Diminutive digits discern delicate details: fingertip size and the sex difference in tactile spatial acuity

Abstract

We have observed that passive tactile spatial acuity, the ability to resolve the spatial structure of surfaces pressed upon the skin, differs subtly but consistently between the sexes, with women able to perceive finer surface detail than men. Eschewing complex central explanations, we hypothesized that this sex difference in somatosensory perception might result from simple physical differences between the fingers of women and men. To investigate, we tested 50 women and 50 men on a tactile grating orientation task and measured the surface area of the participants' index fingertips. In subsets of participants, we additionally measured finger skin compliance and optically imaged the fingerprint microstructure to count sweat pores. We show here that tactile perception improves with decreasing finger size, and that this correlation fully explains the better perception of women, who on average have smaller fingers than men. Indeed, when sex and finger size are both considered in statistical analyses, only finger size predicts tactile acuity. Thus, a man and a woman with fingers of equal size will, on average, enjoy equal tactile acuity. We further show that sweat pores, and presumably the Merkel receptors beneath them, are packed more densely in smaller fingers.

Figure 1.

Perceptual data and finger size. a, Two-interval forced-choice GOT. An adaptive procedure estimated the width of grooves whose orientation the participant could distinguish with 76% probability (GOT threshold). Hand drawing retrieved from www.myctrring.com with permission. b, GOT thresholds by sex (means ± 1 SE). Lower thresholds correspond to better acuity. c, Index finger distal phalanx surface area by sex (means ± 1 SE). d, Scatterplot of threshold versus distal phalanx surface area, with female (red) and male (blue) regression lines. Women: red □; men: blue ○.

Figure 2.

Multivariate Bayesian analysis. Scatterplots and best-fit curves are shown for four models. The data points (participants' GOT thresholds) are identical in the four plots; the models differ in how they consider the data to have been generated. a, Null model: the data derive from a single Gaussian population distribution. b, Sex model: female and male data (left–right offset for clarity) originate from separate (red and blue) Gaussian populations. c, Finger size model: the data derive from a linear trend on fingertip area. d, Finger-size-and-sex model. Women: red □; men: blue ○. Bayes factors (BF) are likelihoods relative to the null model.

Figure 3.

Finger surface microstructure. a, Scans from index fingers of a woman (left) and man (right) traced (yellow) for area measurement (scale bar, 1 cm). b, Portions of 2400 dpi scans taken from boxed regions in a after staining (scale bar, 1 mm). Sweat pores (punctate stain) are more densely distributed in the smaller finger. c, Within-ridge (yellow arrow) and between-ridge (green arrow) pore-to-pore measurements were taken from 15 participants. Dots: sweat pores; lines: finger print grooves. d, Pore-to-pore within-ridge distance (top), between-ridge distance (middle), and sweat pore density (lower) versus fingertip surface area. Women: red □; men: blue ○. *correlation p < 0.05.

Figure 4.

Proposed receptor anatomy and effect of finger size. a, Schematic cross-section through finger. An SA1 axon (red) branches to Merkel cell clusters (M) encircling sweat ducts (SD) beneath papillary ridges (stippled). RA1 axons (uncolored) innervate Meissner corpuscles (Me). b, We propose that SA1 receptive fields (ellipses) are more densely packed in smaller fingers (left).