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Sir,

Giles et al.[1] present interesting data on the relationship between greater frequency of ejaculation during any sexual activity and decreased risk of later prostate cancer. They use these data, and that there was no association of cancer risk with the number of female sexual partners, to argue against an infectious process having a major role in prostate cancer. They also tentatively speculate that perhaps masturbatory ejaculations confer more protection than those during intercourse. Unfortunately, coverage of their research, both in New Scientist and the popular media, trumpeted the ‘news’ that masturbation prevents prostate cancer. The questions (ejaculations summed over all sexual outlets) that were asked in the study by Giles et al. do not allow such a conclusion. However, other researchers have differentiated between different sexual behaviours in their studies of risk factors for prostate cancer, and found evidence that it is intercourse frequency, but not masturbation frequency, that is ‘protective’. A review of several studies found that in general, men who developed prostate cancer had lower rates of penile-vaginal intercourse at age ≥ 50 years than age-matched controls, and a greater frequency of masturbation [2]. This finding is consistent with the inference of Giles et al. that non-infectious factors are more important than any infectious factors in the causes of prostate cancer. The finding is also consistent with other research that found penile-vaginal intercourse but not other sexual behaviour frequency to be associated with longitudinal predictors of better cardiovascular health [3].