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A Related Study

The following article was published after I had collected data, but before my article appeared in print.

H. Wessells, T. F. Lue, & J. W. McAninch: Penile length in the flaccid and erect states: Guidelines for penile augmentation. J. Urol., 156: 995-997, 1996.

This research used as subjects 80 physically normal men who were evaluated for erectile dysfunction. Each man had his penis measured with a tape while flaccid, while flaccid but stretched, and while erect following an intracavernous injection of the vasodilator prostaglandin E1. Of the subjects, 20% were black, 12.5% were Asian, and the remainder were white. The men ranged in age from 21 to 82 years old with a large cluster in their 30s and another in their 60s. The average age was 54.

This study found the average flaccid length to be 3.47 in. with a range from 1.97 to 6.10 in. Stretched length was very similar to erect length. And the average erect length was 5.07 in. with a range of 2.95 to 7.48 in.

The Wessells study appears to provide a cautionary note regarding the average values for length found in the photo sample (6.05 in.) and the Kinsey self-reported sample (6.20 in.) Before comparing these three studies we must acknowledge that subjects were selected to be in the Wessells study because they presented themselves at a urology clinic with erectile problems. It is possible that this, in itself, caused those men to be systematically different in penile length from the men in the main article reported in this site. However, this conclusion seems unwarranted since there are no theoretical reasons or published reports suggesting that individuals with erectile dysfunction might be different in penile length from the general population. The almost 14-year difference in average age between the samples also might be a source of difference in average penile length, yet based on Kinsey data, variation from age 20 to age 60 should account for a decrease of no more than .4 in.

So, if attendance at a urology clinic or slightly older average age is an insufficient basis for explaining the whopping 1-inch difference in the means produced by these studies, what is? I would nominate as the culprit the fact that the photo research and Kinsey subjects self-selected themselves to participate in a sex research study. In other words, these samples were more likely to include a greater proportion of men who were not only willing but anxious to be studied - who had some degree of pride in their own prowess and anatomy. To this, add the fact that the Kinsey subjects mailed in their own penile measurements taken by themselves in the privacy of their homes. If there was any tendency, conscious or unconscious, to exaggerate the measurements, full opportunity was there.

I conclude that self-selection bias in the photo research sample and self-selection plus self-reporting bias in the Kinsey sample have caused these samples to produce a mean for erect penile length that is too high to be a good estimate of length for the general male population. While it is impossible to know if the Wessells sample of urology patients provides any better estimate for the general population mean, I am prepared to believe it does. I believe this because the subjects did not select themselves specifically or solely to take part in the study, but rather where recruited when they attended a urology clinic.

At this point, research has not produced a reliable estimate of average penile length for the general population of all men. The Wessells research feeds our suspicion that the mean, when it is finally measured, will be lower that the values determined in the article: Penile erections: Shape, angle, and length. In that eventual research the key methodological hurdle will be to draw a sample that will adequately reflect the general population of men.