A biomarker that has generated considerable interest is digit ratio, the ratio of the second (index) finger to the fourth (ring) finger, 2D:4D. Men have a longer fourth digit relative to the second digit than do women, and the ratio is lower in boys and men than in girls and women.1 A reduced 2D:4D ratio is assumed to indicate reduced prenatal androgen exposure or sensitivity and high prenatal testosterone levels.

Vivekananda and colleagues2 describe how they used 2D:4D as a surrogate for prenatal testosterone levels, hypothesising that reduced …