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Lookup NU author(s): Dr Gareth RichardsORCiD
This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License (CC BY-NC 4.0).
The Geschwind-Behan-Galaburda (GBG) theory and the sexual differentiation model both predict elevated foetal androgen exposure to be associated with left-handedness whereas the callosal hypothesis predicts that it will be associated with right-handedness. As it is unethical to intentionally manipulate prenatal hormones for research purposes in humans, or to measure the foetal circulation directly, digit ratio (2D:4D) is commonly used as a proxy. This is the ratio of length between the second and fourth fingers of one or other hand and has been hypothesised to indicate individual differences in exposure to prenatal sex hormones. The current article presents a meta-analysis of research examining associations between 2D:4D and handedness and includes 374 effect sizes from 60 samples. Left-handedness was associated with low (i.e., ‘male-typical’) right hand digit ratio (R2D:4D) (k=107, n=226,335; r = -0.014, p < 0.001), high (female-typical) left hand digit ratio (L2D:4D) (k=98, n=222,265; r = 0.023, p < 0.0001), and low R2D:4D relative to L2D:4D (D[R-L]) (k=87, n=220,344; r = -0.044, p < 0.0001). These effects were not moderated by sex (male or female), type of handedness measure (dichotomous or continuous/ordinal) or method of measuring digit ratio (direct, indirect, or self-measurement), and no overall correlation was observed between handedness and the average digit ratio of both hands (M2D:4D) (k=82, n=220,083; r = 0.004, p = 0.118). These effects appear fairly robust, as we observed the same general pattern of findings after excluding a study (Manning & Peters, 2009) with a much larger sample than any other (110,329 males, 90,412 females); however, it is noted that no significant effects relating to R2D:4D were observed once these samples were removed. Taken together these results do not conform to the predictions of any of the main theories linking prenatal androgen exposure with handedness. We therefore speculate that they may instead be explained by the mechanical action of writing causing subtle changes in the musculature and/or fat pads of the fingers. More specifically, gripping a pen/pencil might cause a slight transient or even permanent increase in 2D relative to 4D (and/or decrease in 4D relative to 2D) that results in higher ratios on the writing- as opposed to non-writing hand; furthermore, this process could differ in magnitude between left- and right-handers due to the action of writing in the left-to-right direction (as in English, the language in which most of this research has been conducted) having asymmetrical effects depending on which hand is used.
Author(s): Richards G, Medland SE, Beaton AA
Publication type: Article
Publication status: Published
Journal: Laterality: Asymmetries of Brain, Behaviour, and Cognition
Online publication date: 31/01/2021
Acceptance date: 07/12/2020
Date deposited: 07/12/2020
ISSN (print): 1357-605X
ISSN (electronic): 1464-0678
Publisher: Taylor & Francis
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