Hormonal and Genetic Effects on Sexual Orientation and Gender Identity

Hormonal Effects: Activation vs. Organizational

• Activation: Effects due to current hormone levels.
  • Example: Testosterone levels correlate with sexual motivation.
  • No evidence testosterone changes sexual orientation, only interest.
  • Menstrual cycle: Hormone fluctuations do not change sexual orientation.
• Organizational: Effects due to hormonal differences during development (embryonic/puberty).
  • May organize the brain in relation to sexual orientation.
  • Early development (weeks 8-24 of pregnancy) and mini-puberty after birth are key periods.

Activation of Hormones and Sexual Orientation

• No direct influence on sexual orientation.
• Interest in sex is influenced by hormone levels.
• No correlation between testosterone levels and sexual orientation.

Organizational Hormonal Effects

• Focus on embryonic development.
• Two key periods:
  • Weeks 8-24 of pregnancy (external genitalia formation, brain differentiation).
  • Period just after birth (mini-puberty).
• Possibility that sexual orientation might be set up during these periods.

Indirect Evidence

• Correlates influenced by early testosterone (prenatal hormones).
• Individuals with atypical early hormone levels:
  • 46XX individuals with congenital adrenal hyperplasia (high testosterone levels).
  • 46XY individuals with androgen insensitivity syndrome (do not respond to testosterone).

Correlates Dependent on Prenatal Hormones

• Effects are small but replicable.
  • Cognitive Performance
  • 2D:4D Ratios
  • Autoacoustic Emissions

Cognitive Performance

• Small sex differences in cognitive performance.
  • Males: better spatial rotation
  • Females: better verbal abilities
• Verbal abilities:
  • Better in females.
  • Slightly better in androphilic men.
• Visual-spatial performance:
  • Worse in androphilic men.
  • Gynophilic women are faster at mental rotation.

2D:4D Ratios

• Ratio of index finger length to ring finger length.
  • Dependent on early testosterone.
  • Lower ratio indicates higher prenatal testosterone.
• Consistent results primarily in gynophilic women.
  • Masculine presenting gynophilic have more masculine ratios.
• Gender nonconformity correlates with 2D:4D ratios.

Autoacoustic Emissions

• Ears click back in response to a click next to the ear.
• Stronger in females, depends on low testosterone levels during development.
• Gynophilic women have more masculine autoacoustic emissions.

Androgen Insensitivity Syndrome

• 46XY individuals present as female from birth, do not respond to testosterone.
• Tend to be androphilic.
• Suggests not responding to testosterone might make you more androphilic

Congenital Adrenal Hyperplasia

• 46XX individuals exposed to high testosterone during embryonic development.
• Higher proportion identify as gynophilic or bisexual.
• Suggests early testosterone biases toward gynophilia.

Role of Early Testosterone

• Evidence suggests early testosterone is a factor in sexual orientation development.
• Most males have higher early testosterone and identify as gynophilic.
• Higher testosterone in females increases odds of identifying as gynophilic.

Genetic Effects on Sexual Orientation

• Evidence from twin studies and genetic mapping.

Twin Studies

• Monozygotic twins (genetically identical) vs. dizygotic twins.
• Concordance (both twins share the same trait) is higher in monozygotic twins.
• Indicates a genetic component to sexual orientation.

Genetic Mapping

• Mapping sexual orientation onto family trees.
• Linkage to a region on the X chromosome in families with androphilic males.
• Suggests genes in that region bias development in sexual orientation.

Evolutionary Perspective

• How can there be a genetic basis for sexual orientation if it leads to lower fertility?

Kin Selection

• Genes could be maintained if they cause individuals to help their siblings.

Genes Don't Always Do the Same Thing in Males and Females

• Gene that makes a male more likely to be androphilic might make a female more likely to have more children
• There is some evidence that maternal relatives of male homosexuals have more children.

Heterozygote Advantage

• Gene maintains in the population because in heterozygous states promotes have more offspring, have longer survival, whatever the advantage might be.