Adarq.org

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/20862705

Dev 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.

Lots more science here:

http://www.ncbi.nlm.nih.gov/pubmed?term=%28%22fingers%22%5BMeSH%20Terms%5D%20OR%20%22fingers%22%5BAll%20Fields%5D%20OR%20%22finger%22%5BAll%20Fields%5D%29%20AND%20digit%5BAll%20Fields%5D%20AND%20ratio%5BAll%20Fields%5D%20AND%20%28%22testosterone%22%5BMeSH%20Terms%5D%20OR%20%22testosterone%22%5BAll%20Fields%5D%29&cmd=DetailsSearch

and here:

http://www.ncbi.nlm.nih.gov/pubmed?term=2D%5BAll%20Fields%5D%20AND%20%284D%5BAll%20Fields%5D%20AND%20ratio%5BAll%20Fields%5D%20AND%20%28%22sports%22%5BMeSH%20Terms%5D%20OR%20%22sports%22%5BAll%20Fields%5D%29%29&cmd=DetailsSearch

It's not perfect but, like facial and jaw structure, are one of many indicators that can tell you things.

There are many other interesting anatomical and muscular correlators that one can observe in people.

For example, in women calf size tends to correlate negatively with breast size.

In men, higher calf insertion points correlate with more muscular, broader upper bodies and vice versa.

In men, knockneedness tends to correlate with a soft, feminine physique and bowleggedness tends to correlate with masculinity. 

Okie doke. You and the apparently dozens of researchers that love the 2D:4D theory might be right about all of this. I'm just saying, it's is all a bit too much like eugenicists or craniometrists trying to determine who's more or less likely to be a criminal, or more or less "intelligent," based on cranial cavity size or facial features. That was all exposed as a farce many decades ago.

So I'm inclined to be skeptical.

Yeah. It's not exactly solid science. I wonder for how many of these studies the results occurred purely by chance.

http://www.ncbi.nlm.nih.gov/pubmed/20733526
The Association between Athleticism, Prenatal Testosterone, and Finger Length.

Moffit, DM and Swank, CB (PE) The association between athleticism, prenatal testosterone, and finger length. J Strength Cond Res 24(x): 000-000, 2010-Research suggests that prenatal levels of testosterone are related to finger length development and traits beneficial to athletic skill, such as power, endurance, visual-spatial skills, or sensation seeking and dominance behavior. In men, the second digit to fourth digit ratio (2D:4D) has been shown to correlate with success in competitive levels of football (soccer), which suggests that the 2D:4D ratio is a possible marker for level of attainment in sport. The purpose of this study was to explore the 2D:4D relationships between sports and make comparisons with nonathletes. A multiple group posttest-only design was used. Participants included 138 male volunteers with 92 intercollegiate National Collegiate Athletic Association division I athletes and 46 nonathletes who were not varsity athletes. The independent variable was group (crew, football, gymnastics, soccer, nonathlete). The dependent variable was the 2D:4D ratio. No significant differences were noted between the athletes and nonathletes (p = 0.182). Significant differences were found among the different groups (p = 0.000), with significantly lower ratios between football and crew (p = 0.000), football and nonathletes (p = 0.030), and gymnastics and crew (p = 0.001). This research provides a stronger level of evidence that the 2D:4D ratio may help indicate potential athleticism or competition-level achievement, but the external validity may be limited to only specific sports.

To me that suggests that digit ratio is dependent on something else, possibly height, which correlates with success in football rather than general athleticism.