Comparing indirect methods of digit ratio (2D:4D) measurement

The ratio of the lengths of the second and fourth finger (2D:4D) has been proposed to index prenatal exposure to androgens. Different methods have been utilized to measure digit ratio, however, their measurement precision and economy have not been systematically compared yet. Using different indirect methods (plastic ruler, caliper, computer software), three independent raters measured finger lengths of 60 participants. Generally, measurement precision (intraclass correlation coefficient, technical error of measurement, and relative technical error of measurement) was acceptable for each method. However, precision estimates were highest for the computer software, indicating excellent measurement precision. Estimates for the caliper method were somewhat lower followed by ruler which had the lowest precision. On the contrary, the software-based measurements took somewhat longer to complete than the other methods. Nonetheless, we would favor the use of these tools in digit ratio research because of their relative superior reliability which could be crucial when associations with other variables are expected to be low to moderate or sample size is limited. Software offers several promising opportunities that may contribute to an accurate identification of the proximal finger crease (e.g., zooming, adjusting contrast, etc.).