As any third-grader knows, IF UR A DUDE AND UR INDEX FINGER IS LONGER THEN UR GAYYYYYYYYYYYYYY
My guess is that the digit ratio theory is gonna be disproven, or at least fall out of favor, pretty soon. It just smells too much like craniometry to me. I snooped around a little bit just now and found that people have done studies correlating finger length with everything from competitive performance of female fencers to rates of schizophrenia. But they're just correlations, which are subject to all kind of biases and errors even in the most honest and diligent scientists. The best guess anyone has made as to why the correlations might be there is prenatal androgen exposure, leading to greater testosterone levels and sensitivity in later life, as Kellyb pointed out.
But doubt is already being cast on use of digit ratio as a marker of prenatal androgen levels, which is the supposed basis for its validity as a determinant of masculinization and therefore all kinds of physiological effects. And if there's not even a correlation there, the rest of the supposed effect kind of falls apart:
http://www.ncbi.nlm.nih.gov/pubmed/20862705Dev Psychobiol. 2011 Jan;53(1):69-78.
No association between two candidate markers of prenatal sex hormones: digit ratios (2D:4D and other) and finger-ridge counts.
Dressler SG, Voracek M.
Department of Basic Psychological Research, School of Psychology, University of Vienna, Liebiggasse 5, Vienna, Austria.
Abstract
The second-to-fourth digit ratio (2D:4D), putatively indexing prenatal androgen levels retrospectively, has become increasingly popular as an easily applied measure in research into the prenatal sex-hormonal bases of behavior, health, and disease.
However, its validity has not yet been conclusively demonstrated and in fact is currently debated, because validation tests of 2D:4D with other, prenatally established, presumed markers for prenatal sex-hormone action have yielded mixed evidence or still are unavailable. Hence, the associations of 2D:4D with finger-ridge counts, one such further under-researched marker, were examined in this study. In a sample of 75 male and 75 female normal healthy adults, the six possible finger-length ratios of the human hand (from 2D:3D to 4D:5D, including the classic 2D:4D ratio) were ascertained with two commonly used measurement methods (imaged-based vs. fingers measured directly), along with two traditional dermatoglyphic traits (total and absolute finger-ridge counts). Sex differences in finger-length ratios (lower in men) generally were of moderate size (about .5 SD units), whereas those in finger-ridge counts (higher in men) were small to negligible (about .2 SD units). Within-sex analysis did not indicate theory compliant (i.e., negative) correlations between these two sets of traits that were consistent, noteworthy, or reliable.
Finger-length ratios and finger-ridge counts are ontogenetically overlapping in their prenatal formation and anatomically adjacent. Hence, possible temporal and localized sex-hormonal effects in prenatal life are unlikely to account for their nonassociation. The current findings cast some doubt on the validity of these retrospective pointers to prenatal androgen levels.
http://www.ncbi.nlm.nih.gov/pubmed/19819951
Endocrinology. 2009 Nov;150(11):5119-24. Epub 2009 Oct 9.
Fingers as a marker of prenatal androgen exposure.
Berenbaum SA, Bryk KK, Nowak N, Quigley CA, Moffat S.
Department of Psychology, The Pennsylvania State University, 519 Moore, University Park, Pennsylvania 16802, USA. sab31@psu.edu
Comment in:
* Endocrinology. 2009 Nov;150(11):4819-22.
Abstract
Interest in biological substrates of sex-related variations in psychological and physiological characteristics has led to a search for biomarkers of prenatal hormone exposure that can be measured postnatally. There has been particular interest in digit ratio, the relative lengths of the second and fourth fingers (2D:4D), but its validity as a measure of prenatal androgen has not been established. We report the strongest evaluation of the value of 2D:4D as a biomarker for early androgen exposure. Individuals with 46,XY karyotype but no effective prenatal androgen exposure due to complete androgen insensitivity syndrome had digit ratios that were feminized: they were higher than those of typical men and similar to those of typical women.
Nevertheless, the effect was modest in size, and there was considerable within-group variability and between-group overlap, indicating that digit ratio is not a good marker of individual differences in prenatal androgen exposure.
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