Excellent critique and comparison of the British study below.
"Does Masturbation Increase Risk of Prostate Cancer?
Most research has shown that as ejaculations increase (from both PVI and masturbation), risk of prostate cancer decreases. In a Harvard-National Cancer Institute study, compared with men who reported four to seven ejaculations a month, those who reported 21 were 67 percent less likely to develop prostate cancer. The researchers concluded: “Frequency of ejaculation is not related to increased risk of prostate cancer.”
My critic didn’t mention that study. Instead, he fastened on a British report showing that frequent young-adult ejaculations (PVI and masturbation) were linked to an increased risk of prostate cancer. “Frequent masturbation during men’s 20s and 30s was a marker for increased prostate cancer risk” decades later.
So we have dueling studies. Which one should we believe? When studies disagree, it helps to review their methodology.
• The larger the subject pool the more reliable the results. The Harvard-NCI study showing no increase in prostate cancer from masturbation had 222,426 participants, the British study showing a link between young-adult mastrubation and prostate cancer years later had 840. The Harvard-NCI study is more believable.
• Prospective studies are more reliable than retrospective research. Prospective studies examine participants at the outset and then re-examine them periodically for the duration of the study. Retrospective studies depend entirely on memory. But memory can play tricks, especially when you ask men in their fifties to estimate how much they masturbated during their twenties. The Harvard-NCI study was prospective, the British study retrospective. The Harvard-NCI study is more believable.
• As biological plausibility increases, so does believability. Prostate cancer is to some extent sexually transmitted. A history of sexually transmitted infections (STIs) increases risk, and as the number of STIs increases, so does risk. Ejaculation flushes the prostate, removing pathogens that may contribute to cancer risk. As a result, we would expect frequent ejaculations to reduce risk across the lifespan. That’s what the Harvard-NCI study showed. But not the British report. It showed increased cancer risk from masturbation during men’s 20s and 30s, but decreased risk from masturbation after 50. Huh? Why the change from increased risk to decreased? The British study strains credulity. The Harvard-NCI study is more reliable.
The score: The Harvard-NCI study, three. The British study, zero. The best evidence shows that mastrubation does not increase prostate cancer risk."