Etiology of Sexual Orientation

Why are LBGT population rates changing with time?

A podcast listener recently sent me a thought-provoking question:

What's your best theory as to why the number of people experiencing LGBT issues has increased so much in recent years?

Turns out I had a lot to say. Note that all of this is extremely speculative and informal. In the future I would like to revisit this issue in more depth, but am posting my initial thoughts below. (Substack and peer-review are different for a reason.)

I broke the question into a few forms:

1. What determines sexual orientation biologically?

Sexual orientation as a biological phenomenon is innate, and largely determined by organizational effects of sex hormones during a prenatal critical window of brain development. Sexual behavior can be manipulated in rodents by blocking androgens in males or introducing androgens in females during this critical window. In humans, the intersex conditions I talked about on Mikhaila Peterson’s Podcast map onto this theory. There is substantial heritability to homosexuality, but I believe the genetic effects are all mediated by sex hormones*

2. Can sexual orientation change as a result of hormonal effects outside of the prenatal critical window? 

No, I don’t know of any evidence to support this. The same rodent studies, as well as human evidence from other intersex, transgender, or gonadal cancer patients on hormone replacement therapy don’t show any effects of androgen, estrogen, or progestin exposure on sexual orientation outside of this prenatal window (but it will affect sex drive, and possibly the phenotypes someone is attracted to - see Sarah Hill’s work on birth control). 

3. Can sexual orientation change as a result of social or environmental influences?

The only types of environmental variables that can biologically impact sexual orientation would be mediated by prenatal sex hormones. This probably explains the indirect effects of male birth order, and other possible small effects related to maternal diet or exposure to endocrine disrupting toxins (such as such as atrazine which “turns the frogs gay”). 

Looking to animal research, I don’t know of any environmental variables or plausible mechanisms that would impact sexual orientation other than through prenatal sex hormones. Some species such as bonobos engage in same-sex sex as a form of social bonding, but this is different from homosexuality. You could argue they’re bisexual, but you could also argue that they’re all heterosexual when it comes to attraction and engage in same-sex sex for other reasons. 

4. What are the evolutionary theories for homosexuality?

My hot take: they are all spandrel arguments. There could be any number of selection pressures for prenatal sex hormones which net out as adaptive despite some low or high extreme of the bell curve stably having no offspring. 

5. What then explains demographic shifts in human sexual orientation? 

The only possible biological mechanisms I gave are endocrine disrupters impacting fetal brain development through maternal diet or toxin exposure. Actually, I think there could possibly be a small effect there due to changes in diet and pollution, but highly unlikely to explain the changes from 1-2% of Boomers to 10-20+% of Zoomers. The changes then must be all a problem of measurement or sociocultural influences:

- Stigma: It’s believable to me that the true base rate of homosexual or bisexual attraction is underestimated worldwide and in older generations because people are afraid to come out. My guess is that the true base rate is similar to the 1-2% gay and 5-10% bi observed in Millenials, but overestimated in Gen Z. 

- Measurement: What does it mean to be bisexual? If you have ever once fantasized about someone of the same sex, are you gay or bi? My guess is that sexual orientation truly exists on a skewed bimodal distribution (majority heterosexual, minority homosexual, with overlap in the middle and variability in how straight or gay someone is). With any continuous trait and categorical labels, there is some arbitrary cutoff. Perhaps most people who’d ever rarely fantasized about the same sex, or acknowledged their attractiveness, would not doubt their sexual orientation, but a teenager raised in today’s world of fluid sexuality would be more open to labeling themselves as bi. There can also be perverse incentives. I saw recent data that 20-40% of Ivy League students identified as LBGT. No one can disprove them of their self-identity, and if they believe it will get them affirmative action, it makes sense that it would be overrepresented. 

*Earlier when I said the genetic effects must all be mediated by prenatal hormones, it’s also possible that genes involved in personality trait Openness could predict how willing someone is to experiment with self-identity, even if their underlying biological attraction is the same

 6. What about the T in LBGT? 

The same answers apply for all of the above. I think the percentage of people with innate gender incongruence is likely stable but underestimated in older cohorts and worldwide, overestimated in younger cohorts, and driven by prenatal sex hormones. I believe the overestimate in younger cohorts may be driven by maladaptive response to body dysmorphia in response to puberty, and comorbid mental health conditions. Gender identity development during adolescence may be particular hard for autistic and highly Open youth navigating our evolving social landscape of gender and sexuality.

I experienced mild gender dysphoria during adolescence, likely due to weight-related body dysmorphia, social issues, and a lack of masculine role models, which later resolved as each of those improved.

All of this is speculative but extremely important to be discussed openly if we hope to progress gender affirming care. The chances that no gender dysphoria may have resulted from causes other than innate mismatch between sex and gender identity is the same as the chances there are no people with innate gender dysphoria who would not benefit from transition. Zero.