"The second-to-fourth digit ratio (2D:4D) has been proposed as a biomarker reflecting prenatal androgen effects (PAE),
such that individuals with lower ratios have experienced higher PAE than those with higher ratios.
2D:4D has been correlated with a number of sex-linked traits in humans
such as aggression, promiscuity, and competitiveness.
In addition, polygynous societies reportedly have lower 2D:4D
(higher PAE) than more monogamous populations.
This evidence suggests that PAE may be implicated in the
development of sexually selected behaviors in humans"