Finger, sex, and side differences in fingertip size and lack of association with image-based digit ratio (2D:4D) measurements.

Different methods for measuring the second-to-fourth digit ratio (2D:4D), a putative marker for prenatal androgen exposure, may lead to varying values of 2D:4D. Specifically, it has been hypothesized that fingertip fat pads may be differentially deformed when taking palm images (photocopies or flatbed scans), but not when fingers are measured directly, thus yielding lower 2D:4D values from image-based measurements rather than direct ones. A measurement protocol for fingertip size is unavailable, as are data on finger and sex differences in this trait. Introducing such a protocol, this study found very large finger differences in adult fingertip size (largest for 4D, smaller for 3D and 5D, smallest for 2D), large side differences (larger in the left than in the right hand of right-handed subjects), but only small and not significant sex differences. Fingertip size was unrelated to image-based 2D:4D measurements, suggesting that fingertip size and the extent of finger and sex differences therein are unlikely to explain discrepant 2D:4D values obtained from image-based versus direct measurements.