The ratio between the second and fourth digits is linked to stretched penile length according to a study published online this week in Asian Journal of Andrology. This finding suggests that digit ratio can predict adult penile size and that the effects of prenatal testosterone may in part explain the differences in adult penile length.
Digit ratio has been correlated with a number of aspects of reproductive biology and sexual behaviour over the past decade, including a link with the risk of developing prostate cancer. Digit ratio has also been proposed as a putative biomarker for prenatal testosterone and covaries with the sensitivity of the androgen receptor.
Both prenatal testosterone and the androgen receptor play a central role in penile growth. Tae Kim and colleagues provide support for a relationship between digit ratio and penile length. Men with a lower digit ratio tend to have a longer penile length. The authors concluded that higher prenatal androgen exposure is responsible for both the lower digit ratio and the longer penile length.
As Denise Brooks McQuade highlights in her accompanying Commentary, this finding indicates the potential value of the digit ratio biomarker for predicting clinical conditions that are related to developmental androgenic effects.
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Finger ratio points to penile length - DOI:10.1038/aja.2011.75