IStock Photo 190512 © Ronda Oliver

Why was this survey taken at all? Over three decades, Kinsey and his associates at Indiana University interviewed thousands of people on what had been, up to that point, a taboo subject: their sex lives, including orientations, habits, fantasies, proclivities, memories, and frequencies. Many of these questions were deeply personal, so Kinsey developed a careful, considerate, and essentially anonymous interviewing technique. But he was not merely interested in subjective answers; he also wanted, as much as possible, to obtain objective bodily measurements. Let’s not forget, Kinsey was a trained entomologist, who’d spent years meticulously taking the body measurements of more than 300,000 individual gall wasps—no fewer than 26 measurements per wasp.

How big was the Kinsey sample size? Kinsey and his researchers asked a total of 5,637 men to estimate the size of their erect penis, and 4,429 of those men participated. In addition, they requested the same sample do an actual measurement at home: 2,578 men complied.

Why was the top of the penis measured? Any man who has measured his penis could, if shameless enough, probably answer you this one: Measuring the underside of the penis leads to exaggeration, for one simple reason: Where do you stop? Where, one could say, is the unmistakable root of the penis? On the underside it is, to put it mildly, obscured.

Why 18- to 24-year-olds, and why does it matter that the men were in college? Because that is the group to which Kinsey, a university professor, had the most unfettered access. He also had access to prostitutes and inmates—two other fairly forthcoming groups—but public outcry at conclusions being based (even partly) on “delinquent” data led to those groups later being “cleaned” from the Kinsey data.

Did Kinsey’s researchers measure only white males? The collegiate sample consisted primarily of white males. Out of the 2,578 who submitted measurements, 2,376 of them were white. They also received measurements from 59 black collegians and from 143 white non-collegians.

These measurements were made in 1948. Couldn’t the average penis length have changed significantly since then? More recent studies of penis size (see below) do not confirm any significant change in the average American penis length.

What if Kinsey’s interviewees fudged their numbers? Kinsey and his associates believed that a truly anonymous survey, in which personal information would never be revealed—along with relaxed and carefully neutral questioning—left little reason for an interviewee to lie.

Do other penis-size studies exist? Why did Book of Odds choose this one? Quite a few exist. One of the largest hands-on studies involved over 400 males in Cancun, Mexico, conducted March 11-17, 2001. The males—all nightclub-goers, 18+ years old—were asked to induce an erection, which was then measured by an all-female staff of nurses. Only 300 were able to sustain an erection. As you may have guessed, these men were Americans on spring break; the study was funded by Lifestyles Condoms. It found the average erect penis to be around 5.9 inches long; the average length found by the Kinsey study was 6 inches. Most other studies vary from the Kinsey data in one critical variable or another. Some have small sample sizes. Others concern men from other countries, e.g. Australians or Italians; these generally find an average between 5 and 6 inches.

Are there many penises that measure significantly less than 5 inches? No. For those that do: a very small number of men have a condition known as micropenis, in which the erect penis is 2 inches long or less. During puberty, the condition may be treated with hormone therapy.

What about porn stars? They often advertise double-digit numbers. While adult film actors often do have abnormally large penises, it is very uncommon for any human to have a penis over 9 inches. The largest measurement received by Kinsey was 9.25 inches, reported by less than 0.25% of the study population. It is likely that many porn stars inflate their numbers, or measure the underside of the penis, for publicity purposes.

Why does penis length matter? A good question, but one that men pose more often than women. About 85% of women 18 - 65 are satisfied with their partner’s penis size, while just slightly over half of men are satisfied with their own.