It’s not too difficult to fathom that syphilis would discourage the marginal individual from having “non-traditional” sex. It’s just another example of a cost of engaging in a particular behavior. You often see these relationships in public policy. Tax policy is a prominent example.

Most studies are done with a conclusion in mind which greatly simplifies the variable selection process and analysis.

However, you are right to say that the study had poor data.

It seems the State provided a disincentive to premarital copulation and it does not seem as if it will return to that role in the near future now that we know marriage is about survivor benefits and estate tax rather than children.

“It is important to recognize the limitations of this study. It was challenging to measure sexual behaviors during the period of interest.”

“Also, the study was unable to confirm or reject alternative hypotheses, because it was even more difficult to measure use of contraceptive technology, social attitudes toward sex, and moral values during the period.”

Basically, the study begins with an idea, and looks for data to support that idea without considering that there might be other reasons behind the data. That’s not a very good study

The supposition is that syphilis rates were depressing “non-traditional” sexual encounters. Syphilis has been around for over five hundred years. The sexual mores of society have greatly-fluctuated over that time. It’s hard for me to believe that syphilis played a primary factor in the changing sexual mores when the role of women and, as you cite, the prevalence of artificial contraception were much more significant and unique developments in the same time period.

The role of women in society was certainly changing, but contraception also had effects on premarital sex independent of women’s changing social roles. I suggest reading the piece linked in my last paragraph if you’re curious about how that played out.

I don’t know enough about statistics to argue about correlation/causation in this particular case, but I assume a professional economist would know enough about that to consider and account for it in a study.

Is the idea here that people weren’t having premarital sex nor adulterous affairs before 1950?

The real change was the role of women in society. The middle class woman was no longer locked away and separated from men until she was married off. This greatly increased the opportunity for sex among the unmarried.

People who are going to have sex are usually not thinking, “But what if I get syphilis?” This seems like one of those cases of correlation rather than causation.