Penicillin as an Aphrodisiac?

An interesting new paper (abstract; PDF) by the Emory economist Andrew M. Francis explores penicillin’s role in shaping modern sexuality:

It was not until 1943, amid world war, that penicillin was found to be an effective treatment for syphilis. This study investigated the hypothesis that a decrease in the cost of syphilis due to penicillin spurred an increase in risky non-traditional sex. Using nationally comprehensive vital statistics, this study found evidence that the era of modern sexuality originated in the mid to late 1950s. Measures of risky non-traditional sexual behavior began to rise during this period. These trends appeared to coincide with the collapse of the syphilis epidemic. Syphilis incidence reached an all-time low in 1957 and syphilis deaths fell rapidly during the 1940s and early 1950s. Regression analysis demonstrated that most measures of sexual behavior significantly increased immediately following the collapse of syphilis and most measures were significantly associated with the syphilis death rate. Together, the findings supported the notion that the discovery of penicillin decreased the cost of syphilis and thereby played an important role in shaping modern sexuality.

(HT: Marginal Revolution)


A great example of the laws of unintended consequences.


Hmmm, most religions have strong sexual guidelines restricting sexual practice. Some of that probably was to guard against diseases. The Jewish religion in particular is known for its extensive cleanliness requirements as well as sexual practice limits.

They (and Christians) would probably describe it as God giving good advice. Atheists (or practitioners of other religions) would describe it as practical wisdom being put into a religions framework to improve adherence to good practices.

Either way, it suggests that people weren't nearly so motivated by religion when they were avoiding extra-marital sex as they were by practical motivations of I-might-die-of-a-nasty-disease-if-I-screw-around.


The problem with the religious proscription as way of avoiding nasty disease theory is that this particular nasty disease didn't exist in the Old World at the time the proscriptions were created. Syphilis was introduced by sailors returning from Columbus' voyages. (Nor do I recall reading of any other sexually-transmitted disease in ancient times.)

It's an interesting study in virgin field epidemics. It was apparently endemic in the New World, so the population had acquired resistance, yet it became a fearsome plague in the virgin field of Europe, in the same way that minor European diseases like measles and mumps became mass epidemics when introduced to the Americas.

Cyril Morong

Darn, I'm allergic to penicillin.

Hefner's debt to Fleming

My first thought was that there would be a citation of some recent research stating something along the lines of "as a consequence of penicillin induced attenuation of inflammatory induced nitric oxide (NO) synthesis, sexual response was stimulated by increased blood flow . . ." It sounded intriguing.

Then I got wondering "Again, what's this gotta do with economics?"

Ah, but my mistake . . . Never mind!


I read the pdf file and the researchers mention that other social and demographic factors contributed as well. This research from Emory does conflict with Dan Ariely's research from 'Predictably Irrational' on 'The Influence of Arousal : why hot is much hotter than we realize'. His research demonstrates that people are not able to take rational precautionary actions in the heat of passion.

I would like to see a stronger rejection of the alternate hypothesis of increased social mobility post WWII as the root cause. Increased social mobility would increase opportunity to engage in potentially dangerous behavior as traditional norms carry less significance in a more open society. It could be that the availability of penicillin prevented a major epidemic, so it would be an effect not a cause.

There is also another epidemiological factor at play here: If the type of risk taking people who are most likely to contract syphilis survive long enough to pass it on, the epidemic grows. If on the other hand, death and impairment happens faster, then these primary agents of the epidemic die off and the people left behind are more conservative by nature. This analysis is difficult to prove in humans but is a common approach used in agricultural pest resistance and in some public health situations where insect vectors are the primary agents of distribution.



I guess I am not completely clear on your final point about the risk takers not living long enough to spread it. Maybe I am missing something in my read, but it sounds like people with syphilis live a long time, late (most serious) stages of the disease do not present until 10 to 30 years after initial infection.

On the article's main point, it certainly seams reasonable that when you reduce the perceived cost of something, people consume more of it. In this case, penicillin has clearly reduced the perceived cost of risky sex, at least prior to HIV.

I am pretty sure that when a cheap fix is found for HIV, and all other legal barriers are removed, many people will be at it again, swinging from the chandeliers or whatever, with reckless abandon.


So how do they explain the roaring 20's? More likely both the post WWI and post WWII increases in risky sexual behavior had to do with the changes in many people's attitudes brought by the wars. When so many are dying around you social conventions lose a lot of their force.


Not just the Roaring Twenties, but the periodic swings between (relative) sexual freedom and repression throughout history? As for instance the large number of prostitutes (50,000 and up) in Victorian London.


Can we observe any (reverse) effects due to the discovery of HIV?

Shane L

Yes indeed, there's some sign of that:

"The prevalence of HIV among young people in countries worst-affected by Aids, mainly in Africa, has fallen, new figures from the UN show.

In a report, UNAids says the incidence of HIV has decreased by up to 25% as young people between the ages of 15 and 24 change their sexual behaviour."


It’s an interesting study in virgin field epidemics. It was apparently endemic in the New World, so the population had acquired resistance, yet it became a fearsome plague in the virgin field of Europe, in the same way that minor European diseases like measles and mumps became mass epidemics when introduced to the Americas.


Uh, correlation does not imply causation? Post hoc, ergo propter hoc? Don't economists study elementary scientific inference?


A broader look at current Syphilis and HIV data indicates that Syphilis infections have increased since the early 1990s due to co-infection with HIV. The research indicates that a Syphilis infection increases the risk of HIV infection by 2-5 times at a time when an HIV infection was lethal.

So people were spreading Syphilis and HIV at a time (pre HIV protease inhibitors) when they knew they were risking their life. I somehow doubt that people in the 1937-1947 period (when penicillin wasn't widely available yet) were more rational than people in the early 1990s (when HIV protease inhibitors weren't invented yet).

This seems to point to opportunity for -and acceptability of- risky behavior as the driver after 1947 and that the availability of penicillin merely prevented a massive epidemic.