This study will most likely have men all over examining their right hands. LMAO Digit ratio, most commonly the ratio of the index to ring fingers in humans, is sexually dimorphic (males have lower values than females) with greater differences on their right hand. Ratios are determined early in development and remain relatively stable thereafter. Perhaps the popular appeal is the relative simplicity of the measure, and the fact that researchers interested in any number of behavioral or physiological traits correlated with hormone activity have adopted digit ratio as a convenient biomarker for prenatal androgen exposure. Over the past decade, the correlation of digit ratio with sexual behavior and other aspects of reproductive biology has been well documented and there is a growing list of traits with links to digit ratio, although the associations are less well established. Amidst a flurry of 2D:4D publications, mostly indicative of positive findings, these criticisms were brought to light in a recent Asian Journal of Andrology report. The authors, Choi et al., provide convincing support for a relationship between digit ratio and penile length. The researchers measured right hand digits directly with calipers, averaging repeated measures to calculate digit ratio. Flaccid and stretched penile lengths were measured in anesthetized patients with a rigid ruler, by a second researcher blind to the digit measurements. Digit ratio was strongly (negatively) correlated with stretched penile length (P=0.024). Choi et al. concluded that higher prenatal androgen exposure is responsible for both the lower digit ratio and the longer penile length. McQuade DB. Does digit ratio (2D:4D) predict penile length? Asian J Androl. Asian Journal of Andrology - Does digit ratio (2D:4D) predict penile length? / http://www.nature.com/aja/journal/vaop/ncurrent/pdf/aja201181a.pdf Choi IH, Kim KH, Jung H, Yoon SJ, Kim SW, Kim TB. Second to fourth digit ratio: a predictor of adult penile length. Asian J Androl. http://www.nature.com/aja/journal/vaop/ncurrent/pdf/aja201175a.pdf The second to fourth digit ratio (2D:4D) has been proposed as a putative biomarker for prenatal testosterone and covaries with the sensitivity of the androgen receptor (AR). Both prenatal testosterone and the AR play a central role in penile growth. In this study, we investigated the relationship between digit ratio and penile length. Korean men who were hospitalized for urological surgery at a single tertiary academic centre were examined in this study, and 144 men aged 20 years or older who gave informed consent were prospectively enrolled. Right-hand second- and fourth-digit lengths were measured by a single investigator prior to measurement of penile length. Under anaesthesia, flaccid and stretched penile lengths were measured by another investigator who did not measure nor have any the information regarding the digit lengths. Univariate and multivariate analysis using linear regression models showed that only height was a significant predictive factor for flaccid penile length (univariate analysis:r=0.185, P=0.026; multivariate analysis: r=0.172, P=0.038) and that only digit ratio was a significant predictive factor for stretched penile length (univariate analysis:r=?0.216, P=0.009; multivariate analysis: r=?0.201,P=0.024; stretched penile length=?9.201×digit ratio + 20.577). Based on this evidence, we suggest that the digit ratio can predict adult penile size and that the effects of prenatal testosterone may in part explain the differences in adult penile length.