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Azathoth123 comments on Quantified Risks of Gay Male Sex - Less Wrong Discussion
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Which is a consequence of HIV being harder to transmit through heterosexual sex. Which does not automatically imply HIV is impossible to transmit through heterosexual sex.
That still doesn't explain how HIV spread as much as it did in Southern Africa given how hard it is to transmit heterosexually.
Epidemiologists currently reckon that's mainly down to Southern Africans having multiple concurrent partnerships and low male circumcision rates. (Other factors have likely played a role as well, like South Africa's recent bout of officially-sanctioned HIV/AIDS denialism, and the potentially higher transmissivity of the HIV-1 subtype prevailing in Southern Africa.)
Europe has an even lower circumcision rate.
When I first herd this theory ~15 years ago it was accompanied by a prediction that the HIV-1 subtype would break into the western heterosexual population real soon now. Since that has failed to happen, I'm dubious about this theory.
Pay attention to the word "and" in what you quoted, it is actually quite important. The Southern African Journal of HIV Medicine article I linked explicitly addressed Europe's lower circumcision rate:
It is the conjunction of low male circumcision rates and multiple concurrent partnerships which explains HIV's strong showing in Southern Africa.
I don't give that point any more weight than the last time you raised it (which I can't link because the relevant post got deleted). Now, as then, you haven't cited any specific person or authority who's supposed to have made this prediction, and I still don't see why the failure of that prediction would be strong evidence against the hypothesis that HIV-1 group M subtype C likely has a higher transmissivity than HIV-1 group M subtype B. Non-zero evidence? Yes. Decisive evidence? No.
So why hasn't HIV-1 group M subtype C spread out of Southern Africa?
That question assumes a false premise. HIV-1 group M subtype C has spread out of Southern Africa.
Perhaps what you were trying to ask was why subtype C hasn't spread as aggressively as you personally expect beyond Southern Africa, though the information I gave two comments ago suffices to answer that question. Still, I will build on that information to spell this out.
The transmissibility of an HIV subtype is not the only factor determining how, and how far, that subtype spreads; behavioural differences between populations also matter. Southern African populations more often engage in non-circumcision and multiple concurrent partnerships than people elsewhere, and that combination of behaviours is the most likely reason why subtype C hasn't run riot among heterosexuals outside of Southern Africa (and Ethiopia & India).
If I leave things there, I suspect, I can look forward to a follow-up attempt at a dubious gotcha question along the lines of "So why bring up the transmissibility differences in the first place?". Because the fact remains that relative transmissibility is probably a factor in explaining why subtypes B & C have different spatial distributions. I had thought it clear that I was invoking relative transmissibility as merely a probable secondary factor, since I mentioned it parenthetically and wrote it "likely played a role", not that it was a sufficient, primary explanation in its own right.
What about western groups that have many sexual partners, e.g., the swinger and polyamory communities?
Your question blurs the distinction between promiscuity and multiple concurrent partnerships — I assume that was an accident.
Which precise question are you asking? Are you asking why Western groups with MCPs and low circumcision rates don't have (much) subtype C? If so, I'd like some specific evidence that they don't — it's a good idea to establish a phenomenon occurs before trying to explain it, and I'm having trouble finding systematic evidence on the HIV subtypes found among Western polyamorous people & swingers. If your question is about something subtly different (e.g. if you're asking about HIV in general among those groups, not subtype C in particular), please clarify.
Which are?
Your theory predicts that promiscuous heterosexual westerners should be getting HIV at rates similar to Southern Africa. Near as I can tell this is not the case.