Саусақ ұзындығының қатынасы және сандардағы басты цифрлар жиілігі туралы зерттеулер

Ratio of lengths of fingers
the observation about the frequencies of leading digits of numbers

The digit ratio is the ratio taken of the lengths of different digits or fingers on a hand. In modern sciences, the 2D:4D ratio has become the most commonly studied digit ratio and is calculated by dividing the length of the index finger of a given hand by the length of the ring finger of the same hand. Some studies claimed that males have a lower 2D:4D ratio than females, on average; however, there are studies showed contradictory findings. Although studies have claimed to show a correlation between the 2D:4D digit ratio and various physical and behavioral traits, these studies have drawn criticism for irreproducible or contradictory findings, Measurement of the digits on the dorsal ("back side") hand, from the tip of the finger to the proximal phalange bone protrusion (which occurs when digits are bent at 90 degrees to the palm), has recently also gained acceptance. A study has shown that, compared to the palmar digit ratio, the dorsal digit ratio is a better indicator of bone digit ratio. Other digit ratios are also similarly calculated in the same hand. It has been suggested that because academics have accepted a variety of techniques and equipment (such as calipers, radiography and photocopy), researchers deliberately record multiple measurements and report only those which have significant findings, a form of reporting bias.

The ratio of two digits in particular – the palmar 2nd (index finger) and 4th (ring finger) – is supposedly affected by fetal exposure to hormones, in particular to testosterone, and other androgens; this 2D:4D ratio could therefore be considered a crude measure for prenatal androgen exposure, with lower 2D:4D ratios pointing to higher prenatal androgen exposure. There are also studies that suggest that the palmar 2D:4D ratio is influenced by prenatal estrogen exposure, and that it thus correlates negatively not with prenatal testosterone alone, but rather with the prenatal testosterone to estrogen ratio (T:O). However, various studies have challenged the aforementioned findings. A meta analysis of 54 studies (with 8077 participants) found there was no correlation between digit ratio and adult testosterone levels (44 studies), testosterone change (6 studies) and prenatal testosterone (10 studies). The researchers conclude that due to the link being unsupported, it is concerningly common that papers use 2D:4D as a proxy for testosterone. Another paper has also found no relationship between 2D:4D ratios and testosterone or estradiol levels, or even the testosterone to estradiol ratio. The largest study on 2D:4D as of 2018 failed to find evidence for 2D:4D being a marker for prenatal androgen exposure, and one frequently cited paper on the topic

A study measuring the 2D:4D of deceased fetuses found the ratio to be sexually dimorphic, being lower in men than in women. However, when comparing the extent of the sexual dimorphism to adults and children, the researchers found the extent was similar to children but lower than adults. This led the authors to suggest that the digit ratio is also affected by the postnatal development process. Similarly, a study of people from Poland found a statistically significant correlation between digit ratio and age groups (divided by children, young adults and adults) for both males and females in the left hand, but only for women with the right hand. In palmar digit ratios, strong sexual dimorphism occurs in those of digit 2. In dorsal digit ratios, in contrast, strong sexual dimorphism occurs in those of digit 5, with women having shorter fifth digits on average. Moreover, compared to palmar digit ratio, dorsal digit ratio is a better indicator of bone digit ratio. with the statistically significant sex difference in a sample of 201 men and 109 women established by 1930, after which time the sex difference appears to have been largely forgotten or ignored. In 1983, Glenn Wilson of King's College London published a study examining the correlation between assertiveness in women and their digit ratio, which found that women with a lower 2D:4D ratio reported greater assertiveness. This was the first study to examine the correlation between digit ratio and a psychological trait within members of the same sex. Wilson proposed that skeletal structure and personality were simultaneously affected by sex hormone levels in utero. and further developed the idea that the index was a marker of prenatal sex hormones. Since then, research on the topic has burgeoned around the world. Since 2006, 2D:4D digit ratios have been used alongside other methods in an attempt 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", which may be the basis for the sex difference in digit ratios and/or any putative hormonal influence on the ratios. A 2011 paper by Zhengui Zheng and Martin J. Cohn reports "the 2D:4D ratio in mice is controlled by the balance of androgen to estrogen signaling during a narrow window of digit development". During this period if the fetus is exposed to androgens, the exact level of which is thought to be sexually dimorphic, the growth rate of the 4th digit is increased, as can be seen by analyzing the 2D:4D ratio of opposite sex dizygotic twins, where the female twin is exposed to excess androgens from her brother in utero, and thus has a significantly lower 2D:4D ratio. Importantly, there has been no reliable correlation between the sex hormone levels of an adult and the individual's 2D:4D, which implies that it is strictly the exposure in utero that causes this phenomenon. Marta Kowal alongside other researchers in 2020 published a paper providing more evidence that there is no relationship between adult testosterone levels and 2D:4D ratio, highlighting that testosterone levels are correlated with other factors (such as smoking, body weight, and diet) that would impact the relationship between 2D:4D and an adult's testosterone level. Subsequent researchers failed to replicate their findings, and criticised Manning and Fink for publishing papers that fail under scrutiny despite the urgent need for high quality science during the pandemic that informs medical decisions. In a response, John T. Manning criticized the researchers for not controlling for ethnicity.

The sex difference in 2D:4D is present before birth in humans. The ratio of testosterone to estradiol measured in 33 amniocentesis samples correlated with the child's subsequent 2D:4D ratio. Several studies present evidence that digit ratios are heritable. In a non clinical sample of women, digit ratio correlated with anogenital distance in the expected direction. In other words, women with a greater anogenital distance, indicating greater prenatal androgen exposure, had a smaller digit ratio.

Women with congenital adrenal hyperplasia (CAH), which results in elevated androgen levels before birth, have lower, more masculinized 2D:4D on average. Other possible physiological effects include an enlarged clitoris and shallow vagina. Males with CAH have smaller (more masculine) digit ratios than control males, These measures indicate that males with CAH are exposed to greater prenatal concentrations of total androgens than are control males. A greater digit ratio occurs for men with Klinefelter's syndrome, who have reduced testosterone secretion throughout life compared to control males, than in their fathers or control males. Men with genes that produce androgen receptors that are less sensitive to testosterone (because they have more CAG repeats) have greater, i. e. more feminine, digit ratios. There are reports of a failure to replicate this finding. However, men carrying an androgen receptor with more CAG repeats compensate for the less sensitive receptor by secreting more testosterone, probably as a result of reduced negative feedback on gonadotropins. Thus, it is not clear that 2D:4D would be expected to correlate with CAG repeats, even if it accurately reflects prenatal androgen. XY individuals with androgen insensitivity syndrome (AIS) due to a dysfunctional gene for the androgen receptor present as women and have feminine digit ratios on average, as would be predicted if androgenic hormones affect digit ratios. This finding also demonstrates that the sex difference in digit ratios is unrelated to the Y chromosome per se.

It is not clear why digit ratio might be influenced by prenatal hormones, with studies supporting the notion there is no relationship between digit ratios and prenatal hormones. have been implicated in affecting these multiple traits (pleiotropy). Direct effects of sex hormones on bone growth might be responsible, either by regulation of Hox genes in digit development or independently of such genes. Likewise, it is unclear why digit ratio on the right hand should be more responsive than that on the left hand, as is indicated by the greater sex difference on the right than the left. However, because no right–left difference has been found in sexual dimorphism of bone digit ratios (2D:4D) and dorsal digit ratios and because differential placing of flexion creases contributes to sex differences in palmar digit ratio, This variation is far larger than the differences between sexes; in Manning's words, "There's more difference between a Pole and a Finn, than a man and a woman." The standard deviations associated with each given 2D:4D mean are considerable. For example, the ratio for Han children (0.954±0.032) allows for a ratio as low as 0.922, while the ratio for Jamaican children (0.935±0.035) allows for a ratio as high as 0.970. Thus, some ethnic groups' confidence intervals overlap. 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. In 2007, Manning et al. also found that mean 2D:4D varied across ethnic groups with higher ratios for Whites, non Chinese Asians, and Middle Easterners and lower ratios in Chinese and Black samples. Two studies explored the question of whether geographical differences in 2D:4D ratios were caused by gene pool differences or whether some environmental variable associated with latitude might be involved (e. g., exposure to sunlight or different day length patterns). The conclusions were that geographical differences in 2D:4D ratio were caused by genetic pool differences, not by geographical latitude. Consanguineous parentage (inbreeding) has been found to lower the 2D:4D ratio in offspring, which may account for some of the geographical and ethnic variation in 2D:4D ratios, as consanguinity rates depend on, among others, religion, culture, and geography.

There is some evidence that 2D:4D ratio may also be indicative for human development and growth. 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. Kumar et al. have reported that a similar effect of hand preference on digit lengths and digit ratios occurs oppositely among children and adults. Moreover, studies about 2D:4D correlations with face shape suggest that testosterone exposure early in life may set some constraints for subsequent development. Prenatal sex steroid ratios (in terms of 2D:4D) and actual chromosomal sex dimorphism were found to operate differently on human faces, but affect male and female face shape by similar patterns. Fink et al. (2004) found that men with low (indicating high testosterone) and women with high (indicating high estrogen) 2D:4D ratios express greater levels of facial symmetry.

The digit ratio has received attention within mainstream media because of supposed correlations with health and behavior, Critics have found research to be lacking moderate to strong statistical significance, consistently failing to be replicated, and useless as a proxy variable. Although research into digit ratios appears to find statistical significance, there has been criticism of researchers for failing to account for a majority of the variation within a dataset, with one of the most highly publicized papers on the topic failing to account for 96% of variation. Furthermore, skepticism has also been attributed to an abundance of research lacking a cause and effect relationship between digit ratio and supposed traits, research that often lacks any theoretical motivation. This is especially problematic in cases where the relationship is likely to be weak, such as prenatal testosterone. and physical, medical or behavioral conditions. Researchers have also pointed out that research into the digit ratio is illustrative of the ongoing replication crisis. but have often found contradictory results, or failed closer scrutiny. Nancy Burley's research group has demonstrated sexual dimorphism in zebra finches, and found a correlation between digit ratio in females and the strength of their preference for sexually selected traits in males. Alžbeta Talarovičová and collaborators found in rats that elevated testosterone during the prenatal period can influence 4D length, the 2D:4D ratio, and open field motor activity. Peter L. Hurd, Theodore Garland Jr., and their students have examined hindlimb 2D:4D in lines of mice selectively bred for high voluntary wheel running behavior (see experimental evolution). These high runner mice exhibit increased 2D:4D. This apparent "feminization" is opposite to the relation seen between 2D:4D and physical fitness in human beings, and is difficult to reconcile with the idea that 2D:4D is a clear proxy for prenatal androgen exposure in mice. The authors suggest that 2D:4D may more accurately reflect effect of glucocorticoids or other factors that regulate any of various genes. In pheasants, the ratio of the 2nd to 4th digit of the foot has been shown to be influenced by manipulations of testosterone in the egg. Studies in mice indicate that prenatal androgen acts primarily by promoting growth of the fourth digit.