The 2D:4D literature

The ratio between the length of the index finger and the ring finger, usually written 2D:4D, is a marker researchers use in a fairly wide range of studies as an indirect proxy for prenatal hormone exposure. It has been studied in relation to many traits, not this one specifically - and a smaller number of papers have looked at whether it relates to stretched penile length.

What the small literature reports

A handful of studies report an association between a lower 2D:4D ratio - meaning a relatively shorter index finger compared to the ring finger - and greater stretched length. "A handful" is doing real work in that sentence: this is not a large, repeatedly replicated body of research on the scale of the general clinician-measured pooled data, and this site will not quote a specific coefficient from it, because a small literature is exactly where an unverified figure is most likely to be either wrong or already superseded by a later paper that did not replicate it. The honest summary is that a relationship has been reported, more than once, in more than one sample, using a real anatomical measurement on both sides rather than self-report - which makes it a legitimate research finding worth naming - without it rising to the level of an established, precise, population-wide rule.

Why "research curiosity" is the right label

2D:4D research generally works by treating the ratio as a rough marker of something that happened developmentally, long before either measurement in the comparison was taken, and using it to probe questions that are otherwise hard to study in living adults. That is a legitimate way to generate a hypothesis and test it. It is a different thing from a marker that is reliable enough, and strongly enough related, to use as a stand-in for an actual measurement. Even where a real association is reported, the finger-ratio literature as a whole is smaller, newer, and less consistently replicated across large, diverse samples than the length literature that traces back through Veale et al. (2015) and its component studies.

What this does not license

Measuring your own fingers and drawing a conclusion about anything is not supported by this literature, however the association is described in whatever source you saw it in. A population-level, weak-to-moderate association between two traits does not become a personal predictor by being interesting, and a small number of published findings, even genuine ones, is not the same evidentiary weight as a large pooled meta-analysis with tens of thousands of measured participants. The correct use of this literature is as a data point in a larger developmental-biology conversation, not as a shortcut to a number.

This sits alongside two other correlates worth reading for contrast, treated the same careful way: height, where a real but weak correlation exists in large samples, and shoe size, where a direct test found nothing. The three claims are not equivalent in evidentiary weight and should not be filed together as "the same kind of folk belief," even though people commonly repeat all three in the same breath.

None of this is a substitute for a measurement, and it is a very different exercise from a subjective assessment. A rating platform has no use for a finger ratio, nor does an image model inferring from a photograph, nor a human judge reading one - and a site that ranks scored photographs against each other is not drawing on developmental biology at all. If a hand happens to be in a photo somewhere, that is incidental, not data.