Sexual Orientation

Kinsey scale

7-point scale ranging from 0 (exclusively heterosexual) to 6 (exclusively homosexual) to measure sexual orientation

developmental

• lifespan approach, sociological studies

• “I always knew I was gay”

• Childhood Gender Nonconformity

Neuroanatomical

• brain structure and function

• “Gay brains vs Straight brains”

Neuroendocrine

• hormones

• “gay men have feminized brains, lesbian women have masculinized brains”

Genetic

• “The gay gene”

• evolutionary reasons for gay gene persisting

Fraternal Birth Order

In men, sexual orientation correlates with number of older brothers. Each older brother increases the odds of homosexuality by up to 33%, but this only accounts for 1 in 17 gay men.

Maternal Immune Hypothesis

Attempts to explain significance of fraternal birth order by proposing mothers are immunized to male-specific antigens by male fetuses, increasing the effects of immunization on sexual differentiation of the brain in succeeding male fetuses

Medial preoptic area

Region at the front of the hypothalamus, which is involved in the regulation of sexual behaviors shown by males (eg. preference for female partners)

INAH3

The interstitial nucleus of the anterior hypothalamus - cell group within the medial preoptic area that is sexually dimorphic (larger in males), and is smaller in gay men compared to straight men.

Organizational effect

A hormonal influence that happens very early in life and is permanent

Activational effect

A hormonal influence that happens very early in life and is not permanent (is transient)

Intrauterine position effects / hormone transfer

Due to the transfer of testosterone from male fetuses to adjacent fetuses, adjacent fetuses may have permanently altered hormone levels, reproductive organs, aggressive behaviors, secondary sex ratios and susceptibility to endocrine disruption.

Family studies

• investigating familial aggregation of traits (genetic relatedness - 1st, 2nd, 3rd degree relatives, etc)

• Assessing resemblance between genetically related parents and offspring or siblings living together

• resemblance can be due to heredity or to shared family environment

twin studies

• comparing resemblance of identical (MZ) and fraternal (DZ) twins to estimate genetic and environmental components of variance

• if genetic factors are important, identical twins must be more similar than fraternal twins

• Equal Environments Assumption - statistical assumption to account for MZ twins being treated more similarly than DZ twins

Adoption studies

• use the separation of biological parents and offspring to assess relative importance of genetic and environmental influences

• compare adoptee’s resemblance to parents

• may also compare genetically related and unrelated siblings reared in the same household

Xq28

X-linked gene studied in linkage studies of sexual orientation that assessed whether maternal uncles of gay men had higher rates of homosexuality compared with paternal uncles and other cousins