2D:4D digit ratio
The best, non-invasive, marker of prenatal hormone exposure is the digit ratio
of the second and fourth finger lengths (2D:4D ratio), a known sexually dimorphic measure (males showing lower ratios than females). Patients with androgen over-exposure (such as in congenital adrenal hyperplasia
) show lower 2D:4D ratios,
providing evidence linking prenatal androgen exposure as key to this feature. XY individuals with androgen insensitivity syndrome
due to a dysfunctional gene for the androgen receptor present as women and have feminine digit ratios, as would be predicted if androgenic hormones affect digit ratios. This finding also demonstrates that the sex difference in digit ratio
is unrelated to the Y chromosome per se.
Additionally, the 2D:4D ratio has been shown to be affected by variation in the androgen receptor gene in men.
The ratio of testosterone to estrogen in amniotic fluid has also been found to be negatively correlated with the 2D:4D ratio.
Independent studies indicate that homosexual women have masculinized (lower) digit ratios,
and homosexual men show either hyper-masculinized or feminized ratios. These findings reinforce the prenatal androgen model - abnormal prenatal hormone exposure is related to the development of human homosexuality.