Digit ratio (2D:4D) and its behavioral correlates in infant rhesus macaques (Macaca mulatta)

2D:4D ratios are typically lower in adult human males than females and are linked to numerous sex-differentiated behaviors. 2D:4D ratios are considered an indicator of prenatal androgen exposure; if so, children's 2D:4D ratios would arguably be even better indicators of prenatal androgen exposure since children have experienced fewer postnatal influences than adults. However, sex differences in 2D:4D ratios as well as associations between 2D:4D ratios and sex-typed behaviors in children have been inconsistent. Several studies also report sex differences and behavioral correlates of 2D:4D ratios in adult non-human primates, but little is known about 2D:4D ratios in infant non-human primates. This study examined sex differences in 2D:4D ratios over the first month of life, and associations with behavioral outcomes at 12–24 weeks of age, in N = 304 infant rhesus macaques. An increase in 2D:4D ratios over the first month of life was found, as well as associations with aggression and play behaviors, but no sex differences in 2D:4D ratios were observed. These results highlight the need for future developmental studies of 2D:4D ratios in order to determine not only their stability and predictive value, but also to discern the mechanism connecting prenatal androgen exposure, 2D:4D ratios, and behavioral outcomes.     

宁夏回、汉族医学生指长比与握力关系

目的:分析宁夏回、汉族医学生示指与环指指长比(2D:4D)与握力的关系。方法:依体质测量方法,比较628例宁夏回、汉族医学生(回族277例,汉族351例)左手、右手2D:4D及握力各均值的差异性,并分析2D:4D与握力的相关性。结果:宁夏回、汉族医学生2D:4D各均值在同一民族不同性别均为男性显著低于女性,在同一性别不同民族均无显著性差异;宁夏回、汉族医学生握力各均值在同一民族不同性别均为男性显著高于女性,在同一性别不同民族均无显著性差异;宁夏回、汉族男性及回族女性医学生双手2D:4D与握力均无明显相关性,宁夏汉族女医学生双手2D:4D与握力呈显著负相关。结论:指长比及握力存在明显的性别差异;2D:4D与宁夏汉族女性医学生握力有关,可能是个体某些运动潜能的重要生物学参考指标之一。     

Digit ratio (2D:4D) as an indicator of body size,testosterone concentration and number of children in human males

Objectives: The 2nd to 4th digit ratio (2D:4D) is thought to reflect exposure to androgens during foetal development. This study examined the relationship between low (more masculine) and high (more feminine) 2D:4D and body size at different stages of the life course, adult testosterone levels and number of children among males.

Methods: Five hundred and fifty-eight men from rural Poland at the Mogielica Human Ecology Study Site participated in this study. Life history data and anthropometric measurements were collected. Salivary morning and evening testosterone levels among 110 men from the same population were measured.

Results: Low 2D:4D was related to higher birth weight (p?=?0.04), higher birth length (p?=?0.01), higher body mass during childhood and adolescence (p?=?0.01), higher BMI (borderline significance, p?=?0.06), higher number of children among fathers (p?=?0.04) and higher testosterone levels during adulthood (p?=?0.04).

Conclusions: This study shows, for the first time in a single population, that digit ratio is related to sub-adult body size at different stages of the life course, adult testosterone levels and number of children. The observed results suggest that digit ratio might be a valuable predictor of male body size and reproductive characteristics.     

Second to Fourth Digit Ratio(2D:4D) in North-West Indians: Sexual Dimorphism

The index (2D) to ring finger (4D) ratio less than one (2D:4D < 1) has been considered as a “male finger pattern” and 2D:4D more than or equal to one (2D:4D≥1) has been considered as a “female finger pattern”. The present study was conducted on 100 young adults (m=56, f=44) from northwest population of India to find out the sexual dimorphism and right-left asymmetry in WAD ratio by three different methods namely visual, direct and scan methods. Visual method showed that significantly more number of males were having Type 3 hand i.e. index finger smaller than ring finger compared to females who more often had Type 1 hand i.e. index finger longer than ring finger (p< 0.001). The mean 2D:4D ratios in males were observed to be 0.97 by direct method and 0.96 by scan method. In females the ratio was 1.00 and 0.99 by direct and scan methods respectively; sex difference being statistically significant (p <0.001).Mean values by direct method were slightly higher than scan method but the difference was statistically insignificant. In both sexes there was no significant right left asymmetry.     

Digit ratios (2D:4D), postnatal testosterone and eye contact in toddlers

Previous research has shown an association between eye contact and prenatal testosterone measured in amniocenteses samples. The purpose of this study was to test the association between eye contact and prenatal androgen action measured via second to fourth digit ratios (2D:4D ratios), and to explore the relationship between eye contact and postnatal testosterone levels. Participants included 72 children, between the ages of 18 and 24 months, and their parents. Salivary testosterone levels were obtained when children were 3-months old. At 18-months, 2D:4D ratios were measured and parent–child dyads participated in an 8-min play session that was recorded and later coded for duration and frequency of eye contact. Results indicated that larger 2D:4D ratios (indicative of lower androgen levels) significantly predicted longer duration and more frequency of eye contact, while postnatal testosterone levels were unrelated to eye contact. These novel findings suggest prenatal androgens may influence the emergence of social development.     

A comparison of self-measured and researcher-measured digit ratio (2D:4D)

Abstract

The largest investigation of digit ratio (2D:4D), the BBC Internet Study, reported on finger lengths measured by participants themselves, yet data validating this technique are scarce and the reliability has been questioned. The current study aimed to calculate reliability and repeatability statistics for self-measured 2D:4D and to examine the correlations with researcher-measures. One hundred and seventy-eight undergraduate psychology students attending a practical class self-measured their finger lengths with rulers; a researcher using digital Vernier calipers measured the second and fourth fingers of a random sub-sample (n?=?97). Reliability and repeatability of self-measured 2D:4D were high, as were correlations with researcher-measurements. In each case, lower values were observed for the right–left difference in 2D:4D (D_[R-L]). Self-measured L2D:4D and M2D:4D were significantly higher than the equivalent researcher measurements, suggesting that direct comparison could be problematic. Self-measurements and directly made researcher-measurements of 2D:4D are strongly correlated, though self-measured D_[R-L] is unreliable.     

Endurance running and digit ratio (2D:4D): implications for fetal testosterone effects on running speed and vascular health.

There is anatomical and physiological evidence that endurance running (ER), i.e., running one or more kilometers using aerobic metabolism, originated early in the evolution of Homo, and the consequences of early selection for ER may be important in modern Homo. Here we examine ER performance in competitive ER. ER is sex dependent such that men tend to run faster than women, and the influence of sex on ER suggests that it may be modified by testosterone (T). It is shown that a putative proxy for prenatal T, the ratio of the length of the 2nd and 4th digits (2D:4D), is correlated with ER. Thus performance in training for ER was associated with high prenatal T, as measured by low 2D:4D, in both men and women. In cross-country races from 1 to 4 miles, 2D:4D explained about 25% of the variance in both male and female ER. Therefore, speed in ER was dependent on a proxy for prenatal T. 2D:4D correlates with performance in sport and exercises, which test a mix of strength and fitness, but the associations are in general quite weak with 2D:4D accounting for less than 10% of the variance in performance. Our finding that 2D:4D explains about 25% of the variance in ER suggests that prenatal T is important in determining efficiency in aerobic exercise. Early populations of Homo may have been strongly selected for ER and high prenatal T. The implications of this for patterns of predisposition to cardiovascular disease in modern Homo are discussed.     

Comparing indirect methods of digit ratio (2D:4D) measurement

The ratio of the lengths of the second and fourth finger (2D:4D) has been proposed to index prenatal exposure to androgens. Different methods have been utilized to measure digit ratio, however, their measurement precision and economy have not been systematically compared yet. Using different indirect methods (plastic ruler, caliper, computer software), three independent raters measured finger lengths of 60 participants. Generally, measurement precision (intraclass correlation coefficient, technical error of measurement, and relative technical error of measurement) was acceptable for each method. However, precision estimates were highest for the computer software, indicating excellent measurement precision. Estimates for the caliper method were somewhat lower followed by ruler which had the lowest precision. On the contrary, the software‐based measurements took somewhat longer to complete than the other methods. Nonetheless, we would favor the use of these tools in digit ratio research because of their relative superior reliability which could be crucial when associations with other variables are expected to be low to moderate or sample size is limited. Software offers several promising opportunities that may contribute to an accurate identification of the proximal finger crease (e.g., zooming, adjusting contrast, etc.). Am. J. Hum. Biol., 2009. © 2008 Wiley‐Liss, Inc.     

Low digit ratio (2D:4D) is associated with delayed menarche

The prenatal environment of the human fetus is now known to have lasting physiological and behavioral effects into childhood and adulthood. The objective of this study was to examine the relationship between early testosterone exposure, as measured by the ratio of the length of the 2nd and 4th digits (2D:4D ratio), and age of menarche. Retrospective, self‐report data on menarcheal age were obtained from 206 healthy female college students, as well as finger length ratio data from both hands. Low right‐hand 2D:4D ratios (but not left‐hand) were associated with delayed menarche. The results provide evidence for an androgen‐delayed menarche link and suggest that this relationship can be continuous in nature and not always categorical as occurs in congenital adrenal hyperplasia, hyperandrogenism, or polycystic ovary syndrome. Am. J. Hum. Biol., 2008. © 2008 Wiley‐Liss, Inc.