I.

Did cultural evolution create sexual purity taboos to prevent the spread of STIs? A few weeks ago, I wrote a post assuming this was obviously true; after getting some pushback, I want to look into it in more depth.

STIs were a bigger problem in the past than most people think. Things got especially bad after the rise of syphilis: British studies find an urban syphilis rate of 8-10% from the 1700s to the early 1900s. At the time the condition was incurable, and progressed to insanity and death in about a quarter of patients. If you’ve got a 10% local syphilis rate, you are going to want some major sexual purity taboos. It’s less clear how bad they were in truly ancient times, but given how easily the extent of syphilis has slipped out of our cultural memory, I’m not ruling out “pretty bad”.

Here are some things I think of as basic parts of sexual purity taboos. All of these are cross-cultural – which isn’t to say they’re in every culture, or that some cultures aren’t exactly the opposite, just to say that they seem to pop up pretty often. I’m writing this from the male perspective because most of the cultures I know about thought that way:

1. If your wife has sex with another man, you should be angry

2. Preferably you should marry a virgin. If you think your bride is a virgin, but she isn’t, you should be angry

3. If you’ve got to marry a non-virgin, then marrying a widow is okay, but marrying a former prostitute or somebody known for sleeping around a lot is beyond the pale.

All of these are plausible ways to prevent the spread of STIs. If your wife has sex with another man, she could catch his STI and give it to you. If your bride isn’t a virgin, she might have STIs. If someone’s a widow, they probably slept with one known person whose STI status can be guessed at; if they’re a prostitute or slept around, they slept with many unknown people and have a higher chance of having STIs.

But the counterargument is that at least (1) and (2) are also good ways to prevent false paternity, ie raising another person’s child as your own.

The main argument that it’s more STI than paternity is that (3) doesn’t seem paternity-related; if it’s been more than nine months, you shouldn’t care who they’ve slept with before. Also, the taboos usually explicitly reference ideas of “pure” vs. “gross”; in most other cases, these are disease-related taboos. For example, spoiled food is “gross”, dirt/feces/blood are “gross”, corpses are “gross” – all of these are related to risk of disease transmission.

The main argument that it’s more paternity than STIs is that there’s less concern around men who have slept around being impure and unmarriagable. But that could just be because men are making the taboos and rigging them in their own favor. Yet you’d still think that if 10% of the population had syphilis and cultural evolution worked, men would stick to the purity taboos out of self-interest. Not sure here.

One way to distinguish between these possibilities would be to see how taboos changed as STIs became more common. This paper did some computer modeling and finds that STIs probably started becoming a problem around the rise of agriculture, which was also when a lot of restrictions on female sexuality became stricter. They tie this in with the triumph of monogamy over polygyny, which is especially interesting because false paternity doesn’t have a good explanation for this.

If purity taboos were related to STIs, we would expect them to get stricter and stricter through history, from the ancient through the classical and medieval worlds, maybe a sudden jump around the arrival of syphilis, reaching their peak in the 1800s, and then dropping precipitiously once good public health made the threat of STIs recede. I don’t have any real data on this, but it fits my impressions.

Most likely purity taboos came from both paternity issues and STIs. But I think it’s fair to speculate that STIs played a part.

II.

What about taboos on homosexuality?

Obviously there are no paternity issues here. And the AIDS epidemic proves that STIs transmitted primarily through homosexual contact can be real and deadly. Men who have sex with men are also forty times more likely to get syphilis and about three times more likely to get gonnorrhea (though they may be less likely to get other conditions like chlamydia).

In the previous thread, some people suggested that this could be an effect of stigma, where gays are afraid to get medical care, or where laws against gay marriage cause gays to have more partners. But Glick et al find that the biology of anal sex “would result in significant disparities in HIV rates between MSM and heterosexuals even if both populations had similar numbers of sex partners, frequency of sex, and condom use levels”.

Other people brought up that HIV and syphilis both post-date cultural taboos around homosexuality and so can’t be responsible for them. Were there earlier STIs that might have caused the taboos? This history of venereal diseases suggests ancient origins of gonorrhea, chlamydia, and (at least oral) herpes (the last of which provoked Emperor Tiberius to ban public kissing). But nobody understood that the conditions were spread by sex until the Middle Ages (?!) so the records weren’t great. Overall the ancient maladies seem a lot less worrying than syphilis or anything else that moderns have to deal with, but not completely absent.

Complicating the story, taboos around homosexuality were complicated and in some cases nonexistent. China seems not to have had any rules against it (though it also seems to have been pretty rare). The ancient and medieval Middle East seems to have been somewhat accepting also, assuming modern historians aren’t projecting. Some Greek city states had socially-sanctioned relationships between older men and younger boys. In Rome, it was considered acceptable for a man to be the penetrating partner, but shameful to be the receiving partner (and this tended to be limited to slaves and prostitutes). It wasn’t until the rise of Christianity that homosexuality became definitely taboo in Europe (mostly around 1000 or so), and not until Europeans took over other places that those places became equally strict.

Goodreau et al writes about “role versatility” in homosexual communities – ie whether people switch between being the penetrative vs. receptive partner, or always stick to one or the other. They find that role versatility is responsibility for faster STI spread (especially compared to heterosexuals, who are restricted to one role or the other), with receptive partners most easily infected. That makes it pretty suggestive that many of the ancient cultures that tolerated homosexuality had traditions that limited role versatility, with fixed distinctions between a high-status penetrating partner (freemen or adults), and low-status receptive partners (slaves or young boys) (except wouldn’t the young boys eventually grow into adults? Maybe the ten year delay is important in slowing the spread of epidemics). On the other hand, you could also say that these societies were sexist, and it was considered honorable to have sex in the male-like role and dishonorable to have it in the female-like role.

One plausible story is that there were relatively weak prohibitions on homosexual intercourse (as long as there was limited role versatility) during the period when STIs were rare and weak. Once syphilis started spreading in the late 1400s, these became much stronger. But honestly the strengthening of taboos in Europe was closer to 1000 or 1200 than to 1500, so I don’t know.

I still think it’s pretty likely STIs played a role in the cultural evolution of taboos against promiscuity and homosexuality. But the evidence is still pretty circumstantial. To really be convincing, you’d have to determine that serious STIs predated these taboos, maybe even correlate the STI rate with taboo strength. I don’t know of any research that’s tried this, and given how poor the ancient epidemiological records are it sounds pretty hard.

I haven’t been able to find a lot of real anthropological research on these issues; if you know of any, please tell me.

[Comments will be policed especially carefully here; please stick to discussing the origin of these taboos, not what you think of them personally.]