Paul Joannides Psy.D.

As You Like It

Male Genital Length Study Falls Short

His finger length says nothing about what's in his briefs.

Posted Jul 10, 2011

illustration showing the index and ring fingers, with a ruler over a male crotch
Penis length was in the news this week. This headline from Men's Health was typical: "What Your Hand Says about Your Penis." The truth would have been well served if the corresponding article had said "LITTLE, IF ANYTHING AT ALL."

Fueling the hand-penis stories was a study by Korean researchers titled "Second to fourth digit ratio: a predictor of adult penile length." Unfortunately, there is a large gap between what the authors' write-up implies and what the data from their study shows.

Researchers have long suspected that testosterone in the womb can have an impact on the genital size of the fetus as well as on a person's sexual orientation. A subset of these researchers believes that testosterone in the womb also influences the length of the index and ring fingers. They say the longer the ring finger when compared to the index finger, the more testosterone was in the womb during pregnancy.

I can't begin to tell you how much controversy this hypothesis about finger-length ratios and prenatal testosterone has generated. (It is referred to as "2D:4D," with 2D being the index finger, and 4D being the ring finger.) The researchers who are doing the studies don't always appreciate the limitations of their work, and the reporters who write about it rarely understand the complexity that's involved. This is what seems to have happened this past week.

To get an idea of how inaccurate this week's articles on penis length have been, take a look at this graph from the Korean study. It shows the relationship between finger-length ratios and penis size among the men who were studied.

graph of from the study showing a great deal of randomness

What relationship, you ask? Exactly.* The ratio between the ring and index-finger lengths fails to explain 96% of the variation in penis length. Zen at the NeuroDojo blog crunched the numbers and came to the following conclusion: "To me the lesson from this paper is not how much finger length ratios reveal about penis size, but how little they reveal."

It is virtually impossible to accurately guess the length of an individual's penis based on his ring-and-index finger ratio. Unfortunately, this is not what the authors of the study implied in their write-up, nor is it what the reporters reported.

Not long ago, we would have expected reporters to do background research before publishing an article. This is a pathetically easy undertaking in the day of Google and Bing. With a simple browser search, a reporter would have found the following from the American Journal of Human Genetics by Medland et al.: "Our findings call into question the validity of 2D:4D as a simplistic retrospective biomarker for prenatal testosterone exposure." Talk about a flag of caution!

Many scientists believe that research on prenatal testosterone exposure will yield important findings about human sexuality at some point in the future. However it will never be the kind of simplistic dribble that makes for good headlines.


*If the relationship were as robust as the media has implied, the circles in the graph would have been crowding around the line.

Thanks to Zen at NeuroDojo, and to the scientists on an excellent listserve I belong to for their fascinating 2D:4D debates.

The illustration is by Daerick Gross from the Guide To Getting It On. I hijacked and slightly altered it.

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