Sex Differences, Sexual Orientation, and Gender Identity

Sexual Orientation: Origins of Sex Differences

  • Sexual orientation is one of the most significant sex differences.
  • The majority of male individuals are generally androphilic.
  • The majority of female individuals are generally gynephilic.

Four Potential Sources of Sex Differences

  1. Experiential Cultural Effects
    • There is no solid evidence that cultural influence or experiences influence sexual orientation.
    • Asexuality is also a sexual orientation. Something in the brain and body has to develop to lead to that orientation.
    • Experience does not seem to be a factor.
    • It is important to remember that sex differences are always on a population level and not between two individuals.
    • Cultural effects are detectable on average, but there is usually huge overlap with small differences.
    • Experiential cultural effects don't seem to be involved in sexual orientation.
  2. Hormonal Effects
    • Split into two types: activation and organizational.
    • Activation: Effects due to current hormone levels in the bloodstream.
    • Organizational: Effects due to hormonal differences during embryonic development or puberty, organizing the brain in a certain way.
Activation Role of Hormones
  • To determine the activation role of hormones, we need to see if the trait varies when hormone levels vary.
  • Testosterone levels correlate with sexual motivation in males and females; higher levels increase interest, and lower levels decrease interest.
  • Anti-testosterone drugs reduce interest in sex among sex offenders but do not change orientation.
  • Estrogen levels do not change sexual orientation during menopause or when treated with drugs to reduce estrogen.
  • The menstrual cycle, with its hormone fluctuations, does not affect sexual orientation.
  • Activation of hormones doesn't seem to influence sexual orientation.
  • There are individual differences in testosterone levels in gay men compared to straight men, but they don't correlate with orientation whatsoever.
Organizational Hormonal Effects
  • Usually related to early development (embryonic development).
  • Two periods of higher testosterone levels in males during early development:
    • Weeks 2-24 of pregnancy: early on, external genitalia formation; late, brain differentiation.
    • Period just after birth: mini-puberty, short testosterone peak, potentially influencing brain differentiation.
  • Sexual orientation might be set up during these periods if early testosterone predicts sexual orientation in adulthood.
  • In an ideal world, we would look for actual measures of prenatal hormones.
  • Measuring prenatal hormones has risks to the pregnancy when sampling amniotic fluid.
  • There are no studies that have looked at prenatal hormones and followed those children later on to see what their sexual orientation was.
  • Indirect evidence can be seen from things influenced by early tesosterone.
  • People with atypical early hormone levels can be studied.
    • 46 XX individuals with congenital adrenal hyperplasia have unexpectedly high testosterone levels during embryonic development.
    • 46 XY people with androgen insensitivity syndrome. They do not respond to testosterone.
Correlates in Adulthood Dependent on Prenatal Hormones
  • Three correlates that are quite small but replicable:
    1. Cognitive Performance
      • Small differences in cognitive performance.
      • Males on average, have better spatial orientation, spatial rotation abilities.
      • Females, on average, have better verbal abilities.
      • Sexual orientation may vary with these cognitive differences under the influence of early testosterone.
      • Verbal abilities, better in females, are also slightly better in androphilic men than in gynephilic men.
      • Visual spatial performance is worse in androphilic men than in gynephilic men.
      • Gynephilic women tend to be faster at mental rotation (something males tend to be faster at).
    2. 2D:4D Ratios
      • Ratio of the length of the second digit (index finger) to the fourth digit (ring finger).
      • Males have a slightly shorter index finger relative to their ring finger than females of the same age, as a population.
        • In males, the index finger is 95% of the ring.
        • In females, the index finger is 97 to 98% of the ring.
      • There is evidence this happens in other species as well.
      • Most consistent results have been in gynephilic women.
      • Gynephilic women who present more masculine tend to have more masculine 2D:4D ratios.
      • Gynephilic men who have more feminine 2D:4D ratios tend to correlate with gender nonconformity.
    3. Autoacoustic Emissions
      • Providing a click next to someone's ear and measuring the ear's response (clicking back).
      • Stronger in females.
      • Depends on low testosterone levels during development.
      • Gynephilic women have more masculine autoacoustic emissions, which points to slightly higher testosterone levels during development.
Varying Prenatal Hormone Levels
  • 46 XY individuals with androgen insensitivity syndrome present female from birth.
    • They tend to be androphilic.
    • The proportion of androphilic androgen insensitivity syndrome people 46 XY is no different from other females.
    • Not being able to respond to testosterone might make one more androphilic.
  • Congenital Adrenal Hyperplasia
    • 46 XX individuals are exposed to high testosterone during embryonic development.
    • In this study, about 25% said