Previous research has suggested that offspring sex ratio may be influenced by the actions of prenatal sex steroids, principally androgens. The relative length of the second (index finger) to the fourth digit (ring finger) has been reported to be a proxy to prenatal testosterone levels. This trait is sexually dimorphic, such that males display a significantly lower 2D:4D ratio (indicating higher testosterone exposure), and this dimorphism appears robust across different populations. We suggest that digit ratio (2D:4D) may form a useful marker to help explain variation in sex ratio and sociosexuality.