Digit ratio

A lower ratio has been associated with higher androgen exposure, which would be the physiological norm for males but may also occur in some exceptional circumstances in females.

[3] Studies in this field have drawn criticism over questionable statistical significance and difficulties in reproducing their findings[4][5][6] as well as lack of high quality research protocols.

The digit length is typically measured on the palmar (ventral, "palm-side") hand, from the midpoint of the bottom crease to the tip of the finger.

2D:4D digit ratios have been used alongside other methods in an attempt to understand Palaeolithic hand stencils found in prehistoric European and Indonesian cave painting.

A 2009 study in Biology Letters argues: "Sexual differences in 2D:4D are mainly caused by the shift along the common allometric line with non-zero intercept, which means 2D:4D necessarily decreases with increasing finger length, and the fact that men have longer fingers than women",[28] which the authors claim may be the basis for the sex differences in 2D:4D and/or any putative hormonal influence on the ratios.

[39] A 2011 paper by Zhengui Zheng and Martin J. Cohn reports that "the 2D:4D ratio in mice is controlled by the balance of androgen to estrogen signaling during a narrow window of digit development".

[30] The formation of the digits in humans, in utero, is thought to occur by 13 weeks, and the bone-to-bone ratio is consistent from this point into an individual's adulthood.

[41] However, an attempt to replicate these findings with a larger sample of dizygotic twins (867 individuals) found no differences in the variance or co-variance of same-sex and opposite-sex pairings to support the theory, though it did confirm female 2D:4D to be significantly higher than male as expected.

[42] Researchers have raised concerns that, although the general trend points towards a correlation between digit ratio and early androgen exposure, many results have not been statistically significant.

One study of 66 children that attempted to replicate the findings of a frequently cited paper on the topic[37] found no association between prenatal testosterone and estrogen levels and 2D:4D in childhood.

[46] Women with congenital adrenal hyperplasia (CAH), who have elevated androgen levels before birth, have lower (more masculine) 2D:4D on average[31][32][47] along with other possible physiological effects such as an enlarged clitoris and shallow vagina.

A greater (more feminine) digit ratio occurs for men with Klinefelter's syndrome, who have reduced testosterone secretion throughout life compared to their fathers or to controls.

[52] Men with genes that produce androgen receptors that are less sensitive to testosterone (because they have more CAG repeats) have greater digit ratios, though there have also been reports of failure in replicating this finding.

[60] A 2008 study by Lu et al. found that the mean values of 2D:4D of the Hui and the Han in Ningxia were lower than those in European countries like Britain.

[65] The 2D:4D ratio has been associated with a variety of traits including academic ability, personality as well as physical attributes such as health outcomes.

Lower (masculinized) 2D:4D ratios have been associated with higher mathematical ability as well as personality traits such as aggressiveness (in males), assertiveness (in females) and reduced agreeableness.

[71] Prenatal androgen exposure has been implicated as a determinant in left-handedness, though contradictory and inconclusive findings have been reported concerning an association between 2D:4D and left-hand dominance.

[72][73][74] Kumar et al. have reported that hand preference is most strongly associated with 3D:4D digit ratios and that the effect is reversed between children and adults.

[78] Ronalds et al. (2002) showed that men who had an above average placental weight and a shorter neonatal crown-heel length had higher 2D:4D ratios in adult life.

A cross sectional study" followed similar research methodologies adopted by others in the field and came to conclude that low digit ratio is associated with good luck.