & Lopez, 2014;Cooke et al, 2007) are repeatedly shown to be dimorphic in postmortem and in vivo imaging studies (Baldinger-Melich et al, 2020).…”

“…The dimorphic sexual brain is a substrate of psychosexual development, and the main determinants of cerebral differentiation are likely multimorphic. Genetic and hormonal influences have all been proposed (Ristori et al, 2020;Gliske, 2019;Legato, 2018). Further highlighting the possible developmental disconnect between sex and gender, sexual differentiation of the genitals takes place before sexual differentiation of the brain, making it possible that they are not always congruent (Tasopoulos, 2018;, Roselli, 2018).…”

“…Genes may play a role very early in development, and it may be the case that hormones simply reinforce an already established gender identity (Legato, 2018). Nonetheless, within transgender studies, the pre-frontal cortex (Kreukels & Guillamon, 2016), hippocampus (Manzouri et al, 2017;Smith et al, 2015), amygdala (Beking, et al, 2020;Manzouri et al, 2017), hypothalamus (Guillamon et al, 2016;Bao & Swaab, 2011;Swaab & Garcia-Falgueras, 2009), epigenetics (Bailey et al, 2016;Diamond, 2013;Rice et al, 2012), and hormones (Ristori et al, 2020;Fernandez et. al.…”