Updated August 8, 2019
It is "common knowledge" within the anti-circumcision movement that the foreskin contains more than 20,000 highly erogenous nerve endings that provide most of the erotic pleasure that a man experiences during sexual activity. Intactivists say that circumcision removes the most highly sensitive area of the penis, depriving a man of as much as 75% of sexual pleasure.
While every intactivist is taught this 20,000 figure, very few know the source, and none of them question its validity. Circumcision Choice and a few of our friends have taken the time to investigate these claims, and we’re happy to share the findings.
STUDIES OF FORESKIN SENSITIVITY
First let’s consider the claim that circumcision removes the most sensitive area on the penis. It is supposed that a circumcised man experiences just 25% of the sexual pleasure that an uncircumcised man enjoys.
Intactivists refer to Meissner’s corpuscles, the fine touch sensory receptors. They often compare the sensitivity of various body parts to show that the presence or absence of these fine touch receptors determines the level of sensitivity. The Whole Network explains,
Gently run your fingers over the back of your hand. Now, turn your hand over and gently run your fingers over the palm of your hand. Feel the difference? This is because the palm of your hand has Meissner's corpuscles, just like the foreskin and frenulum. They are what make our fingers and palms so sensitive, as well as our lips, anus, and other opening [sic] of the body. 
This explanation implies that the back of the hand is less sensitive because it lacks the fine touch receptors. But the back of the hand does contain Meissner’s corpuscles – just fewer than those in the palm. It turns out that the size and concentration of Meissner’s corpuscles vary at different points on the body. An area with larger and more concentrated Meissner’s corpuscles has greater sensitivity, while an area with lower density and smaller size has less sensitivity. 
A 2008 study by Indian researchers  ranked eight hairless skin sites in terms of density of Meissner's corpuscles. Beginning with the site with the greatest density, they were: fingertips, palm of hand, sole of foot, lips, front of forearm, back of hand, top of foot, and foreskin. When comparing these sites, the researchers found the foreskin to have the lowest density and the smallest size of Meissner’s corpuscles. In other words, the foreskin was the least sensitive skin site to fine touch. 
A 2007 study led by Morris Sorrells  measured response to fine touch and found that several areas "on the uncircumcised penis that are routinely removed at circumcision were more sensitive than the most sensitive location on the circumcised penis." However there were several problems with the study. The researchers admitted that the study suffered from self-selection bias and demographic differences. The uncircumcised men were younger than the circumcised men and were from different ethnic backgrounds.
Although Sorrells measured sensitivity to fine touch, sexual activity and sexual gratification involve other types of penile sensitivity. Common sexual activities include vaginal intercourse, fellatio, anal sex, and masturbation. As a man inserts his penis inside his partner's vagina or anus, most or all of the shaft is enveloped within the partner’s body. As he moves his penis in and out, the man feels sexual stimulation from the frictional contact and the warmth of his partner's body. Fellatio is similar, with the partner usually controlling the oenetration and contact. Manual stimulation (masturbation) also involves frictional contact, moving the hand along the penile shaft. In other words, Sorrells measured neither the types of penile sensitivity that typically occur during sexual activity nor the effect on sexual satisfaction or pleasure.
The researchers who designed the study and analyzed the results hold prominent positions in the anti-circumcision movement. Marilyn Milos is the founder of NOCIRC; Mark Reiss is the vice president of Doctors Opposing Circumcision; and Robert Van Howe is the author of many papers critical of circumcision. The study itself was funded by NOCIRC. Sorrells admitted that he conceived the study because "we kinda set out to change" - what they perceived as a pro-circumcision culture. The researchers hoped to give people "reasons to leave their sons intact or uncircumcised."  While researcher bias doesn't automatically discredit a study, readers should be particularly skeptical when considering a study in which the data, analysis, and conclusions support the partisan agendas of researchers and sponsors.
A 2016 study by Queens University PhD candidate Jennifer Bossio  confirmed that the foreskin is more sensitive to fine touch. But Bossio also measured other types of sensitivity. She found that the foreskin, while more sensitive to heat and pain than the glans, was not more sensitive than areas on the penile shaft. She concluded that "circumcision does not appear to remove the most sensitive part of the penis."
Bossio explained that heat and pain are probably more relevant for sexual sensation than light touch. "We measured heat detection and heat pain by attaching a thermode to the penis. Men would indicate either when they would feel a change in temperature or when it hurt. The nerve fibers in the penis that are activated by temperature and pain are more relevant in sexual functioning — or the feel of a sexy touch — than the light touch that past researchers had done. Even though [the foreskin] is more sensitive to light touch, I suspect that isn't implicated in sexual pleasure.” 
To review: not only does the foreskin contain fewer fine touch receptors than other areas of the body, but those fine touch receptors are unlikely to factor into erotic stimulation and pleasure.
FLEISS MANIPULATED BAZETT’S FINDINGS TO REACH 20,000
Now let’s look for the source of the 20,000 nerve endings. This claim was first advanced by physician and anti-circumcision advocate Paul Fleiss  in a 1997 article in Mothering magazine.  Fleiss wrote that “careful anatomical investigations have shown that circumcision cuts off … more than 20,000 nerve endings.” In a footnote he cited as his source a 1932 paper by physiologist Henry Bazett. 
However Bazett did not report the presence of 20,000 nerve endings in the foreskin. In fact the number 20,000 appears nowhere in the paper.
Bazett counted 212 nerve endings of all types in a single square centimeter. Only 2 of the 212 nerve endings were fine-touch receptors (i.e. Meissner’s corpuscles), and none were genital corpuscles, the ones that most experts attribute to erogenous sensation. 
Bazett had a sample size of 1 and no comparison to any other area of skin to provide a control. So we have no idea how representative the sample is – either across the entire body or among different individuals. Nerve ending density may vary from one part of the foreskin to another. But Bazett didn’t indicate the location on the foreskin for the sample that he used. And although nerve ending density may change with age, Bazett didn’t state the age of the donor.  It’s likely that the sample came from a newborn – which would typically have the highest density – and which cannot indicate the nerve density in an adult. 
None of these issues mattered for the purposes of Bazett’s study in 1932, but they are critical for Fleiss’s calculation.  In using Bazett’s figure, Fleiss made several unsupported assumptions about the age of the donor, the normalcy of the donor (as representative of an average male), and the uniformity of fine touch receptors across the foreskin. Furthermore, in order to arrive at a figure of 20,000 nerve endings, Fleiss must have assumed a surface area of at least 94.3 sq cm (14.62 sq. in.)  A study in Uganda found the foreskin area to range from a low of 7 sq. cm (1.1 sq. in.) to a high of 99.8 sq. cm (15.5 sq. in.) The average was about 38.5 sq. cm (6.0 sq in.) Even counting both inner and outer surface areas, 94.3 sq. cm would be near the top end of the range. 
Fleiss’s figure of more than 20,000 nerve endings was based on a sample size of one, several dubious assumptions, and inflation to the largest foreskin in the human range. The 20,000 statistic, which has gone viral in the intactivist community, is based on nonsense. 
No study currently available identifies the number of nerve endings on an adult foreskin. And the foreskin is among the least sensitive areas of skin to fine touch, based on the size and density of the fine touch receptors. Even if the foreskin is more sensitive than other areas on the male genitals, fine touch is not typically involved in sexual activities.
After researching in an attempt to validate the figure, a German economics professor and circumcision opponent concluded that the number has no basis in fact.
"It is time to take leave of the exaggerated number of 20,000 nerve endings in the foreskin. This legend is very wide spread by the opponents of circumcision. As long as the number cannot be scientifically proven, it is sufficient to say that the prepuce is very densely innervated and for fine touch is more sensitive than the glans. Otherwise, the credibility and persuasive power of the Intactivist movement suffers and makes itself unnecessarily vulnerable." 
For additional discussion on this topic, see:
 “The Touch Test: Can You Feel the Difference?”; The Whole Network; January 20, 2012
 “Intactivist Lie of the Week: 20,000 Nerve Endings”; Circumcision Facts and Science; January 16, 2017
 G. M. Bhat, et al; “Density and structural variations of Meissner’s corpuscles at different sites in human glabrous skin”; Journal of the Anatomical Society of India; 2008
 M. Sorrells, et al., “Fine-Touch Pressure Thresholds in the Adult Penis,”; BJU International 99 (2007): pp 864-869. Waskett and Morris detailed several flaws in the Sorrells study. See their analysis here.
 James Loewen; "Circumcision - Penis SensitivityTest" - interview with Morris Sorrells; YouTube; August 26, 2009
 JA Bossio et al; “Examining Penile Sensitivity in Neonatally Circumcised and Intact Men Using Quantitative Sensory Testing”; The Journal of Urology; 2016
 Gabby Bess, “How Circumcision Affects the Sensitivity of Your Penis"; Vice; April 15, 2016
 Stephen Moreton PhD; “’10,000, 20,000, 70,000 nerve endings’: a myth that keeps on growing”; CircFacts.org; November 2016
 Paul M. Fleiss, “Case Against Circumcision”; mothering; Issue 85, Winter 1997
 H.G. Bazett et al.; “Sensation: Depth, Distribution and Probable Identification in the Prepuce of Sensory End-Organs Concerned in Sensations of Temperature and Touch, Thermometric Conductivity”; Archives of Neurology and Psychiatry, pp 489-517; March 1932
 Another commonly promoted intactivist statistic is that the foreskin measures 15 square inches in area. It’s extremely likely that Fleiss used that figure in his calculation. It explains how he would derive a figure in excess of 20,000 for a whole foreskin from 212 in one square centimeter.
 G Kigozi et al; “Foreskin surface area and HIV acquisition in Rakai, Uganda (size matters); AIDS; October 23, 2009
 Hannes Mueller; "Demythologization of the '20,000 nerve endings' Legend"; October 11, 2017 (PDF download accessible via Facebook)
Stephen Moreton confirmed via personal correspondence with Kigozi that the measurements included both outer and inner surfaces.
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