Digit ratio (2D:4D) and physical fitness in males and females: Evidence for effects of prenatal androgens on sexually selected traits

It has been suggested that male achievement in sports and athletics is correlated with a putative measure of prenatal testosterone the 2nd to 4th digit ratio (2D:4D). It is not known whether this association also extends to females, or whether the association results from an effect of testosterone on behavior (such as exercise frequency) or on physical fitness. Here, we report for the first time data from two studies which consider associations between 2D:4D and physical fitness in females in addition to males: Study I--in a sample of teenage boys (n = 114) and girls (n = 175), their 'physical education grade' was negatively associated with 2D:4D of the right hand (boys), and right and left hand (girls), and Study II-among a sample of young men (n = 102) and women (n = 77), a composite measure of physical fitness was negatively related to right hand 2D:4D in men and left hand 2D:4D in women. We conclude that 2D:4D is negatively related to physical fitness in both men and women. In Study II, there was evidence that the relationship between physical fitness and 2D:4D in men was mediated through an association with exercise frequency. Thus, 2D:4D in males may be a negative correlate of frequent exercise which then relates to achievement in sports and athletics.     

A longitudinal study of digit ratio (2D:4D) and other finger ratios in Jamaican children

It has been hypothesised that the ratio between the length of the 2nd and 4th digits (2D:4D) is a correlate of prenatal sex steroids, and this relationship is strongest for the right hand. Furthermore, it has been suggested that 2D:4D is sexually dimorphic, the dimorphism is determined early, and 2D:4D among children is stable with growth. Here, we present the first longitudinal study of right and left hand 2D:4D. Our sample was 108 (54 males) Jamaican children. The first measurements were made in 1998 when mean age was 9.68 +/- 1.39 years, and a second set of measurements were made in 2002. We found that: (i) there was a small increase in 2D:4D with age which was lowest in the right hand; (ii) 2D:4D was sexually dimorphic, the means for males and females differed in the same direction in the 1998 and 2002 samples, and the sex difference was significant in the 1998 but not in the 2002 sample; (iii) the correlation between the 1998 and 2002 measurements of 2D:4D was high, indicating that rank order of the ratio was stable across year groups; and (iv) the rate of change in 2D:4D did not differ significantly across year groups. We conclude that 2D:4D increases slightly with age in children with the effect less marked for the right hand (i.e. the hand which is likely to show the strongest association with prenatal steroids), 2D:4D is sexually dimorphic from an early age, and the rank order of 2D:4D is stable in children. We discuss the implications of our findings for the status of 2D:4D as a correlate of prenatal sex steroids. The patterns of change in other finger ratios are also considered.     

Digit ratio (2D:4D) and rowing ergometer performance in males and females

Fetal and adult testosterone may be vital in the establishment and maintenance of sex-dependent abilities associated with male physical competitiveness. It has been shown that digit ratio (2D:4D) is negatively associated with prenatal testosterone, and it is also negatively associated with ability in sports such as football, skiing, middle distance running, and endurance running, which are dependent upon an efficient cardiovascular system. The relationship between digit ratio and sports requiring high power (physical strength) output in addition to well-developed cardiovascular systems has not been defined. This study investigated this association in male and female young adult rowers. Participants (77 male and 70 female) were student rowers encompassing a range of abilities from the University of Cambridge. Bilateral digit measurements were taken blind from each subject using Mitutoyo vernier calipers. Rowing performance over 2,000 m was assessed using the Concept 2 rowing ergometer. Significant negative correlations were observed between 2,000 m ergometer performance and male digit ratios, which persisted following adjustment for rowing experience and height. However, no such significant association was found in females despite a comparable sample size. Our data indicate that digit ratio is a predictor of ability in rowing, a sport which requires both cardiovascular efficiency and high power output, in males but not females. This in turn suggests that fetal testosterone exposure has long-term effects on traits associated with physical power in males but not females, suggesting a sex-difference in the capacity to respond to such exposures.     

Digit ratio (2D:4D) and behavioral differences between inbred mouse strains

Digit ratio (2D:4D) is a trait, which is sexually differentiated in a variety of species. In humans, males typically have shorter second digits (2Ds) (index fingers) compared to fourth digits (4Ds) (ring fingers) whereas females' fingers are more equal in length. Smaller, more masculine, digit ratios are thought to be associated with higher prenatal testosterone levels, greater sensitivity to prenatal androgens or both. Men with more masculine digit ratios have shown increased ability, achievement and speed in sports and tend to report that they are more physically aggressive. Previous research has shown the same sexually differentiated pattern in the hind paws of laboratory mice as in human hands, males have lower 2D:4D than females. We measured hind paw digit ratio in mice of eight inbred strains. These measurements were made while blind to strain, sex and whether the paw was from the left or right side. We found large differences in digit ratio between the strains and suggest that inbred mice are a promising system for investigating the correlation between digit ratio and behavioral traits.     

Digit ratio (2D:4D) and performance in male surfers

The relative length of the second and fourth digits (2D:4D) is thought to be negatively related to prenatal testosterone and positively related to prenatal estrogen. Low 2D:4D has been linked to various measures of performance in a range of sports (e.g., soccer, rugby). In this study, we consider the relationship between 2D:4D and performance among male surfers. Our sample comprised 46 competitors in the Men's 5-star Professional World Qualifying Series surfing competition in Newquay, United Kingdom, in 2009. Three experienced surfing coaches rated the participants for overall surfing ability. The coach's ratings were significantly correlated with one another and an overall measure of surfing performance was obtained by calculating the mean of the 3 ratings. In addition, the final placing of the Newquay competition was used as an additional performance measure. Mean 2D:4D (SD) was as follows: right 0.994 (0.023) and left 0.976 (0.028). We found that right 2D:4D (but not left 2D:4D or right-left 2D:4D) was significantly negatively correlated with coaches' ratings (r(s) = 0.58) and the competition result (r(s) = 0.30). It appears that in line with other sports that low right 2D:4D (high prenatal testosterone and low prenatal estrogen) correlates to high surfing ability in men.     

Digit ratio (2D:4D) predicts facial, but not voice or body odour, attractiveness in men

There is growing evidence that human second-to-fourth digit ratio (or 2D:4D) is related to facial features involved in attractiveness, mediated by in utero hormonal effects. The present study extends the investigation to other phenotypic, hormone-related determinants of human attractiveness: voice and body odour. Pictures of faces with a neutral expression, recordings of voices pronouncing vowels and axillary odour samples captured on cotton pads worn for 24 h were provided by 49 adult male donors. These stimuli were rated on attractiveness and masculinity scales by two groups of 49 and 35 females, approximately half of these in each sample using hormonal contraception. Multivariate regression analyses showed that males' lower (more masculine) right 2D:4D and lower right-minus-left 2D:4D (Dr-l) were associated with a more attractive (and in some cases more symmetrical), but not more masculine, face. However, 2D:4D and Dr-l did not predict voice and body odour masculinity or attractiveness. The results were interpreted in terms of differential effects of prenatal and circulating testosterone, male facial shape being supposedly more dependent on foetal levels (reflected by 2D:4D ratio), whereas body odour and vocal characteristics could be more dependent on variation in adult circulating testosterone levels.     

2D:4D ratios in the first 2 years of life: Stability and relation to testosterone exposure and sensitivity

The relative lengths of the 2nd and 4th digits (2D:4D) may provide an easily measurable and stable anthropometric index of prenatal androgen exposure, but no study has examined the development of 2D:4D in infancy and the potential impact of neonatal testosterone levels. We collected 2D:4D ratios from 364 children between 0 and 2 years of age. Saliva samples were collected from 236 of these children 3 months after birth and analyzed for testosterone. In addition, 259 children provided DNA samples which were genotyped for the CAG repeat polymorphism in the androgen receptor. There was substantial variability across age in 2D:4D. Sex differences were small compared to adults and did not consistently reach statistical significance. This suggests that 2D:4D may not function well as a proxy measure of prenatal testosterone exposure in infancy. In addition, the interaction of salivary T and CAG repeats predicted right hand digit ratio at 12 months and left hand digit ratio at 12 months and 24 months in males. The interaction of salivary testosterone and CAG repeat length also predicted change in left hand 2D:4D from 2 weeks to 12 months in males. This suggests that 2D:4D in adults may reflect, in part, neonatal testosterone exposure. No significant relationships were observed within females. No significant relationships were observed when salivary testosterone and CAG repeats were examined independent of each other. Results have important implications for the design and interpretation of studies which use 2D:4D as a proxy measure of prenatal testosterone exposure.     

Digit ratio (2D:4D) moderates the impact of sexual cues on men's decisions in ultimatum games

Three experimental studies demonstrate that 'sex-related cues' impact human decision-making in ultimatum games. In the ultimatum game, two individuals divide a sum of money. The proposer offers a portion of the money to the other player, the responder. If the responder accepts the offer, the money is distributed in agreement with the proposer's offer. If the responder rejects the offer, neither player receives anything. Our studies show that exposure to pictures of sexy women or lingerie increases the likelihood of accepting unfair offers. Digit ratios of responders are reliably associated with their behaviour: males with lower digit ratios are more likely to reject an unfair split in neutral contexts, but more likely to accept unfair offers in sex-related contexts.     

指长比与乳腺癌的相关性研究

本文研究了宁夏汉族女性256例(正常对照:128例,乳腺癌患者:128例)左右手指长比(2D∶3D、2D∶4D、2D∶5D、3D∶4D、3D∶5D、4D∶5D),比较其均值的差异性;分析了指长比与年龄间的关系。结果表明:1)宁夏汉族正常女性与乳腺癌患者组指长比均值呈现2D∶3D<2D∶4D<3D∶4D<2D∶5D<4D∶5D<3D∶5D的趋势;2)乳腺癌患者组指长比均值均高于正常对照组,2D∶3D(P<0.05)、2D∶4D(P<0.01)、2D∶5D(左手P4D的比例高于对照组;3)乳腺癌患者组指长比均值与发病年龄呈高度负相关(P<0.001)。     

Brief communication: Familial resemblance in digit ratio (2D:4D)

Familial resemblance in the second‐to‐fourth digit ratio (2D:4D), a proxy for prenatal androgen action, was studied in 1,260 individuals from 235 Austrian families. In agreement with findings from twin studies of 2D:4D, heritability estimates based on parent–child and full‐sib dyad similarity indicated substantial genetic contributions to trait expression (57% for right hand, 48% for left hand 2D:4D). Because twin studies have found nonadditive genetic as well as shared environmental effects on 2D:4D to be negligible or nil, these family‐based estimates in all likelihood reflect the narrow‐sense (additive genetic) heritability of the trait. Directional (right‐minus‐left) asymmetry in 2D:4D was only weakly heritable (6%). The pattern of same‐sex and different‐sex parent–child and full‐sib correlations yielded no evidence for X‐linked inheritance. This is surprising, considering evidence for associations of male 2D:4D with sensitivity to testosterone (functional variants of the X‐linked androgen receptor gene). 2D:4D was particularly strongly heritable through male lines (father–son and brother–brother correlations), thus raising the possibility that Y‐linked genes (such as the sex‐determining region SRY) might influence 2D:4D expression. Am J Phys Anthropol, 2009. © 2009 Wiley‐Liss, Inc.     

Digit ratio (2D:4D) and handgrip strength in subjects of Han ethnicity: Impact of sex and age

The ratio of the length of the second finger to the fourth finger (2D:4D) is considered to be a putative proxy of prenatal exposure to testosterone, and has been increasingly used as a promising tool to evaluate the impact of prenatal androgenization in humans in such traits as physical performance. In this study, for the first time, we present 2D:4D data on adult participants of Han ethnicity. We consider the sexual dimorphism of 2D:4D and handgrip strength, and also report the relationship between 2D:4D and handgrip strength of males and females. The sample consisted of 54 males and 55 females recruited from a remote village in the Qinling Mountains, China. We found sexual dimorphism of both 2D:4D and handgrip strength, i.e., males had lower 2D:4D and right‐left 2D:4D than females and greater handgrip strength than females. There was a sex‐specific correlation between 2D:4D and handgrip strength, i.e., 2D:4D in the right hand was negatively correlated with handgrip strength in males but not in females. This relationship may be driven by sexual selection operating on fetal programming. Am J Phys Anthropol 149:266–271, 2012. © 2012 Wiley Periodicals, Inc.     

Digit Ratio (2D:4D) and Handgrip Strength in Hani Ethnicity

Introduction

The ratio of the length of the second finger to the fourth finger (2D:4D) in humans is considered as a putative marker of prenatal exposure to testosterone, and has been progressively adopted as one useful tool to evaluate the effect of prenatal hormones in some traits such as physical ability. Handgrip strength is one authentic measure of physical ability and is generally used on the anthropological research within an evolutionary viewpoint.

Methods

Here we present the first evidence on 2D:4D and handgrip strength on adult participants of Hani ethnicity and explore the relationship between digit ratio (2D:4D) and handgrip strength. We examined 2D:4D and handgrip strength of 80 males and 60 females at Bubeng village, in the Yunnan province of China.

Results

The mean 2D:4D in females was higher than that in males for each hand. Females showed significantly higher 2D:4D than males in the right hand rather than in the left hand. Males displayed significantly higher handgrip strength than females for both hands. Handgrip strength decreased with age for both sexes. A significant negative correlation between 2D:4D and handgrip strength was found in the right hand of males.

Conclusion

The relationship between 2D:4D and handgrip strength may be attributed to evolutionary drive of sexual selection operating on fetal programming.     

2D:4D Digit Ratio Predicts Delay of Gratification in Preschoolers

We replicate the Stanford marshmallow experiment with a sample of 141 preschoolers and find a correlation between lack of self-control and 2D:4D digit ratio. Children with low 2D:4D digit ratio are less likely to delay gratification. Low 2D:4D digit ratio may indicate high fetal testosterone. If this hypothesis is true, our finding means high fetal testosterone children are less likely to delay gratification.     

Meta-Analysis Shows Associations Of Digit Ratio (2D:4D) And Transgender Identity Are Small At Best

Abbreviations: 2D:4D = digit ratio; CI = confidence interval; F = female; FtM = female-to-male transgender; M = male; MtF = male-to-female transgender; TGI = transgender identity     

Digit ratio (2D:4D) in Lithuania once and now: testing for sex differences, relations with eye and hair color, and a possible secular change

The second-to-fourth digit ratio (2D:4D) is a sexually dimorphic somatic trait and has been proposed as a biomarker for the organizational, i.e., permanent, effects of prenatal testosterone on the human brain. Accordingly, recent research has related 2D:4D to a variety of sex-dependent, hormonally influenced traits and phenotypes. The geographical variation in typical 2D:4D is marked and presently poorly understood. This study presents the first investigation into the 2D:4D ratio in a Baltic country. A contemporary sample of 109 Lithuanian men and women was compared with data from a historical sample of 100 Lithuanian men and women, collected and published in the 1880s and rediscovered only now. The findings included the following lines of evidence: (i) seen in an international perspective, the average 2D:4D in Lithuania is low; (ii) there was a sex difference in 2D:4D in the expected direction in both samples; (iii) a previously adduced hypothesis of an association of lighter eye and hair color with higher, i.e., more feminized, 2D:4D received no support in both samples; and (iv) the average 2D:4D in the contemporary sample was higher than in the historical sample. In view of a hypothesized increase in 2D:4D in modern populations, owing to increased environmental levels of endocrine disruptors such as xenoestrogens, this latter finding appears to be of particular notice. However, because finger-length measurement methods differed across the samples, it cannot be safely ruled out that the apparent time trend in Lithuanian 2D:4D in truth is an artifact. The puzzling geographical pattern seen in the 2D:4D ratio and the question of possible time trends therein deserve further investigations.     

Bonobos have a more human-like second-to-fourth finger length ratio (2D:4D) than chimpanzees: a hypothesized indication of lower prenatal androgens

The ratio of the second-to-fourth finger lengths (2D:4D) has been proposed as an indicator of prenatal sex differentiation. However, 2D:4D has not been studied in the closest living human relatives, chimpanzees (Pan troglodytes) and bonobos (Pan paniscus). We report the results from 79 chimpanzees and 39 bonobos of both sexes, including infants, juveniles, and adults. We observed the expected sex difference in 2D:4D, and substantially higher, more human-like, 2D:4D in bonobos than chimpanzees. Previous research indicates that sex differences in 2D:4D result from differences in prenatal sex hormone levels. We hypothesize that the species difference in 2D:4D between bonobos and chimpanzees suggests a possible role for early exposure to sex hormones in the development of behavioral differences between the two species.     

Born to be wild: Second-to-fourth digit length ratio and risk preferences

The second-to-fourth digit length ratio of an individual’s hand (digit ratio) is a putative biomarker for prenatal exposure to testosterone. We examine the hypothesized negative association between the digit ratio and the preference for risk taking within a large U.S. population survey. Our statistical framework provides a cardinal proxy for the true digit ratio based on ordinal digit ratio measurements and accounts for measurement error under the assumptions of Gaussianity and time-invariant true digit ratios. Our empirical findings support the hypothesis and suggest a meaningful biological basis for risk preferences.