Study Notes on the Biology of Gender and Sexual Orientation

Gender

Recognized as a spectrum rather than a binary classification, influenced by biological factors, including prenatal hormone exposure.

Prenatal Hormone Exposure

A critical period during gestation where the balance of male and female hormones imprints sexual identity and preferences onto the developing brain and body. All fetuses initially develop as female unless exposed to male hormones.

Androgen Insensitivity Syndrome (AIS)

A condition where genetically male individuals (XY) lack the receptors to respond to male hormones, leading to the development of female physical and psychological characteristics, often resulting in attraction to men.

Adrenogenital Syndrome (Congenital Adrenal Hyperplasia)

A condition in women due to exposure to high levels of androgens in utero, associated with a significantly higher prevalence of bisexual or homosexual identities.

Finger Ratios (2D:4D ratio)

A physical trait influenced by prenatal hormone exposure; lesbian women exhibit finger ratios similar to males, and gay males with multiple older brothers show lower ratios, correlating with females.

Amygdala

A brain region involved in fear and aggression (fight-or-flight response) that shows differing activity patterns based on an individual's sexual orientation.

Spatial Abilities

Cognitive tasks where men often excel over women. Lesbian women outperform heterosexual women, aligning with men's performance, while gay men typically perform lower than straight men, aligning with stereotypical female levels.

Verbal Fluency

Cognitive tasks where women generally outperform men. Gay males perform better than straight males, nearing the performance level of straight females.

Pheromone Responses (Hypothalamus)

Distinct brain responses to pheromones based on sex and sexual orientation, primarily mediated by the hypothalamus, which plays a significant role in sexual behavior.

Homosexuality Concordance in Identical Twins

Research findings indicating a 52% concordance rate for homosexuality in identical twin brothers and a 48% rate in identical twin sisters.