Digit ratio (2D:4D) and handedness: A meta-analysis of the available literature

ABSTRACT

The Geschwind-Behan-Galaburda and sexual differentiation models predict an association between elevated foetal androgen exposure and left-handedness whereas the callosal hypothesis predicts the opposite. We present a meta-analysis of correlations between handedness and digit ratio (2D:4D), a putative marker of prenatal testosterone. Left-handedness predicted low (male-typical) right-hand digit ratio (R2D:4D), high (female-typical) left-hand digit ratio (L2D:4D), and low R2D:4D-L2D:4D directional asymmetry (D_[R−L]). Effect sizes were extremely small and not moderated by sex or method of measuring handedness or 2D:4D. The same general pattern was observed after excluding the very large study (110,329 males, 90,412 females) of Manning and Peters ([2009]. Digit ratio (2D:4D) and hand preference for writing in the BBC Internet Study. Laterality: Asymmetries of Body, Brain and Cognition, 14(5), 528–540. doi:10.1080/13576500802637872); however, no significant effects for R2D:4D were observed once these samples were removed. The results do not confirm any theory linking prenatal androgens with handedness, so we speculate they instead reflect the mechanical action of writing causing subtle changes in the musculature and/or fat pads of the fingers. Gripping a pen/pencil might cause an increase in 2D relative to 4D (and/or decrease in 4D relative to 2D) resulting in higher ratios on the writing-hand; furthermore, this could differ between left- and right-handers due to writing in the left-to-right direction (as in English) having asymmetrical effects depending on which hand is used.

KEYWORDS:

• 2D:4D
• digit ratio
• handedness
• meta-analysis
• testosterone

Acknowledgements

The authors would like to extend their gratitude to each of the researchers who responded to our request to identify published and unpublished datasets relevant to this review. In particular, we would like to thank the following for their willingness to share their data and/or calculate effect sizes for inclusion in our meta-analysis: Shuntaro Ando, Bart Boets, Lori Brotto, Carolyn Choudhary, Colton Christian, Senol Dane, Murat Diramali, Lee Ellis, Marco Fabbri, Bernhard Fink, Andrzej Galbarczyk, Grazyna Jasienska, Leonid Kalichman, Yoshiki Kaneoke, Kiyoto Kasai, Tomoki Kiyono, Magdalena Klimek, Rengin Kosif, Sanjay Kumar, John Manning, Robert Matchock, Lynnea Myers, Emma Nelson, Ronan O’Carroll, Sebastian Ocklenburg, Marietta Papadatou-Pastou, Michael Peters, Simon Sanwald, Oliver Schultheiss, Sandip Shah, Azim Shariff, Cornelia Sindermann, Briton Trabert, Jennifer Wernicke, and Yin Xu. Our inclusion of data from the Western Australian Pregnancy Cohort (Raine) Study was made possible by a British Academy/Leverhulme Trust Small Research Grant (SRG1819\190620) awarded to Gareth Richards, and was approved under the project “A meta-analysis of digit ratio (2D:4D) and handedness” (reference number HOR0686); we also extend our gratitude to the Raine team in general and our collaborators in particular (Martha Hickey, Lauren Lawson, Melissa Licari, Murray Maybery, I. Chris McManus, Diana Tan, and Andrew Whitehouse) for allowing us to incorporate these data here. Additionally, we thank Dr Patrick Rosenkranz for his help translating and interpreting the German language publication of Rösler (1957), and Dr Tommy van Steen and Dr Steven Watson for helpful statistical advice.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

Data and R code are available on the Open Science Framework (doi:10.17605/OSF.IO/Z6CXY).

Correction Statement

This article has been republished with minor changes. These changes do not impact the academic content of the article.

Additional information

Funding

This work was supported by the British Academy: [Grant Number SRG1819\190620].