Neurobiology of gender identity and sexual orientation

Question 1

Sex chromosomes

Answer 1

DNA is packed in chromosomes
23 pairs with 22 autosomes and one pair of sex chromosomes
Female xx
Male xy

Question 2

Y chromosome

Answer 2

On Y chromosome a single gene for transcription factor - testis determining factor aka sex determining region
This gene product is the sole determinant for development male gonadal tissues
Deletion if SRY - phenotypic female

Question 3

SRY and default pathway

Answer 3

Default pathway = female - genital ridges - ovaries - other organs
SRY - genital ridges - testis - testis secrete testosterone - masculinization of body and brain

Question 4

Androgens

Answer 4

Group of male sex hormones, testosterone is an androgen

Question 5

Development of sex organs

Answer 5

Early on internal sex organs are bisexual. All embryos contain precursors for both male and female.
Mullerian - female
Wolffian - male

Third month one of them develops determined by the prescence of testis

Question 6

Anti-mullerian hormone

Answer 6

A peptide secreted by the fetal testes that inhibits development of the mullerian system

Question 7

Androgen insensitivity syndrome

Answer 7

A condition caused by a congenital lack of functioning androgen receptors. in a person with xy sex chromosomes, causes development of a female with testes but no internal sex organs

Question 8

Persistent mullerian duct syndrome

Answer 8

A condition caused by a congenital lack of anti-Müllerian hormone or receptors for this hormone; in a male, causes development of both male and female internal sex organs (small uterus in abdomen / can become tumorous)

Question 9

Sexual orientation

Answer 9

Largely biologically determined as there’s no evidence for nurture
Many studies have examined levels of sex steroids in male homosexuals and found levels to be the same as heterosexuals

Question 10

Prenatal exposure to androgens

Answer 10

Evidence suggests that prenatal androgens can affect social behaviour and sexual orientation.
Different evidence:
congenital androgen hyperplasia
Androgen insensitivity syndrome
Other markers for prenatal androgen exposure

Question 11

Congenital androgen hyperplasia

Answer 11

Abnormally high level of androgen secretion by adrenals

Boys develop normally

Girls > Prenatal ‘masculinization’
Enlarged clitoris, partly fused labia
Increased likelihood homo- / bisexuality (30%)
CAH-girls more likely to have ‘stereotypical-male toy preference’ as compared to non-CAH sisters or female cousins

Question 12

Biology?

Answer 12

For example, even at 1 day of age, baby boys prefer to watch a moving mobile and baby girls prefer to look at a female face

Alexander and Hines (2002) found that young vervet monkeys showed sexually dimorphic preferences in choice of toys (much like in humans): Males chose to play with active toys, females show greater variablilty

Toy preference can develop without explicit gendered socialization

Question 13

Androgen insensitivity syndrome

Answer 13

Most commonly caused by a mutation in the androgen receptor which resides on the X chromosome

Heterozygous XX females are carriers and are minimally affected

XY individuals develop as females > external female genitalia, without uterus or fallopian tubes, with testis in abdomen

Question 14

Sexual orientation in AIS

Answer 14

It was initially reported that there is no occurrence of bi- or homosexuality among XY women with androgen insensitivity syndrome

NO effect of androgens on brain: associated with heterosexual orientation in AIS women

Of course, it is also possible that rearing an XY child with androgen insensitivity syndrome as a girl can play a role in that person’s sexual orientation

Question 15

Markers for prenatal androgen exposure

Answer 15

Otoacoustic emissions (sounds that are generated from within the inner ear; McFadden & Pasanen, 1998; McFadden & Pasanen, 1999)

Finger length patterns (Williams et al., 2000)

Patterns of eye blinks (Rahman, Kumari, & Wilson, 2003)

Skeletal features (Martin & Nguyen, 2004)

All show that women with a homosexual orientation were on average exposed to slightly more fetal androgen than were heterosexual women

Question 16

Sexual orientation and genes

Answer 16

This is exactly what Bailey and Pillard found: The concordance rate was 52 percent for identical twins and only 22 percent for fraternal twins—a difference of 30 percent.

Question 17

Paradox

Answer 17

Male homosexuals 80% fewer children than male heterosexuals
Strong selective pressure against genes that predispose for male homosexuality
Female maternal relatives of homosexual males higher fertility rates than those of heterosexual males
Gene on X-chromosome increases both female’s fertility as male’s likelihood of homosexuality