Sex With 21 (Not 20) Women Lowers Risk Of Prostate Cancer. It’s Science!

Not too pretty.
Not too pretty.

Today’s headline was modified from the Telegraph, one of the least lurid and sensationalistic of those generated by the peer-reviewed paper “Sexual partners, sexually transmitted infections, and prostate cancer risk” by Andrea Spence, Marie-Claude Rousseau, and Marie-Élise Parent in Cancer Epidemiology.

The Telegraph reported, “The University of Montreal has found that men who had sex with more than 20 women lower their prostate cancer risk.” Twenty wouldn’t do it; neither 19. Had to be more than 20. But what, you ask, about sodomy?

In contrast, men who slept with 20 men doubled their risk of developing prostate cancer compared with men who have never had sex with another man.

That level of specificity can only arise via models, neighbors and friends, statistical models. Let’s see what happened.

Some 3,208 Canadian men, half with and half without cancer, were asked in-person personal questions. Now since these were extremely intimate questions about highly sensitive topics (sexual “orientation”, STDs, etc.), it is assured, academically speaking, that everybody told the truth. Right? Right?

Turns out that men who reported having had only 1 sexual partner over their lifetime, 25% of men in each group, half had cancer. The same thing was found for people who had 2-3 partners (about 16% of men; cancer rate about 50%). Those who reported 4-7, 19% of men with cancer and 16% of men without, 54% had cancer. Oho! Another similarity for those reporting 8-20 partners (about 20% of men, cancer rate about 50%). Only 25 men claimed celibacy, 15 with cancer and 10 without: too few to say anything about (though this didn’t stop anybody).

Say, how did we jump from just 1, then 2-3, then 4-7, then 8-20? Hey! That was peer reviewed. So don’t ask. These divisions may seem arbitrary to us, but not to scientists.

Finally, those men who reported having greater than 21 partners, 14% versus 16% of men, the cancer rate was 47%. A wee p-value confirmed that this was “statistically significant”, meaning…what, exactly? We’ll come to that. First, it’s interesting that of those who refused to tell how many “partners” they had, 5% of the men with cancer and 7% without, the cancer rate was only 40%!

That’s just “partners” so far, mind. Not man on woman, but man on anything. So the clever authors split the data to count just those who admitted having had female “partners”—which was all but 10 of the people surveyed. Since this was nearly everybody, the overall results are scarcely different. Yet they are reported as being a whole new separate part of the study, especially in the media.

Stay with me.

The authors then split the cancer cases into those with high and low Gleason scores. A Gleason score is a subjective rating (2 to 10) about how ugly the prostate cancer tissue looks under a microscope. The authors chose 7 as the demarcation point because why not?

According to the hypothesis tests, no “significant” differences between the groups in any “partner” category except the greater than 21 “partners”, and only for those with Gleason scores less than 7. Say, that’s a lot of testing! Any adjustment of p-values? Nope.

The authors then broke the data out by those who admitted having committed sodomy. This was 141 men. Of these, 55% had cancer. 22% of those 141 who admitted having 21+ “partners” had cancer and 11% did not, for a rate of 68%.

Question: was it sexual activity, or its lack, that caused these differences in prostrate cancer rates? Wasn’t that the point of the study? To show what caused observed differences? If activity is causal, then having sex with 4-7 women causes more cancers and having sex with 21+ causes fewer, at the same rates. (This may be why men notch belts: so that they don’t lose count. We already knew sodomy is destructive to health.)

Parent told the Telegraph, “It is possible that having many female sexual partners results in a higher frequency of ejaculations”, so she certainly thought the statistics demonstrate causality. Sort of.

But when asked whether public health authorities should recommend men to sleep with many women in their lives Dr Parent added: “We’re not there yet.”

However, the authors in the paper say, “ours is the first study to report a protective effect of having several female sexual partners over the lifetime.” That’s cause-and-effect language, sisters and brothers. Except for a guessed-at more frequent emptying of seminal fluids (any why can’t that happen inside a marriage?), they know not how this cause works. But they’re sure it’s there.

Yet the authors had no awareness that people could have lied—misremembered, maybe, but not lied. The arbitrary cuts were never acknowledged as arbitrary. The huge number of tests hunting for wee p-values were not mentioned. They did admit that only 63% of those without cancer agreed to participate, but 86% of those with cancer did. And since we’re talking about differences between these groups, this large difference might be meaningful.

And they never realized that statistical models aren’t causal.


Thanks to reader and contributer Bob Kurland for bringing this paper to our attention.


  1. Sheri

    “Except for a guessed-at more frequent emptying of seminal fluids”. I don’t remember the study asking about how frequently said individuals emptied seminal fluids. There’s more than one way that can happen and according to various internet sources, sex can occur without emptying the fluids. (Trying to remember this is a family page….)

    “But when asked whether public health authorities should recommend men to sleep with many women in their lives Dr Parent added: “We’re not there yet.”
    However, the authors in the paper say, “ours is the first study to report a protective effect of having several female sexual partners over the lifetime.” ”
    Must be politicians, not researchers.

    Protection from prostate cancer, not STD’s and homicidal spouses who catch you cheating!

  2. Sander van der Wal

    Having sex with 21 different women, is once enough, or do you need to do it more than once with each woman? The article is not particularly clear on this, and it could be important.

  3. How could your prostate tell whether you’re emptying you seminal fluids into different receptacles? Seems it could only tell how often, not where.

  4. Hal44

    Why should we get our boxers all scrunched up over a Canadian study? I thought all quality health investigations had to involve a New England state or a Northern European nation.

  5. The seeming plethora of papers such as the one dissected (in style) here cause me to feel fortunate that I chose a field of science that aspires to construct causal models almost exclusively. I’ve never calculated a p-value and don’t know anyone in physics who has (although I’ve no doubt it has happened). Having to regularly read this kind of literature would endow me with a permanent headache.

  6. Forget economics: statistics is the dismal science.

  7. Hey, I see another research project to go from here; as I read the article it has just number of times as frequency, not rate (i.e. number of ejaculations/partners per time). So wouldn’t 21 partners in two days be more “cleansing” than 21 partners in 60 years? And we need a new Mozart to write a modern day catalog aria for this. Note: Don Giovanni (don Juan for you unmusical types) died of fright, not prostate cancer, so that’s one statistic supporting the hypothesis (or was there a hypothesis?)

  8. Doug M

    Toally off topic, but I came across this today…

    “Quinnipiac University shows that men and women disagree markedly on the question of marijuana legalization. While men surveyed strongly favor legalization by a margin of 59 to 36 percent, women oppose it by a clear majority of 52-44 percent. This 15-point gender gap


    “President Barack Obama won the two-party vote among female voters in the 2012 election by 12 points, 56% to 44%, over Republican challenger Mitt Romney. Meanwhile, Romney won among men by an eight-point margin, 54% to 46%. That total 20-point gender gap…”

    Here we have two different polls with two different ways to measure the “gender gap.”
    one Poll takes (Men For – Women Against)
    the other takes (Women For – Women against) – (Men For – Men Against)

    Any opinions on what is correct, or what the gender gap even means?

  9. Eric

    I’m making up my sign now… “Sleep with me and save me from prostate cancer! “

  10. HarryG

    So all those male members of celibate religious orders should be suffering from Prostate Cancer should they?
    If not should they be making a booking at the next inquisition?



  11. Hatchetman

    Perhaps they selected twenty-one because that is as high as they can count, using all appendages available while naked.

  12. Gabriel Chase

    Hmmm… how about anal intercourse with 21 women? Does that increase or decrease your chances of prostate cancer?

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