Can the extent to which a penis will expand, from its flaccid state to its fully erect state, be predicted by certain factors? An international research team investigates.

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An international research team answers a few questions about penis sizes.

In 1966, William Masters and Virginia Johnsons, both pioneers of sexual education, refuted a series of “phallic fallacies” — or false notions about the human penis — in their book Human Sexual Response.

One of these was the idea that penises that appear larger when in flaccid state are larger when erect than penises that appear smaller when in flaccid state. Masters and Johnsons proved that this was not a solid notion.

However, the two did find that there was more of a size difference between penises when they are flaccid and when they are erect.

Also, they noted that men with penises that are shorter in size when flaccid tended to experience a greater increase in both length and girth when their penises became erect. This did not appear to be true for men whose penises were on the longer side when in flaccid state.

Recently, researchers from various international institutions — the Tulane University School of Medicine in New Orleans, LA, the University of California at Irvine in Orange, and the Mahidol University in Bangkok, Thailand — set out to investigate whether any telling physiological or lifestyle factors could predict the extent to which a penis would expand from flaccid to erect.

“Men generally focus undue attention on the size and appearance of their penises, both in the flaccid and the erect state,” the researchers write in the introduction to their paper, which is now published in the International Journal of Impotence Research. Its first author is Dr. Faysal Yafi.

“This attention on penile size has led to the colloquial terms ‘grower’ and ‘shower,'” they add.

A ‘shower’ can be loosely defined as a man who displays more penile size when flaccid and does not gain as much when erect. A ‘grower’ is a man who proportionally gains more length and girth on erection.”

But can anyone accurately predict which of these groups a man may fall into? The researchers explain that it’s not that easy.

The study was a retrospective review, analyzing data provided by 274 participants with erectile dysfunction. The men’s various penis sizes were measured in flaccid state, as well as when erect.

To achieve erect state, all the participants “received an intracavernosal injection [injection into the base of the penis] of prostaglandin E1 [used to treat erectile dysfunction], in combination with audiovisual sexual simulation.”

Following their initial analysis, the researchers found that, on average, there was a 4 centimeter difference between penis length in its flaccid state vs. its erect state.

Men whose penises grew 4 centimeters or more from soft to erect were deemed to be growers, whereas participants whose penises extended by under 4 centimeters were deemed showers.

Overall, 26 percent of the participants showed an average — or larger-than-average — growth in penis length, and 74 percent of the men showed a lower-than-average expansion.

The scientists then looked at a series of factors that had previously been suggested as probable predictors of how much a penis would expand. These included: smoking, a diagnosis of diabetes, and having had a radical prostatectomy (the surgical removal of the prostate gland).

However, the new analysis yielded no such associations. “In our study, […] neither the rates of smoking, diabetes mellitus, nor previous radical prostatectomy, were different between ‘showers’ and ‘growers,’ nor were they able to significantly predict being a ‘grower’ on univariate analysis,” the researchers write.

The only more significant association the research team was able to find was that “growers” tended to be younger and single — and the investigators link marital status to age, as well.

So what does it mean that younger men are more likely to experience a greater difference in penis size between flaccid state and erect state? It probably, the researchers believe, has something to do with a range of physiological changes that often occur with increasing age.

“[As] men get older,” the authors write, “physiological changes associated with aging occur, such as increased inflammation and fibrosis, decreased tissue elasticity and smooth muscle content, and compromised penile blood flow, and potentially lead to impaired penile enlargement.”

But the scientists also add that theirs was a small study, with a restricted cohort, and so further research should try to replicate and validate these findings.