Neurobiology of Sexual Orientation and Transsexuality

Sexual Orientation in Women

Limited Research: The field has a much smaller body of research compared to men.
Findings:
- Finger Ratio: On average, homosexual women exhibit a finger length ratio (4th to 2nd digit) more similar to straight men than straight women.
- Autoacoustic Reflex: Similar auditory reflex patterns observed.

Historical Perspective:
- Previously considered a domain of psychopathology.
- Homosexuality was classified as a psychiatric disorder until the 1970s before being reconsidered by the American Psychiatric Association.
Key Studies and Findings:
- Brain Structure: The bed nucleus of the striae terminalis shows sex differences in size, with trans individuals having a brain structure size consistent with their identified gender rather than their birth sex.
- Impact of Hormones: Studies controlled for hormonal treatments associated with gender transition, showing changes were not due to medical transition but inherent identity.
- Phantom Limb Syndrome: Transgender individuals who undergo surgery to remove male genitalia do not experience phantom sensations, unlike individuals who undergo similar procedures due to cancer. This suggests a normative bodily perception aligning with identified gender.

Landmark and Follow-up Studies: Initial studies were well-controlled and have been replicated, supporting the notion of inherent gender identity differing from physical sex.
Social and Medical Implications:
- Changes in insurance policies, like those in San Francisco covering transgender operations, reflect societal shifts.
- Ongoing research and societal changes highlight the complex interplay between biology, identity, and social constructs.

Environmental Triggers: Further exploration into environmental factors affecting sexual behavior and brain areas like the medial preoptic area and amygdala.
Continued Novel Research: Emphasis on new and challenging areas of study that expand understanding of sexual orientation and gender identity.