Do hand, foot, or finger proportions predict penile length or girth?

1. The headline study: finger ratio predicts length, by a hair

A 2011 clinical study of 144 Korean men found stretched penile length negatively correlated with the right‑hand 2D:4D ratio (r = −0.216, P = 0.009), meaning men with relatively shorter index fingers versus ring fingers tended to have longer stretched penises; the authors proposed this as circumstantial evidence linking prenatal androgen exposure to both traits ^{[1] [5]}. The measurement protocol—digits by caliper, penile length measured under anesthesia using standardized stretching—strengthens internal validity but does not erase other limits ^{[5] [4]}.

2. What journalists and press releases emphasized — and why that matters

Major outlets summarized the finding as “index vs. ring finger predicts penis size,” which is accurate to the study result but understates effect size and generalizability ^{[3] [6] [7]}. News pieces and health blogs often present the correlation as a simple rule of thumb; the original authors and commentators warned the sample was ethnically homogeneous (Korean men undergoing urological surgery) and modest in size, so the result may not hold across populations ^{[4] [6]}.

3. Competing evidence: size of hands or feet is not a trustworthy predictor

Separate research and fact‑checks conclude there is no robust correlation between overall hand size or foot size and penis size. Fact‑checkers and prior clinical work comparing hand/foot dimensions to penile measures found no reliable link, undermining broad pop‑culture claims that hands or shoe sizes reveal penile dimensions ^[2]. Thus, the 2D:4D result should not be conflated with claims about hand or foot length or shoe size.

4. Biological rationale offered — plausible but incomplete

Authors frame 2D:4D as a biomarker of prenatal testosterone exposure; prenatal androgens also influence penile development, forming a plausible shared biological mechanism ^{[1] [4]}. However, the degree to which digit ratio (“soft” proxy) quantitatively predicts adult anatomy is limited; the study’s correlation coefficient (r≈−0.216) indicates a weak relationship that explains only a small portion of individual variation ^[1].

5. Measurement, sampling and interpretation limitations

The Korean study used intra‑operative stretched penile length (reducing variability from temperature, anxiety or erection variability) and caliper digit measures, which is methodologically rigorous for a small clinical study ^{[5] [4]}. Still, selection bias (men attending urological surgery), ethnic homogeneity, modest sample size (n=144), and modest effect size constrain external validity; the authors themselves and editorial commentators cautioned against overgeneralizing to other ethnic or general populations ^{[4] [6]}.

6. What the evidence does—and does not—support for readers

Available sources support the specific claim that lower right‑hand 2D:4D correlates modestly with longer stretched penile length in one clinical Korean sample ^[1]. Available sources do not mention a reliable ability to predict penile girth from digit ratios, nor do they support predicting penis size from overall hand or foot size ^{[1] [2]}. Generalized “rule of thumb” use is not supported by the cited literature.

7. Bottom line for practical and scientific use

Digit ratio may carry information about prenatal androgen exposure and shows a reproducible but weak association with stretched penile length in limited samples ^{[1] [4]}. This is scientifically interesting but clinically and socially noisy: it cannot serve as an accurate, individual predictor of penile length or girth across populations, and conflating finger‑ratio science with popular myths about hand or shoe size is unsupported by the available reporting ^{[2] [3]}.

Limitations: this summary uses only the provided sources; broader literature or newer large, multisite replications (not in the supplied set) may change the balance of evidence.