Ratios often lead to biased conclusions concerning the actual relationships between examined traits and comparisons of the relative size of traits among groups. Therefore, the use of ratios has been abandoned in most comparative studies. However, ratios such as body mass index and waist-to-hip ratio are widely used in evolutionary biology and medicine. One such, the ratio of the 2nd to the 4th finger (2D : 4D), has been the subject of much recent interest in both humans and animals. Most studies agree that 2D : 4D is sexually dimorphic. In men, the 2nd digit tends to be shorter than the 4th, while in women the 2nd digit tends to be of the same size or slightly longer than the 4th. Nevertheless, here we demonstrate that the sexes do not greatly differ in the scaling between the 2nd and 4th digit. Sexual differences in 2D : 4D are mainly caused by the shift along the common allometric line with non-zero intercept, which means 2D : 4D necessarily decreases with increasing finger length, and the fact that men have longer fingers than women. We conclude that previously published results on the 2D : 4D ratio are biased by its covariation with finger length. We strongly recommend regression-based approaches for comparisons of hand shape among different groups.

Department of Ecology Faculty of Science Charles University Prague Vinicná 7 128 44 Praha 2 Czech Republic

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Flegr J., Hrušková M., Hodný Z., Hanušová J.2005Body height, body mass index, waist–hip ratio, fluctuating asymmetry and second to fourth digit ratio in subjects with latent toxoplasmosis. Parasitology 130, 621–628 (doi:10.1017/S0031182005007316) PubMed DOI

Flegr J., Lindová J., Pivoňková V., Havlíček J.2008Latent toxoplasmosis and salivary testosterone concentration—important confounding factors in second to fourth digit ratio studies. Am. J. Phys. Anthropol. 20, 487–489 PubMed

Garamszegi L. Z., Hegyi G., Szöllösi E., Balázs R., Török J., Eens M., Møller A. P.2007Phenotypic correlates of digit ratio in a wild bird: implications for the study of maternal effects. Anim. Behav. 74, 641–647 (doi:10.1016/j.anbehav.2006.11.023) DOI

Gayon J.2000History of the concept of allometry. Am. Zool. 40, 748–758 (doi:10.1668/0003-1569(2000)040[0748:HOTCOA]2.0.CO;2) DOI

Holub D., et al. 2008Vigilance/sustained attention abnormalities in subjects with latent toxoplasmosis. Psychiatrie 12(Suppl. 3), 21–25

Huxley J., Teissier G.1936Terminology of relative growth. Nature 137, 780–781 (doi:10.1038/137780b0) DOI

Levinton J. S.1988Genetics, paleontology, and macroevolution Cambridge, UK: Cambridge University Press

Lindová J., Hrušková M., Pivoňková V., Kuběna A., Flegr J.2008Digit ratio (2D : 4D) and Cattell's personality traits. Eur. J. Personality 22, 347–356 (doi:10.1002/per.664) DOI

Malas M. A., Dogan S., Evcil E. H., Desdicioglu K.2006Fetal development of the hand, digits and digit ratio (2D : 4D). Early Hum. Dev. 82, 469–475 (doi:10.1016/j.earlhumdev.2005.12.002) PubMed DOI

Manning J. T.2002Digit ratio: a pointer to fertility, behavior and health NJ, USA: Rutgers University Press

Manning J. T.2008The finger book. London, UK: Faber & Faber

Manning J. T., Wilson D. J., Lewis-Jones D. I.1998The ratio of 2nd to 4th digit length: a predictor of sperm numbers and concentration to testosterone, luteinizing hormone and oestrogen. Hum. Reprod. 13, 3000–3004 (doi:10.1093/humrep/13.11.3000) PubMed DOI

McIntyre M. H., Ellison P. T., Lieberman D. E., Demerath E., Towne B.2005The development of sex differences in digital formula from infancy in the Fels Longitudinal Study. Proc. R. Soc. B 272, 1473–1479 (doi:10.1098/rspb.2005.3100) PubMed DOI PMC

McIntyre M. H., Cohn B. A., Ellison P. T.2006Sex dimorphism in digital formulae of children. Am. J. Phys. Anthropol. 129, 143–150 (doi:10.1002/ajpa.20240) PubMed DOI

Poehlman E. T., Toth M. J.1995Mathematical ratios lead to spurious conclusions regarding age- and sex-related differences in resting metabolic rate. Am. J. Clin. Nutr. 61, 482–485 PubMed

Puts D. A., Gaulin S. J. C., Sporter R. J., McBurney D. H.2004Sex hormones and finger length: what does 2D : 4D indicate? Evol. Hum. Behav. 25, 182–199 (doi:10.1016/j.evolhumbehav.2004.03.005) DOI

Puts D. A., McDaniel M. A., Jordan C. L., Breedlove S. M.2008Spatial ability and prenatal androgens: meta-analyses of congenital adrenal hyperplasia and digit ratio (2D : 4D) studies. Arch. Sex. Behav. 37, 100–111 (doi:10.1007/s10508-007-9271-3) PubMed DOI PMC

Shingleton A. W., Frankino W. A., Flatt T., Nijhout H. F., Elen D. J.2007Size and shape: the developmental regulation of static allometry in insects. BioEssays 29, 536–548 (doi:10.1002/bies.20584) PubMed DOI

SPSS Inc. 1993–2006 SPSS, version 16.0. www.spss.com .

StatSoft Inc. 2001. Statistica, version 6.0. www.statsoft.com .

Talarovičová A., Kršková A., Blažeková J.2009Testosterone enhancement during pregnancy influences the 2D : 4D ratio and open field motor activity of rat siblings in adulthood. Horm. Behav. 55, 235–239 (doi:10.1016/j.yhbeh.2008.10.010) PubMed DOI

Voracek M., Loibl L. M.2009Scientometric analysis and bibliography of digit ratio (2D : 4D) research, 1998–2008. Psychol. Rep. 104, 922–956 PubMed

Voracek M., Manning J. T., Dressler S. G.2007Repeatability and interobserver error of digit ratio (2D : 4D) measurements made by experts. Am. J. Hum. Biol. 19, 142–146 (doi:10.1002/ajhb.20581) PubMed DOI

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