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Why are some people straight? Bisexual? Is it nature or nurture? Is there a gay gene? How is it passed on? This episode of Sexplanations doesn't have all the answers but it does run through a list of the hypotheses proposed around sexual orientation's origins.

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When the question is asked ‘why are some people gay?' I respond “why are some people straight?

If they say reproduction and pair-bonding, I answer “attraction isn't required for procreation or family commitment.” So why then is there so much diversity in sexual attraction? Is it nature or nurture?

Biology or choice? If it's hereditary, how is it passed on? Where does bisexuality fit into all of this?

While there isn't a lot of research on sexual orientation at large, hypotheses about homosexuality attempt to answer these questions and reinforce the value of staying curious. [WHIP CRACKING, COUGH]. It's our souls. One of the first ideas recorded, in 1864, was Karl-Heinrich Ulrichs' proposal that homosexuality is a gender variation.

A man attracted to men was an urning: a heterosexual female soul in a male body. We're all bisexual embryos. Another sexologist of the 1800s, Magnus Hirshfeld, hypothesized that embryos start off with a biological attraction to all sexes and as they develop, attraction to the the same sex diminishes. (Most of the time.) If same sex attraction persisted it was thought to be the result of alcoholism or syphilis, or exceptional health and good upbringing, depending on who it was.

There's a sexual spectrum. n 1903 Hirshfeld took a different position that sex is on a continuum and homosexuality is where male and female physical features and attractions begin to overlap, a kind of intersex. He suggested we look for menstrual blood in gay mens' urine and sperm in lesbians' vaginal fluid. Inversion.

The psychoanalyst Sigmund Freud suspected that we're all born bisexual (like Hirshfeld's original idea) and that we mature toward heterosexuality. Your growth could be stunted by a seductive mother, or hostile father which would cause an “inverted instinct” - inversion, Freud's name for homosexuality. By the way, Freud did therapy with patients to help them ‘develop into heterosexual' but asserted, “In general to undertake to convert a fully developed homosexual into a heterosexual does not offer much more prospect of success than the reverse." Gonads.

Eugen Steinach thought gay men's testes released abnormal secretions that made males more like females. that made males more like females. He reported that when the testicles of a heterosexual man were transplanted to a homosexual man, he'd become heterosexual. This was later found to be untrue.

Archetypes. Carl Jung proposed that males have a female archetype in their psyches called anima, and females have a male archetype animus. It's with these characters that Jung claimed we navigate the world, a balance of our biosexes and our archetypes.

Depending on how well we balanced, we'd either lead to passion or aversion to the same biosex. Phobia. Sandor Rado argued that sexual orientation isn't genetic; it's a phobia to people of another biosex.

Normal variation. Evelyn Hooker asserted that whatever the cause, homosexuality is normal. Heterozygous advantage.

Two years later in 1959 an ecologist, G. Evelyn Hutchinson, explained that homosexuality is genetically possible because of a heterozygous advantage. Here's what that Punnett Square of sexual orientation would look like.

Basically that having two different alleles increases fitness so a gay allele and a straight allele will be healthier, more reproductive, and more likely to pass on homosexuality. Parents. Irving Bieber recognized that the onset of sexual orientation was before puberty but opposed a "born with it" stance.

To him, male homosexuality in particular was brought on by a strong bond to one's mother bond or a weak bond with one's father. Deductive Reasoning. Frederick Whitam deduced that if children were raised to be heterosexual, and some of them identified as gay despite this, then sexual orientation must be biological.

Kin selection. My favorite hypothesis is Edward Wilson's. He offered that homosexuality is good for the world, building off of JBS Haldane's theory of kin selection.

Here's a summary: homosexual genes are in entire families. The homosexual person may not have kids to pass the gene on but then they're more available to help raise their siblings or siblings' offspring, which contributes to a higher success their survival and passing the gene on through them. Gender nonconformity.

Alan Bell and Martin Weinberg conducted one of the most comprehensive studies of sexual orientation to date and showed it isn't pathological. They categorized orientation more by the relationship dynamics: "close coupled" , "open coupled", "functional" ,"dysfunctional" and "asexual". Possibly a polygenic trait.

J. Michael Bailey and Richard Pillard thought that sexual orientation is determined by a combination of multiple genetic mechanisms. Height, for example, is a polygenic trait, controlled by about seven hundred variants, some accounting for a millimeter of growth.

Why can't sexual orientation be the same? With 699 mysterious messages on our DNA to like men, women, all, neither? Brain Structure.

In 1991 Simon LeVay published an article in Science magazine, "A Difference in Hypothalamic Structure Between Heterosexual and Homosexual Men.” Essentially identifying a structural difference in the nucleus that strengthens the position that sexual orientation is biological. There's a gay gene. Dean Hamer and his colleagues interviewed 76 gay brothers and learned that the maternal sides (mom's side) of their families had far more gay relatives than the paternal side (dad's side), suggesting gay is on the X chromosome.

Sure enough, Xq28 became known as the gay gene. There's a gay germ. In 1999 Paul Ewald and other evolutionary scientists went with the hypothesis that homosexuality is the result of a viral or bacterial infection.

In 2014 Dr. Turhan Canli proposed a similar hypothesis to explain depression. It's Complex.

Professor of Neurology George Rice wrote, "The basis of sexual orientation remains uncertain, but the pathways involved can be expected to be complex. The controversies and methodological difficulties notwithstanding, the study of sexual orientation contains fascinating riddles, and further careful systematic study has the potential to reveal important insights about who we are." It's complicated - let's leave it for the next millennium. In the next episode of Sexplanations and the second part of this series,.

I'll explain more recent hypotheses that bring us to 2018. Stay curious. This episode of Sexplanations is brought to you by our supporters on sexplanations.

They're people like us who help fund Sexplanations so that it's free for others to have a comprehensive sex resource. To join them as a sexpla(i)naut and support our work, please visit and learn more. you.