Comparison of External Measurement Methods of the 2D:4D Digit Ratio

External methods for measuring digit length differ in the hand surface and anatomical landmarks used, but the comparability of resulting 2D:4D ratios is unclear. This study compared 2D:4D ratios obtained using four external measurement methods and evaluated between-method differences, correlations, and reliability.

Digit lengths were measured in 231 Turkish university students (103 males, 128 females). Measurements were obtained directly with calipers from the dorsal and ventral surfaces and indirectly from photocopies and photographs. The 2D:4D ratio was calculated as second-digit length divided by fourth-digit length. Right and left hands were analyzed separately for effects of measurement method, sex, and hand side.

Males had lower 2D:4D ratios than females for all methods and hand sides except direct dorsal measurement of the left hand. Mean ratios were lowest with direct dorsal measurement and highest with direct ventral measurement. Differences among methods were significant for both hands (p < 0.001). Between-method correlations ranged from r = 0.529-0.771 for the right hand and r = 0.440-0.776 for the left hand (all p < 0.001). Intraobserver and interobserver reliability in the pilot sample was high (ICC = 0.85-0.95).

The 2D:4D ratio varies systematically according to the external measurement method used. Measurement protocols should therefore be reported explicitly, and absolute 2D:4D values obtained using different methods should be compared cautiously. These findings highlight the importance of standardizing measurement methods in 2D:4D research and suggest that the detection of sex differences may vary according to both the measurement method and hand side.