Finger length ratio (2D:4D) is a sexually dimorphic trait. The ratio of second digit
(index finger) to fourth digit (ring finger) is smaller for males than females in humans,
mice, and baboons (Brown et al., 2002b; Manning, 2002a; Manning et al., 2000; McFadden
and Bracht, 2003; McFadden and Shubel, 2002; Peters et al., 2002).
Sexual dimorphism in digit ratio is seen by the age of two
and is thought to be stable thereafter, even through puberty (Manning et al., 1998; Brown et
al., 2002b; Manning, 2002a). Index to ring finger, or 2D:4D is the most strongly dimorphic
of all human digit ratio combinations (McFadden and Shubel, 2002).
Variation in finger length ratio is thought to reflect the influence of prenatal testosterone
during development (Manning, 2002a; Manning et al., 2003a). While this correlation is
somewhat conjectural, two non-exclusive causes have been posited. The first is that
common genes (Hoxa and Hoxd) underlie development of both fingers and gonads (Kondo
et al., 1997; Peichel et al., 1997). The second is that allelic variation in androgen receptor
sensitivity influences digit ratio. More masculine finger ratios are associated with androgen
receptor alleles with fewer CAG base-pair microsatellite repeats in the terminal domain
(Manning et al., 2003a). Increased number of such repeats produces receptors with lower
androgen sensitivity (Chamberlain et al., 1994; Kazemi-Esfarjani et al., 1995).
More evidence for a relationship between androgen concentration during development
and finger ratio comes from children with congenital adrenal hyperplasia (CAH). CAH
causes the individual to be exposed to increased levels of androgens from early in gestation
to the early neonatal period (Berenbaum and Reinisch, 1997). Both males and females with
CAH, and therefore high developmental androgens, exhibit more masculine finger length
ratios than controls (Brown et al., 2002c; Okten et al., 2002), but not necessarily when
measured on the left hand (Buck et al., 2003).
Digit ratio has consistently been shown to be more dimorphic on the right hand than on
the left in humans (Manning et al., 1998; McFadden and Shubel, 2002; Williams et al.,
2000), baboons (McFadden and Bracht, 2003) mice (Brown et al., 2002b), and finches
(Burley and Foster, 2004). Several authors have suggested that androgenization affects the
right hand more than the left (McFadden and Shubel, 2002;Williams et al., 2000; Brown et