Lesson on Confounds: When comparing circumcised to intact, don't forget the foreskin

Confound (freedictionary.com):

1. To cause to become confused or perplexed.
2. To fail to distinguish; mix up: confound fiction and fact.
3. To make (something bad) worse: Do not confound the problem by losing your temper.
4. To cause to be ashamed; abash: an invention that confounded the skeptics.

The first time I heard the word "confound" was in a graduate class where the professor was teaching us to be critical of published papers. She was talking about how to look for "confounds" which would render the results of the studies or experiments in the papers to be caused by something other than what the authors listed in their conclusions.

What makes me think of confounds today is the sudden appearance of news articles saying "Scientists Disagree about Circumcision Sensation" quoting two recent studies that contradict each other on whether or not a circumcised penis has less sensation than an uncircumcised one. Since I have a healthy distrust of the news media (they are trained and paid to find sensational stories, not be critical thinkers), I decided to go straight to the sources.

First, the news clip:

Kimberley Payne of the Riverside Professional Centre in Ottawa, Canada, and her colleagues tested the sensitivity of 20 intact
and 20 circumcised men's penises as they watched erotic movie
clips, by touching the penises with filaments that press down
with predetermined amounts of pressure.... They found no
difference in penile sensation between circumcised and
uncircumcised men.
However, when Robert Van Howe of Michigan State University used a similar method to measure sensitivity at 19 points along the penises of 163 men, he found that the five most sensitive points were all in portions of the penis removed by circumcision,
especially those in folds exposed as the penis becomes erect....

Something should already be fishy: Howe used more subjects, and we know how many points along the penises he measured. The reporter tells us how Payne measured sensitivity, but was that 1 point? 3? 10?

Now for the first study:

Sensation and Sexual Arousal in Circumcised and
Uncircumcised Men.
Kimberley Payne , Lea Thaler , Tuuli Kukkonen , Serge Carrier
, Yitzchak Binik
Introduction. Research, theory, and popular belief all suggest
that penile sensation is greater in the uncircumcised as
compared with the circumcised man. However, research involving
direct measurement of penile sensation has been undertaken only
in sexually functional and dysfunctional groups, and as a
correlate of sexual behavior. There are no reports of penile
sensation in sexually aroused subjects, and it is not known
how arousal affects sensation. In principle, this should be
more closely related to actual sexual function. Aim. This
study therefore compared genital and nongenital sensation as a
function of sexual arousal in circumcised and uncircumcised
men. Methods. Twenty uncircumcised men and an equal number of
age-matched circumcised participants underwent genital and
nongenital sensory testing at baseline and in response to
erotic and control stimulus films. Touch and pain thresholds
were assessed on the penile shaft, the glans penis, and the
volar surface of the forearm. Sexual arousal was assessed
via thermal imaging of the penis.
Results. In response to the erotic stimulus, both groups
evidenced a significant increase in penile temperature, which
correlated highly with subjective reports of sexual arousal.
Uncircumcised men had significantly lower penile temperature
than circumcised men, and evidenced a larger increase in
penile temperature with sexual arousal. No differences in
genital sensitivity were found between the uncircumcised and
circumcised groups. Uncircumcised men were less sensitive to
touch on the forearm than circumcised men. A decrease in
overall touch sensitivity was observed in both groups with
exposure to the erotic film as compared with either baseline
or control stimulus film conditions. No significant effect
was found for pain sensitivity. Conclusion. These results
do not support the hypothesized penile sensory differences
associated with circumcision. However, group differences in
penile temperature and sexual response were found.

OK, so Payne measured two points on the penis and one on the forearm (?) at different stages of arousal, also measuring temperature on the penis. They measured sensitivity on the shaft and head, but hmmmm, isn't there another part of the penis? the foreskin? isn't that cut off with circumcision? why not measure that, Ms. Payne?

Now for the other study:

Fine-touch pressure thresholds in the adult penis.

Morris L Sorrells , James L Snyder , Mark D Reiss ,Christopher Eden , Marilyn F Milos , Norma Wilcox , Robert S Van Howe

OBJECTIVE To map the fine-touch pressure thresholds of the adult
penis in circumcised and uncircumcised men, and to compare the
two populations.
SUBJECTS AND METHODS Adult male volunteers with no history
of penile pathology or diabetes were evaluated with a
Semmes-Weinstein monofilament touch-test to map the
fine-touch pressure thresholds of the penis. circumcised
and uncircumcised men were compared using mixed models for
repeated data, controlling for age, type of underwear worn,
time since last ejaculation, ethnicity, country of birth,
and level of education.
RESULTS The glans of the uncircumcised men had significantly
lower mean (sem) pressure thresholds than that of the
circumcised men, at 0.161 (0.078) g (P = 0.040) when
controlled for age, location of measurement, type of underwear
worn, and ethnicity. There were significant differences in
pressure thresholds by location on the penis (P < 0.001).
The most sensitive location on the circumcised penis was the
circumcision scar on the ventral surface. Five locations on
the uncircumcised penis that are routinely removed at
circumcision had lower pressure thresholds than the ventral
scar of the circumcised penis. CONCLUSIONS The glans of the
circumcised penis is less sensitive to fine touch than the
glans of the uncircumcised penis. The transitional region
from the external to the internal prepuce is the most
sensitive region of the uncircumcised penis and more
sensitive than the most sensitive region of the circumcised
penis. Circumcision ablates the most sensitive parts of the
penis.

Ouch, the most sensitive parts are cut off in circumcision and the circumcision scar is the most sensitive part left over? That's not very nice. They measured many locations on the penis, seeming to use the same method as Ms. Payne. But first, they eliminated all volunteers with penis problems. And then they ask how old they are, when they last ejaculated, what kind of underwear they had, their country of birth, education level, ethinicity.. dang they really want to make sure that nothing can confound their results. And by the way, those P values are results of statistical tests to prove the data is statistically significant, i.e. it's not a result of random chance.

Although, I do see one confound - Howe doesn't tell us what state the penis is in. Was it soft, hard, both? Looks like both parties may need further research.

And that, folks, is the lesson in confounds. If you don't control for everything that might change your results, critical thinkers won't believe your assertions. Since Ms Payne's group didn't measure sensitivity on the portions of the penis that are removed in circumcision, can she really say that circumcised penises have the same sensitivity? The only thing I get from her study is that the shaft and head of the penis feel the same, but I'm still curious about the foreskin. Hell, she didn't even say whether she turned away volunteers that had penis problems, but she did have age matched volunteers.

When using research to guide serious decisions, like whether or not to cut off the end of your newborn son's penis, you might want to look for confounds before you form your final opinion.