Previous: 18 Sexual Orientation Hypotheses - Part 1
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This episode about sexual orientation is packed and it goes fast. Play it multiple times to really learn what's being taught and stay curious.

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In the late 1800s sexologists started publishing their ideas about sexual orientation: what causes sexual attraction? At first they thought it was our souls, then there was the idea that we're all bisexual embryos that develop into more diverse attractions.

It wasn't that long ago people postulated sexual orientation was a symptom of a viral infection -- or that homosexuality in particular afforded families an extra caregiver to help raise their siblings' kids. The new millennium has brought with it its own set of hypotheses starting with:. Exotic Becomes Erotic.

In 2000 Daryl Bem a professor at Cornell University theorized that genes and hormones don't directly determine sexual orientation. They script the kind of temperament a kid has and the activities that kid'll prefer. He continued: if a girl is into typically boy-ish games and styles, she will feel different than and other girls creating a psychological arousal toward them because they're exotic.

With time exotic becomes erotic and voila you have lesbians. Maternal Immune Hypothesis. According to researchers Ray Blanchard and Anthony Bogaert, “Each additional older brother increased the odds of homosexuality by 33%.” But how?

In 1996 and then again in 2001 they proposed every time a male child is conceived the mother's immune system detects a signal from the Y chromosome, (specifically H-Y antigens) that says something like “another male.” The mother's immune system then creates blood proteins or antibodies to respond, “noted.” As the levels of the proteins rise - child to child to child to child, the likelihood of having offspring that will go on to reproduce goes down. Researchers have hypothesized that it's so these gay males can be helpful uncles or support staff for the family at large. Maybe it's a form of population control?

And homosexuality is nature's condom? It's All Sorts of Factors. Joseph Nicolosi said sexual orientation isn't brought on by one specific family dynamic.

It could be a combination of a scary older brother, nice mom, gross mom, or masculine mom, emotionally unavailable dad, a same sex experience in childhood, being called gay, encouragement to accept gay culture, or a personality that is sensitive, fragile, and passive. But people of all sexual orientations have those family dynamics and personal traits so.... Antagonistic Pleiotropy.

Andrea Camperio Ciani and colleagues came up with the Antagonistic Pleiotropy hypothesis of sexual orientation. Pleiotropy is when a single gene controls for more than one trait, like how the gene for albinism affects skin color, hair color, and eye color. When at least one of the traits quote “benefit” survival and reproduction and at least one that quote “harms” it that's antagonistic pleiotropy.

So in the case of sexual orientation, Ciani argued that homosexuality reduces reproduction for the individual (trait that “harms”), but might account for really high reproductive rates in the individual's relatives (trait that benefits). Little known fact about sheep, 7-11% of the males or rams are gay, higher than the average species. So Charles Roselli and colleagues compared the brains of rams who like rams and rams who like ewes, the females, to detect differences.

The found the hypothalamus which regulates temperature, physiology, emotions, appetite, hormones and sexual sexual behavior was smaller in the gay rams than it was in the straight ones. Leading to the hypothesis that sexual orientation “is neurologically hard-wired and may be influenced by hormones.” Which is not to say the size of one's hypothalamus causes their sexual orientation, only that there seems to be a correlation. Bromance.

Frank Muscarella believed that homoerotic relationships were evolutionarily positive because it meant less aggression toward each other. This would lead to a higher rate of survival because there wouldn't be fighting to the death, and being alive would mean it's easier to reproduce or at least support the offspring of others. Sexual Fluidity.

Lisa Diamond's studies showed that women are more fluid in their sexual orientation than men. But because the bulk of research on sexual orientation has been about male homosexuality, the framework for understanding sexuality is mostly fixed and rigid. In her manuscript, Diamond wrote, “...there is no point at which sexuality completely finishes developing, neatly tying off loose ends and therefore ruling out the prospect of unexpected future transitions.

Because of fluidity, same-sex sexuality remains an unpredictable possibility for all women throughout the life course, albeit an unlikely one (though this is due more to culture than to biology).” By this time, scientists all over the world had put together that sexual orientation is at the very least hereditary. It's biology, not socialization. Ivanka Savic and colleagues asserted that testosterone surges in first half of pregnancy affect genital development, while a surge in the second half of pregnancy affects gendered brain development.

So there are diverse combinations of anatomy, physiology, gender, and attraction. More on Survival of the Family. Remember Antagonistic Pleiotropy and the reproductive success of a gay man's maternal relatives?

Eight years later the same research team found female relatives of homosexual males are funnier, happier, more relaxed, outgoing, and more likely to get along with others. And they have fewer gynecological problems than female relatives of heterosexuals males. As in they're not just more fertile like it was previously hypothesized.

They're more attractive! They can be more choosy with partners to produce the fittest offspring. Epigenetics.

Epigenetics is the study of what turns genes on and off. According to researchers Tuck C. Ngun and Eric Vilain we all have a network of genes for sexual attraction: bi, pan, hetero, homo, ace etc. and epigenetics determines the expression of this network.

On on off, on off, off off off, without changing anything about the DNA sequence. It's like clownfish that can switch from male to female depending on the reproductive needs of the school. We become sexually oriented based on the needs of our environment.

Unfortunately this couldn't be a comprehensive list of all the hypotheses out there. My hope is that we did a fair job of showing the diversity of speculation and study and my intention with this two part series of historical ideas around sexual orientation is to understand each other. Why do we hold the beliefs we have about sexual orientation?

Is it because of what we were taught? What do we know for sure and what is still uncertain or wrong? Asking questions is important.

Ask your friends what they think about these hypotheses. Confront the weaknesses in the research and the breach of ethics in them. Question scientists about why they don't study heterosexuality, bisexuality, asexuality, and women more.

Where's demisexuality in the research or the distinction because sexual and romantic attraction? What do you want to know? Stay curious.

In the description there are links to dozens of other resources on the topic as well as our Patreon page. We appreciate any support you're able to give be it sharing this video or subscribing as a sexplanaut. It means a lot and gets the information out to more people.