The ratio of 2nd to 4th digit length: a predictor of sperm numbers and concentrations of testosterone, luteinizing hormone and oestrogen

The differentiation of the urinogenital system and the appendicularskeleton in vertebrates is under the control of Hox genes. The commoncontrol of digit and gonad differentiation raises the possibility thatpatterns of digit formation may relate to spermatogenesis and hormonalconcentrations. This work was concerned with the ratio between the lengthof the 2nd and 4th digit (2D:4D) in humans. We showed that (i) 2D:4D inright and left hands has a sexually dimorphic pattern; in males mean 2D:4D= 0.98, i.e. the 4th digit tended to be longer than the 2nd and in femalesmean 2D:4D = 1.00, i.e. the 2nd and 4th digits tended to be of equallength. The dimorphism is present from at least age 2 years and 2D:4D isprobably established in utero; (ii) high 2D:4D ratio in right hands wasassociated with germ cell failure in men (P = 0.04); (iii) sperm number wasnegatively related to 2D:4D in the right hand (P = 0.004); (iv) in mentestosterone concentrations were negatively related to right hand 2D:4D andin women and men LH (right hand), oestrogen (right and left hands) andprolactin (right hand) concentrations were positively correlated with 2D:4Dratio and (v) 2D:4D ratio in right hands remained positively related toluteinizing hormone and oestrogen after controlling for sex, age, heightand weight.