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, so 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.