In Dr. Abraham Morgentaler's 26 years as a urologist who treats issues of male sexuality, he has seen thousands of patients, and "probably there hasn't been a single one who hasn't paid attention to his penis size on some level," he says.

"Most men tend to believe they're smaller than average, and there's some distortion about what reality is," says Morgentaler, director of Men's Health Boston and author, most recently, of "The Truth About Men and Sex: Intimate Secrets From the Doctor's Office."

A new study could help combat some of that reality distortion.

Combining 17 previous published studies for a total of 15,521 men, it amounts to the biggest review to date of medically measured penis size, says its lead author, Dr. David Veale of King's College London. It processed the data into "nomograms," or graphical diagrams, like the one above, familiar to parents as the typical form for the growth charts that pediatricians use.

From the press release on the paper (metric conversions mine), which is titled "Am I normal? A systematic review and construction of nomograms for flaccid and erect penis length and circumference in up to 15,521 men":

The nomograms revealed that the average length of a flaccid penis was 9.16 cm [3.6 inches], the average length of a flaccid stretched penis was 13.24 cm [5.21 inches], and the average length of an erect penis was 13.12 cm [5.165 inches]. The average flaccid circumference was 9.31 cm [3.66 inches], and the average erect circumference was 11.66 cm [4.59 inches]. There was a small correlation between erect length and height.

So those are the averages, but the great beauty of a nomogram is that it can also give you a sense of the distribution of the variation, and you may have already noticed that the curve above looks strikingly flat. That is, there's just not much difference, except at the extreme edges.

If your erect penis is 11 centimeters, that puts you down in the 10th percentile; if your erect penis is 15 centimeters, that puts you way up in the 85th percentile. Quite a jump, for a little over an inch.

"What's interesting is, when you look at the curves, you see that most penises actually are fairly similar in size," Dr. Morgentaler says. "You really have to go to the extremes — the top or bottom 5 or 10 percent — to really see some big differences. And truthfully, in my practice, I would say that's exactly right. Most men have penises roughly the same size."

But somehow, many men who are average think they're below average. The study notes:

"Men may present to urologists or sexual medicine clinics with a concern with their penis size, despite their size falling within a normal range. This type of concern is commonly known as ‘small penis anxiety’ or ‘small penis syndrome.’ Some men who are preoccupied and severely distressed with the size of their penis may also be diagnosed with body dysmorphic disorder (BDD), where the preoccupation, excessive self-consciousness and distress is focussed on their penis size or shape. The diagnosis of BDD or small penis anxiety excludes 2.28% of the male population who are abnormally small as less than 2 standard deviations below the mean.

A recent New York Times piece on what our Google searches reveal about our sex lives documents the depth and breadth of (mostly sub-clinical) penis anxiety.

"Men Google more questions about their sexual organ than any other body part: more than about their lungs, liver, feet, ears, nose, throat and brain combined," Seth Stephens-Davidowitz writes. "Men make more searches asking how to make their penises bigger than how to tune a guitar, make an omelet or change a tire."

Size means more than just centimeters, Dr. Morgentaler emphasizes. "The whole issue is how much men actually look at their penis size as a stand-in or surrogate for their degree of masculinity. And we can argue that it shouldn't — that we should be more highly evolved than that — but whatever we may think, we're left with what guys actually do, and they are concerned about it."