A comparison of finger 2D:4D by self-report direct measurement and experimenter measurement from photocopy: methodological issues

Digit Ratio (2D:4D; Right-Left 2D:4D) and Multiple Phenotypes for Same-Sex Attraction: The BBC Internet Study Revisited

Same-sex attraction may be linked to low prenatal androgen (in men) and high prenatal androgen (in women). Digit ratio (2D:4D) is thought to be a negative correlate of prenatal androgen and right-left 2D:4D (Dr-l) to reflect lateralized differences in sensitivity to prenatal androgen. Lower 2D:4D has been reported for lesbians compared to heterosexuals, but links to high 2D:4D in gay men are less clear. The largest study thus far (the BBC Internet study) found no significant difference between the 2D:4D of lesbians and heterosexual women but a higher 2D:4D in gay men compared to heterosexual men. Here we consider the possibility that low and high prenatal androgen is associated with same-sex attraction in men (n = 108,779) and women (n = 87,742), resulting in more than two phenotypes. We examined the associations between 2D:4D, Dr-l, and same-sex attraction scores in the BBC Internet study. In contrast to the earlier report, which considered sexual orientation in categories, there were positive linear associations in men (right and left 2D:4D, but not Dr-l) and negative linear associations in women (right 2D:4D and Dr-l, but not left 2D:4D). There were no curvilinear relationships for right and left 2D:4D. However, Dr-l showed a U-shaped association with same-sex attraction in men. Thus, (1) high prenatal androgen may be implicated in female homosexuality, while both low and high prenatal androgen may be implicated in male homosexuality, and (2) large side differences in sensitivity to androgen may be associated with elevated same-sex attraction in men.

Use of crowdsourced images for determining 2D:4D and relationship to pro-environmental variables

The primary purpose of this study was to examine whether 2D:4D ratios (a putative measure of prenatal androgen exposure) could be determined using participant-submitted hand images. The secondary purpose was to examine whether 2D:4D ratio was associated with pro-environmental behaviors, attitudes, and empathy, given the recent literature linking sex to environmental attitudes and actions. Participants (N = 1065) were asked via an online survey to submit a clear photograph of their right hand, palm side up. Participants also completed a questionnaire to assess (a) demographics, (b) dispositional empathy, and (c) environmental attitudes and behavior. A 2D:4D ratio was calculated for each participant, and the quality of each image was classified as poor, moderate, or good. We then examined the reliability of the 2D:4D image measurements, and the relationship between 2D:4D and our environmental measures. 2D:4D ratios fell somewhat outside of previously reported ranges, but the measurements did show acceptable intra-rater consistency. Although we did not find a sex difference in 2D:4D, we did find a sex by ratio interaction for both empathy and the number of pro-environmental behaviors in which individuals had engaged. Specifically, as 2D:4D ratio increased, males reported lower empathy and less engagement in pro-environmental behaviors, whereas females reported more engagement in pro-environmental behaviors (but no differences in empathy). These findings were contrary to expectations, as we anticipated that greater digit ratios (i.e., feminized) would be associated with greater empathy and pro-environmental behaviors. Overall, the findings of this study present a preliminary examination of the utility of measuring digit ratio with online samples. Furthermore, our results provide information regarding the complex relationship between sex and pro-environmental behaviors.

Digit ratio (2D:4D) and maternal testosterone-to-estradiol ratio measured in early pregnancy

The ratio of index to ring finger (2D:4D) has been hypothesised to indicate prenatal androgen exposure, yet evidence for its validity is lacking. We report the first pre-registered study to investigate mothers' early pregnancy sex hormone concentrations in relation to their children's digit ratios measured at 18-22-month follow-up. Although the testosterone (T) to estradiol (E) ratio correlated negatively with right hand digit ratio (R2D:4D) and directional asymmetry (right-minus-left) in digit ratio (D_[R-L]), neither effect remained statistically significant once demographic and obstetric covariates were controlled for. Nevertheless, the multivariate level of analysis did reveal that T correlated positively with left hand digit ratio (L2D:4D) and negatively with D_[R-L]. However, the first of these effects is in the opposite direction to that predicted by theory. Taken together, the results of our study suggest research with larger samples is required to determine whether digit ratios are valid proxies for maternal sex hormone exposure.

Can Digit Ratio and Gender Identity Predict Preferences for Consumption Options With a Distinct Gender Image?

This study investigated whether individuals' preferences for masculine (vs. feminine) consumption options could be predicted by a biological sex cue (the 2D:4D digit ratio; a biomarker linked to prenatal testosterone exposure), and a psychological gender cue (self-perceived gender identity). Chinese participants (N = 216) indicated their preferences for a series of binary options that differed in their perceived gender image (e.g., romantic comedy vs. action thriller; pop music vs. hard rock), with one of the options evaluated as relatively more feminine and the other viewed as comparably more masculine. Participants also self-reported their gender identity and the length of their index and ring fingers, which was used to calculate their 2D:4D digit ratios. A low (male-typical) digit ratio and a masculine gender identity were both associated with more masculine preferences, regardless of participants' biological sex. However, a low digit ratio predicted preferences for masculine consumption options only in female participants with a masculine gender identity, but not in those with a feminine gender identity. These findings add to the literature on whether and when biological sex cues and psychological gender cues can predict preferences for options with a distinct gender image and suggest that the connection between these cues is more complex in women than in men.

An anthropometric study of sexual orientation and gender identity in Thailand

The biodevelopment of psychological sex differentiation is putatively reflected in several anthropometrics. We examined eight anthropometrics in 1404 Thai participants varying in sex, sexual orientation, and gender identity/expression: heterosexual men and women, gay men, lesbian women, bisexual women, sao praphet song (transgender birth-assigned males), toms (transgender birth-assigned females), and dees (birth-assigned females attracted to toms). Exploratory factor analyses indicated the biomarkers should be analyzed independently. Using regressions, in birth-assigned males, less male-typical second-to-fourth digit ratios in the left hand were associated with sexual orientation towards men regardless of gender identity/expression, whereas shorter height and long-bone growth in the arms and legs were more evident among sao praphet song-who are both sexually oriented towards men and markedly feminine. In birth-assigned females, there were no clear sexual orientation effects, but there were possible gender-related effects. Groups of individuals who tend to be more masculine (i.e., toms, lesbians) showed more male-typical patterns on weight and leg length than some groups of individuals who tend to be less masculine (i.e., heterosexual women, dees). Thus, it appears the various anthropometrics inform separate biodevelopmental processes that differentially relate to sexual orientation and gender identity/expression depending on the measure in question as well as birth-assigned sex.

No evidence for a difference in 2D:4D ratio between youth with elevated prenatal androgen exposure due to congenital adrenal hyperplasia and controls

The second-to-fourth digit ratio (2D:4D) has been associated with sexual dimorphism, with a lower 2D:4D in males. A large body of research has relied on the 2D:4D as a proxy for prenatal androgen exposure, and includes reports of relationships between 2D:4D and a wide range of human traits. Here, we examine the validity of the 2D:4D proxy by studying the association between 2D:4D and classical Congenital Adrenal Hyperplasia (CAH) due to 21-hydroxylase deficiency, a condition characterized by excessive prenatal exposure to androgens during most of the gestational period. To this end, we retrospectively examine 513 serial radiographs of the left hand obtained clinically in 90 youth with classical CAH (45 female) and 70 control youth (31 female). Replicating previous reports, we observe associations of the 2D:4D with sex (lower 2D:4D in males) and age (increase of 2D:4D through development). However, we find no evidence for differences in 2D:4D between CAH and controls (full sample: β = -0.001 (-0.008, 0.006); females: β = -0.004 [-0.015, 0.007]; males: β = 0.001, [-0.008, 0.011]). Although our findings do not rule out a small association between the 2D:4D and CAH, they cast doubt on the usefulness of the 2D:4D as a biomarker for prenatal androgen exposure in behavioral research.

Association patterns between 2D:4D ratio and age at menarche: the impact of methodology

The ratio of index finger to ring finger length (2D:4D) is a sexually dimorphic feature and widely used as an indicator of prenatal androgen-estrogen exposure. Several studies have investigated the relationship between the 2D:4D ratio and the onset of the first menstrual period (menarche) in women.

The present study tested the association patterns between 2D:4D ratio and age at menarche. Furthermore, the impact of methods of measuring finger lengths was considered.

Two samples were used to conduct the study. One sample consisted of 110 women who self-measured their finger lengths. The finger length measurements in the second sample (88 women) were taken from one trained observer using the caliper-based technique. Age at menarche was determined using a retrospective method.

Women from the first sample reported an average age for the onset of the first menstrual bleeding at 12.9 (SD = 1.4) years of age. There was no significant association between the left and right 2D:4D ratio and the age at menarche. The second sample showed a mean age at menarche at 12.8 (SD = 1.3) years of age. Here, the right hand 2D:4D ratio and age at menarche were significantly correlated (p<0.001).

A more feminine 2D:4D ratio was significantly associated with an earlier menarcheal age only in the sample with direct finger length measurements by a trained investigator. The sample using self-measurements yielded no significant associations between menarcheal age and 2D:4D. The use of a reliable and well-founded methodology is essential for obtaining meaningful results.

Do sex hormones at birth predict later-life economic preferences? Evidence from a pregnancy birth cohort study

Economic preferences may be shaped by exposure to sex hormones around birth. Prior studies of economic preferences and numerous other phenotypic characteristics use digit ratios (2D : 4D), a purported proxy for prenatal testosterone exposure, whose validity has recently been questioned. We use direct measures of neonatal sex hormones (testosterone and oestrogen), measured from umbilical cord blood (n = 200) to investigate their association with later-life economic preferences (risk preferences, competitiveness, time preferences and social preferences) in an Australian cohort (Raine Study Gen2). We find no significant associations between testosterone at birth and preferences, except for competitiveness, where the effect runs opposite to the expected direction. Point estimates are between 0.05-0.09 percentage points (pp) and 0.003-0.14 s.d. We similarly find no significant associations between 2D : 4D and preferences (n = 533, point estimates 0.003-0.02 pp and 0.001-0.06 s.d.). Our sample size allows detecting effects larger than 0.11 pp or 0.22 s.d. for testosterone at birth, and 0.07 pp or 0.14 s.d. for 2D : 4D (α = 0.05 and power = 0.90). Equivalence tests show that most effects are unlikely to be larger than these bounds. Our results suggest a reinterpretation of prior findings relating 2D : 4D to economic preferences, and highlight the importance of future large-sample studies that permit detection of small effects.

Digit ratio, a proposed marker of the prenatal hormone environment, is not associated with prenatal sex steroids, anogenital distance, or gender-typed play behavior in preschool age children

Prenatal hormones have been proposed as key factors impacting child development as well as long-term health and disease. Digit ratio (the ratio of the lengths of the second to fourth digits; 2D:4D) has been proposed as a sexually dimorphic, noninvasive marker of prenatal androgen exposure that can be reliably measured in children and adults. To date, few longitudinal pregnancy cohort studies have examined childhood digit ratio in relation to other relevant measures including prenatal hormones and androgen-sensitive outcomes. To augment the current literature on this topic, we measured right-hand digit ratio in 4-year-old children participating in The Infant Development and the Environment Study, a multicenter longitudinal cohort study that has been following mother-child dyads since the first trimester of pregnancy (n = 321). We assessed sex differences in digit ratio and fit multivariable linear regression models to examine digit ratio in relation to: (1) child sex; (2) maternal sex steroid hormone concentrations in early pregnancy; (3) newborn anogenital distance, another proposed measure of sensitivity to prenatal androgens; and (4) gender-typical play behavior as measured by the Preschool Activities Inventory (PSAI) at age 4. We observed no sex difference in digit ratio; the mean 2D:4D was 0.97 ± 0.05 mm in both sexes. Furthermore, digit ratio was not associated with maternal sex steroid concentrations in early pregnancy, anogenital distance in either sex, or PSAI scores in either sex in covariate-adjusted models. In conclusion, we observed no evidence that early childhood digit ratio was associated with child sex or hormone-sensitive measures in this cohort.

Digit ratio (2D:4D) and transgender identity: new original data and a meta-analysis

Previously reported associations between second-to-fourth digit length ratio (2D:4D), a proxy for prenatal androgen load, and transgender identity have been inconsistent. The objectives of the present study were to provide additional original data and an updated meta-analysis concerning this association. In a study of 464 participants, we compared the 2D:4D of transgender individuals with age- and sex-matched controls. Patients were recruited at a specialized psychiatrist's medical office, whereas controls were hired via flyers, advertisements, and as convenience sample. A random-effects meta-analysis of the literature (17 samples, n = 3674) also quantifies the overall magnitude of the difference in 2D:4D between transgender individuals and controls. In our study providing new original data, we found a significantly higher (i.e. feminized) left-hand 2D:4D in the male-to-female transgender (MtF) identity [mean age: 32.3 (18; 61)] than in the male control group [mean age: 34.5 (18; 65)] with a Cohen's d = 0.271. Concordantly, the meta-analytic results suggest a significant difference in 2D:4D among MtF individuals compared to male controls [g = 0.153; 95% CI (0.063; 0.243)], which was even more pronounced when individuals had been diagnosed by a clinician instead of self-identified as transgender [g = 0.193; 95% CI (0.086; 0.300)]. In both studies, no significant results were revealed for female-to-male transgender individuals [mean age: 26.1 (18; 53)] versus female controls [mean age: 27.2 (18; 55)]. This original investigation and the updated meta-analysis clarify the association between transgender identity and 2D:4D indicating the influence of prenatal androgen on the development of gender identity in subjects born as males.

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

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.

Revisiting the relationships of 2D:4D with androgen receptor (AR) gene and current testosterone levels: Replication study and meta-analyses

The relationships of digit ratio (2D:4D) with the length of AR (CAG)n, and testosterone levels from saliva and blood have been extensively debated over the years. This research including three studies further clarifies such controversies. To do so, we re-examined the relationships between the length of AR (CAG)n, 2D:4D, and current testosterone levels, through replication study and meta-analysis for each study. The results indicate: (a) the length of AR (CAG)n is not significantly associated with 2D:4D; (b) current testosterone levels are not significantly associated with the ratio; and (c) the length is not significantly associated with testosterone levels. Thus, AR (CAG)n and current testosterone levels are not significantly related to 2D:4D at individual level.

Direct versus indirect measurement of digit ratio: New data from Austria and a critical consideration of clarity of report in 2D:4D studies

BACKGROUND

Digit ratio (2D:4D), has been the subject of many studies. However, the best method of measuring digit length, either directly (d2D:4D) or indirectly (i2D:4D), is controversial. In many reports i2D:4D has been found to show a directional effect such that d2D:4D > i2D:4D. The exception is three studies from one group of researchers in Austria. Thus, it is unclear whether this effect is nation- or lab-specific.

AIMS

To examine evidence for effects of direct versus indirect measurements of mean 2D:4D in Austrians.

STUDY DESIGN

We compared 2D:4D based on direct and indirect measurements of digit lengths of Austrians.

SUBJECTS

There were 80 participants, 21 adults and 59 children.

OUTCOME MEASURES

2D:4D of right and left hands, measured directly (from the palm) and indirectly (from hand scans).

RESULTS

Repeatability was high for both d2D:4D and i2D:4D, with the latter slightly higher than the former. d2D:4D and i2D:4D correlated strongly and the sex difference in 2D:4D (males < females) was greater for i2D:4D. With regard to directional differences, we found d2D:4D > i2D:4D for both right (d = -0.53) and left hands (d = -0.80).

CONCLUSION

There was no evidence of an "Austrian" effect on direct versus indirect measurements of 2D:4D, i.e. mean d2D:4D was greater than i2D:4D. We discuss our findings in the light of issues regarding "clarity of report" from earlier Austrian studies.

Does the index-to-ring finger length ratio (2D:4D) differ in amyotrophic lateral sclerosis (ALS)? Results from an international online case-control study

OBJECTIVES

The ratio of the length of the index finger (2D) to the ring finger (4D) (2D:4D) has been reported to be lower (ie, 2D<4D) in people with amyotrophic lateral sclerosis (ALS) than non-ALS controls. This has led to suggestions that exposure to increased prenatal testosterone, which also lowers this ratio, could be a risk factor for ALS. In an attempt to test this hypothesis, we examined 2D:4Ds from large numbers of patients with ALS and controls.

SETTING

An online multilingual questionnaire enabling respondents to measure their own index and ring finger lengths.

PARTICIPANTS

Of the initial 949 respondents, 572 remained for analysis after elimination for inability to straighten fingers, not answering the question, statistical outliers and aged <40 years. Respondents remaining for analysis were 202 patients with ALS (125 males, 77 females) and 370 non-ALS controls (112 males, 258 females).

RESULTS

Unpaired t-tests with 95% CIs were used to assess differences in mean 2D:4Ds. Males had significantly lower mean 2D:4Ds than females, in both ALS and control groups, for both left and right hands. No significant differences were found in 2D:4Ds between ALS and control groups, in either males or females, for either left or right hands. Receiver operating characteristic curves showed no power for 2D:4Ds to predict ALS status in either males or females.

CONCLUSIONS

2D:4Ds did not differ between patients with ALS and controls in this study. This was despite the dataset being large enough to confirm the established finding of lower 2D:4Ds in males compared with females. These findings do not support the hypothesis that exposure to increased prenatal testosterone is a risk factor for ALS. A putative lower 2D:4D has been proposed to explain the link between ALS and exercise, but our results indicate that other exercise-related factors are more likely to explain this association.

Asymmetry and empathy: Higher asymmetry is associated with lower levels of mentalizing

People differ in their mentalizing abilities. Though past research suggests that individual differences in exposure to prenatal testosterone may be able to explain why some people excel at mentalizing, while others struggle, meta-analyses yield a null relationship between 2D:4D ratio (a proxy for prenatal testosterone) and mentalizing. Importantly, however, past research has not examined the asymmetrical differences between the digit ratios on the right and left hands. In the current work, we test whether the difference between the digit ratios of the left and right hands may function as a better predictor of mentalizing than digit ratio alone. In Study 1, we begin by validating an online, self-report measure of 2D:4D ratio, providing test-retest reliability, convergent, and concurrent validity for our measure. In Study 2, we demonstrate that a) 2D:4D is quadratically related to asymmetry, b) asymmetry is negatively associated with mentalizing, and c) the relationship between asymmetry and mentalizing cannot be explained by the relationship between asymmetry and short-term memory. Taken together, our results paint a more nuanced picture of the relationship between digit ratio and mentalizing ability.

Prenatal testosterone exposure is related to sexually dimorphic facial morphology in adulthood

Prenatal testosterone may have a powerful masculinizing effect on postnatal physical characteristics. However, no study has directly tested this hypothesis. Here, we report a 20-year follow-up study that measured testosterone concentrations from the umbilical cord blood of 97 male and 86 female newborns, and procured three-dimensional facial images on these participants in adulthood (range: 21-24 years). Twenty-three Euclidean and geodesic distances were measured from the facial images and an algorithm identified a set of six distances that most effectively distinguished adult males from females. From these distances, a 'gender score' was calculated for each face, indicating the degree of masculinity or femininity. Higher cord testosterone levels were associated with masculinized facial features when males and females were analysed together (n = 183; r = -0.59), as well as when males (n = 86; r = -0.55) and females (n = 97; r = -0.48) were examined separately (p-values < 0.001). The relationships remained significant and substantial after adjusting for potentially confounding variables. Adult circulating testosterone concentrations were available for males but showed no statistically significant relationship with gendered facial morphology (n = 85, r = 0.01, p = 0.93). This study provides the first direct evidence of a link between prenatal testosterone exposure and human facial structure.

The Relationship Between Digit Ratio (2D:4D) and Sexual Orientation in Men from China

We examined the relationship between 2D:4D digit ratio and sexual orientation in men from China and analyzed the influences of the components used to assess sexual orientation and the criteria used to classify individuals as homosexual on this relationship. A total of 309 male and 110 female participants took part in a web-based survey. Our results showed that heterosexual men had a significantly lower 2D:4D than heterosexual women and exclusively homosexual men had a significantly higher left 2D:4D than heterosexual men whereas only exclusively homosexual men had a significantly higher right 2D:4D than heterosexual men when sexual orientation was assessed via sexual attraction. The left 2D:4D showed a significant positive correlation with sexual identity, sexual attraction, and sexual behavior, and the right 2D:4D showed a significant positive correlation with sexual attraction. The effect sizes for differences in 2D:4D between homosexual and heterosexual men varied according to criteria used to classify individuals as homosexual and sexual orientation components; the more stringent the criteria (scores closer to the homosexual category), the larger the effect sizes; further, sexual attraction yielded the largest effect size. There were no significant effects of age and latitude on Chinese 2D:4D. This study contributes to the current understanding of the relationship between 2D:4D and male sexual orientation.

Direct Versus Indirect Measurement of Digit Ratio (2D:4D)

Digit ratio (2D:4D: the relative lengths of the second and fourth digits) is widely used as a correlate of prenatal sex steroids. There are two common methods of measuring 2D:4D, the direct method and the indirect method. The modern interest in 2D:4D began 16 years ago when finger lengths were measured directly, but many studies now report 2D:4D calculated from indirectly measured fingers from photocopies or scans. However, there are concerns about the accuracy of the latter in comparison to the former. The purpose of this article was twofold: to review these concerns and to add new data to the debate. Our review shows that in 2005, directional effects in indirect 2D:4D were reported such that direct 2D:4D > indirect 2D:4D. This finding was challenged by a 2006 report that direct 2D:4D was lower (not higher) than indirect 2D:4D for male right-hand 2D:4D. Two further studies from the same group have claimed that indirect 2D:4D may be lower, higher, or comparable to direct 2D:4D. More recent comparisons of direct 2D:4D versus indirect 2D:4D and a meta-analysis of Chinese studies have replicated the finding of direct 2D:4D > indirect 2D:4D. We considered an additional sample and found significant direct 2D:4D > indirect 2D:4D for three of four ratios. The overall literature is discussed within the context of standards of research (sample size) and publishing (clarity of report). It is concluded that direct 2D:4D does tend to be greater than indirect 2D:4D. Implications for comparative studies and other aspects of research in 2D:4D are discussed.

Prenatal testosterone exposure is related to sexually dimorphic facial morphology in adulthood

Prenatal testosterone may have a powerful masculinizing effect on postnatal physical characteristics. However, no study has directly tested this hypothesis. Here, we report a 20-year follow-up study that measured testosterone concentrations from the umbilical cord blood of 97 male and 86 female newborns, and procured three-dimensional facial images on these participants in adulthood (range: 21-24 years). Twenty-three Euclidean and geodesic distances were measured from the facial images and an algorithm identified a set of six distances that most effectively distinguished adult males from females. From these distances, a 'gender score' was calculated for each face, indicating the degree of masculinity or femininity. Higher cord testosterone levels were associated with masculinized facial features when males and females were analysed together (n = 183; r = -0.59), as well as when males (n = 86; r = -0.55) and females (n = 97; r = -0.48) were examined separately (p-values < 0.001). The relationships remained significant and substantial after adjusting for potentially confounding variables. Adult circulating testosterone concentrations were available for males but showed no statistically significant relationship with gendered facial morphology (n = 85, r = 0.01, p = 0.93). This study provides the first direct evidence of a link between prenatal testosterone exposure and human facial structure.

Validation of digit-length ratio (2D:4D) assessments on the basis of DXA-derived hand scans

Background

The second-to-fourth digit-length ratio (2D:4D) may be a correlate of prenatal sex steroids, and it has been linked to sporting prowess. The aim of the study was to validate dual-energy X-ray-absorptiometry (DXA) as a technique to assess 2D:4D in soccer players under 15 years of age (U-15).

Methods

Paired X-ray and DXA scans of the left hands of 63 male U-15 elite soccer players (age: 14.0 ± 0.3 years) were performed, and 2D:4D was then compared between the two techniques. The 2D:4D measurements were performed twice by two blinded raters. Intrarater and interrater reliability, as well as agreement between the X-ray and the DXA assessments, were tested.

Results

Intrarater reliabilities of both raters using X-ray with intraclass correlation coefficients (ICCs) of 0.97 and 0.90 were excellent. Using DXA, the ICCs were 0.90 and 0.91 thus also showing excellent reliability. Interrater reliabilities were excellent using both the X-ray (ICC of 0.94) and the DXA (ICC of 0.90), assessments respectively. Bland-Altman plots demonstrated that the 2D:4D ratios of the two raters did not differ significantly between the X-ray and the DXA assessments. The standard errors of estimate were 0.01 for both techniques. The 95% limits of agreement of ±0.018 (±2.0%) and ±0.023 (±2.6%), respectively, were within the acceptable tolerance of 5%, and showed very good agreement.

Conclusion

DXA offered a replicable technique for assessing 2D:4D in youth soccer players. Therefore, the DXA technique seems to be an alternative method for evaluating 2D:4D in youth sports.

Adult digit ratio (2D:4D) is not related to umbilical cord androgen or estrogen concentrations, their ratios or net bioactivity

BACKGROUND

Ratio of second digit length to fourth digit length (2D:4D) has been extensively used in human and experimental research as a marker of fetal sex steroid exposure. However, very few human studies have measured the direct relationship between fetal androgen or estrogen concentrations and digit ratio.

AIMS

We investigated the relationships between both androgen and estrogen concentrations in umbilical cord blood and digit ratio in young adulthood. In addition we calculated measures of total serum androgen and total estrogen bioactivity and investigated their relationship to digit ratio.

STUDY DESIGN

Prospective cohort study.

SUBJECTS

An unselected subset of the Western Australian Pregnancy Cohort (Raine) Study (159 female; 182 male).

OUTCOME MEASURES

Cord serum samples were collected immediately after delivery. Samples were assayed for androgen (testosterone, Δ4-androstenedione, dehydroepiandrosterone) and estrogen (estrone, estradiol, estriol, estetrol) concentrations using liquid-chromatography mass-spectrometry. Digit ratio measurements were taken from hand photocopies at age 19-22years.

RESULTS

For both males and females, there were no significant correlations between digit ratio and any androgen or estrogen concentrations considered individually, the testosterone to estradiol ratio, total androgen bioactivity measure or ratio of androgen to estrogen bioactivity (all p>.05). In males, but not females, total estrogen bioactivity was negatively correlated with left hand digit ratio (r=-.172, p=.02), but this relationship was no longer significant when adjusted for variables known to affect sex steroid concentrations in cord blood.

CONCLUSIONS

Our findings indicate that digit ratio is not related to fetal androgens or estrogens at late gestation.

A comparison of measurement methods and sexual dimorphism for digit ratio (2D:4D) in Han ethnicity

The digit ratio (2D:4D) is sexually dimorphic and has been considered an indicator of prenatal sex hormone exposure. Previous studies have shown that males tend to have lower 2D to 4D ratio than females, and this sexual dimorphism has been reported across different ethnic groups and different countries. However, digit ratio data are missing from the Han ethnicity in China. Furthermore, most of the previous studies used direct measurement for digit ratio. In this article, we used multiple measurement methods, including the direct measurement and two X-ray measurement methods to examine the trait of 2D:4D in Chinese Han. Our sample consisted of 128 men and 122 women from Liaoning Medical University. They were 18-20 years old. The direct measurement and two types of X-ray measurements of the length of their 2nd and 4th fingers were used separately to calculate digit ratios. Soft tissue thickness of 2D and 4D fingertips were also assessed from the two X-ray methods. The results suggest that (1) sex differences in 2D:4D tend to be stronger in the two X-ray measurements in comparison to the direct measurement; (2) 2D:4D ratios from X-ray measurements tend to be lower than that from the direct measurement; (3) Han ethnicity have a lower mean value of 2D:4D than other ethnic groups; (4) no sex difference in the soft tissue of finger tips. In conclusion, the digit ratio is lower in both men and women in Han, and the sexual dimorphism in digit ratio was stronger with X-ray measurements in comparison to the direct measurement.

Biological markers of asexuality: Handedness, birth order, and finger length ratios in self-identified asexual men and women

Human asexuality is defined as a lack of sexual attraction to anyone or anything and it has been suggested that it may be best conceptualized as a sexual orientation. Non-right-handedness, fraternal birth order, and finger length ratio (2D:4D) are early neurodevelopmental markers associated with sexual orientation. We conducted an Internet study investigating the relationship between self-identification as asexual, handedness, number of older siblings, and self-measured finger-lengths in comparison to individuals of other sexual orientation groups. A total of 325 asexuals (60 men and 265 women; M age, 24.8 years), 690 heterosexuals (190 men and 500 women; M age, 23.5 years), and 268 non-heterosexuals (homosexual and bisexual; 64 men and 204 women; M age, 29.0 years) completed online questionnaires. Asexual men and women were 2.4 and 2.5 times, respectively, more likely to be non-right-handed than their heterosexual counterparts and there were significant differences between sexual orientation groups in number of older brothers and older sisters, and this depended on handedness. Asexual and non-heterosexual men were more likely to be later-born than heterosexual men, and asexual women were more likely to be earlier-born than non-heterosexual women. We found no significant differences between sexual orientation groups on measurements of 2D:4D ratio. This is one of the first studies to test and provide preliminary empirical support for an underlying neurodevelopmental basis to account for the lack of sexual attraction characteristic of asexuality.

Second to fourth digit ratio, handedness and testicular germ cell tumors

BACKGROUND

Research on early life exposures and testicular germ cell tumors (TGCT) risk has focused on a possible perinatal etiology with a well-known hypothesis suggesting that hormonal involvement during fetal life is associated with risk. Second-to-fourth digit ratio (2D:4D) and left-hand dominance have been proposed as markers of prenatal hormone exposure.

AIM

To evaluate associations between 2D:4D digit ratio, right minus left 2D:4D (ΔR-L), and left-hand dominance and TGCT in the U.S. Servicemen's Testicular Tumor Environmental and Endocrine Determinants Study.

METHODS

A total of 246 TGCT cases and 236 non-testicular cancer controls participated in the current study, and completed a self-administered questionnaire. Associations between digit ratio, hand dominance and TGCT were estimated using unconditional logistic regression adjusting for identified covariates.

RESULTS

Right 2D:4D was not associated with TGCT [odds ratio (OR) for a one-standard deviation (SD) increase in right-hand 2D:4D: 1.12, 95% confidence interval (CI): 0.93-1.34]. The results were consistent when evaluating the association based on the left hand. The difference between right and left-hand 2D:4D was also not associated with TGCT risk [OR for a one-SD increase in ΔR-L: 1.03, 95% CI: 0.87-1.23]. Compared to men who reported right-hand dominance, ambidexterity [OR (95% CI)=0.65 (0.30-1.41)] and left-hand dominance [OR (95% CI)=0.79 (0.44-1.44)] were not associated with risk.

CONCLUSIONS

These results do not support the hypothesis that prenatal hormonal imbalance is associated with TGCT risk. Given the limited sample size, further evaluation of the relationship between TGCT and prenatal hormonal factors using digit ratio, ΔR-L, or left-hand dominance and larger sample size are warranted.

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.

Digit ratio 2D:4D in relation to autism spectrum disorders, empathizing, and systemizing: a quantitative review

Prenatal testosterone (PT) effects have been proposed to increase systemizing (the drive to understand lawful input-output relationships), to decrease empathizing (the drive to understand others), and to cause autism via hypermasculinization of the brain. Digit ratio 2D:4D is a putative marker of PT effects in humans. An online study (n = 1896) into the relationship between the Reading the Mind in the Eyes Test (a widely used measure of empathizing) and self-measured 2D:4D in a nonclinical sample is reported. No evidence for a link between empathizing and 2D:4D in either females or males emerged. Further, three meta-analyses are presented that look into the relationships of 2D:4D with autism spectrum disorder (ASD), systemizing, and empathizing. 2D:4D was substantially lower (more masculine) in ASD-affected individuals than in normal controls (d = -0.58, P < 0.001). However, 2D:4D was found to be virtually unrelated to systemizing and empathizing in normal adults. The results support the idea that high PT is a risk factor for autism, but they challenge the view that PT substantially contributes to sex differences in systemizing and empathizing. Possibly, this pattern reflects an interaction effect, whereby PT drives ASD characteristic changes only in brains with a specific damage.

Patterns of second-to-fourth digit length ratios (2D:4D) in two species of frogs and two species of lizards at La Selva, Costa Rica

It is now well documented that androgen and estrogen signaling during early development cause a sexual dimorphism in second-to-fourth digit length ratio (2D:4D). It is also well documented that males of mammalian species have a smaller 2D:4D than females. Although there are discrepancies among 2D:4D studies in birds, the consensus is that birds exhibit the opposite pattern with males having a larger 2D:4D than females. The literature currently lacks substantial information regarding the phylogenetic pattern of this trait in amphibians and reptiles. In this study, we examined 2D:4D in two species of frogs (Oophaga pumilio and Craugastor bransfordii) and two species of lizards (Anolis humilis and Anolis limifrons) to determine the existence and the pattern of the sexual dimorphism. Male O. pumilio and C. bransfordii displayed larger 2D:4D than females in at least one of their two forelimbs. Male A. humilis had larger 2D:4D than females in both hindlimbs, but smaller 2D:4D than females in both forelimbs. Male A. limifrons may also have smaller 2D:4D than females in the right forelimb. Finally, digit ratios were sometimes positively related to body length, suggesting allometric growth. Overall, our results support the existence of the 2D:4D sexual dimorphism in amphibians and lizards and add to the knowledge of 2D:4D trait patterning among tetrapods.

Organizing effects of testosterone and economic behavior: not just risk taking

Recent literature emphasizes the role that testosterone, as well as markers indicating early exposure to T and its organizing effect on the brain (such as the ratio of second to fourth finger, ), have on performance in financial markets. These results may suggest that the main effect of T, either circulating or in fetal exposure, on economic behavior occurs through the increased willingness to take risks. However, these findings indicate that traders with a low digit ratio are not only more profitable, but more able to survive in the long run, thus the effect might consist of more than just lower risk aversion. In addition, recent literature suggests a positive correlation between abstract reasoning ability and higher willingness to take risks. To test the two hypotheses of testosterone on performance in financial activities (effect on risk attitude versus a complex effect involving risk attitude and reasoning ability), we gather data on the three variables in a sample of 188 ethnically homogeneous college students (Caucasians). We measure a digit ratio, abstract reasoning ability with the Raven Progressive Matrices task, and risk attitude with choice among lotteries. Low digit ratio in men is associated with higher risk taking and higher scores in abstract reasoning ability when a combined measure of risk aversion over different tasks is used. This explains both the higher performance and higher survival rate observed in traders, as well as the observed correlation between abstract reasoning ability and risk taking. We also analyze how much of the total effect of digit ratio on risk attitude is direct, and how much is mediated. Mediation analysis shows that a substantial part of the effect of T on attitude to risk is mediated by abstract reasoning ability.

Is low digit ratio linked with late menarche? Evidence from the BBC internet study

OBJECTIVES

Age at menarche (AAM) in the general population may be influenced by prenatal sex steroids. The ratio of the second and fourth digits (2D:4D) is a putative correlate of prenatal sex steroids, but the relationship of 2D:4D to AAM is controversial. Matchock ([2008]: Am J Hum Biol 20:487-489) has reported that right 2D:4D (but not left) was negatively related to AAM, but Helle ([2010a]: Am J Hum Biol 22:418-420) found no relationship. Here the association between 2D:4D, AAM, and self-reported rate of pubertal development (RPD) is considered.

METHODS

The sample consisted of self-measured finger lengths and AAM and RPD reported by women in the BBC internet study.

RESULTS

There were 70,658 white women who reported a mean (± SD) AAM of 12.54 (1.48) years. Right 2D:4D was negatively related to AAM and positively related to RPD. These relationships were independent of left 2D:4D, age and height. Between-country variation in right 2D:4D was also significantly related to AAM such that in countries with low 2D:4D women mean AAM was higher.

CONCLUSIONS

In support of the findings of Matchock, right 2D:4D was found to be negatively related to AAM. In addition, right 2D:4D was positively related to RPD. In a sample of 19 countries, mean right 2D:4D was negatively correlated with mean national AAM. These findings suggest that women with high prenatal testosterone and low prenatal estrogen tend to show late menarche and slow pubertal development.

Meta-analysis of digit ratio 2D:4D shows greater sex difference in the right hand

OBJECTIVES

Our aims are, first, to describe the sex difference in the length ratio of the second and fourth digits (2D:4D), which likely reflects prenatal testosterone levels in humans. Second, to infer the loss of reliability observed in 2D:4D based on self-measured finger lengths.

METHODS

We used random-effects meta-analysis of 2D:4D based on expert-measured finger lengths (116 samples with 13,260 females and 11,789 males).

RESULTS

We find a moderate sex difference (with lower 2D:4D for males), which shows substantial heterogeneity (which is unrelated to age). The sex difference is moderated by the type of finger length measurement and by hand. Measurement involving the distortion of soft tissue leads to a significantly larger sex difference than finger length measurement avoiding this. The sex difference in 2D:4D is larger in the right hand than in the left. The reliability of self-measured 2D:4D in the BBC internet study, by far the largest study on 2D:4D, is estimated to be 46% of that of expert-measured 2D:4D.

CONCLUSIONS

Right-hand 2D:4D might be a better indicator of prenatal androgenisation than left-hand 2D:4D. The view that 2D:4D has allometric properties (Kratochvil L, Flegr J. 2009. Differences in 2nd to 4th digit length ratio in humans reflect shifts along the common allometric line. Biol Lett 5:643-646.) is not supported. Bone lengths contribute to the sex difference in 2D:4D. In addition, there might be a sex difference in fingers' soft tissue, which should be investigated. Because of measurement unreliability, correlations between 2D:4D and variables of interest are about one-third smaller in the BBC internet study than in studies in which 2D:4D is based on expert-measured finger lengths.

Hand pattern indicates prostate cancer risk

BACKGROUND

The ratio of digit lengths is fixed in utero, and may be a proxy indicator for prenatal testosterone levels.

METHODS

We analysed the right-hand pattern and prostate cancer risk in 1524 prostate cancer cases and 3044 population-based controls.

RESULTS

Compared with index finger shorter than ring finger (low 2D : 4D), men with index finger longer than ring finger (high 2D : 4D) showed a negative association, suggesting a protective effect with a 33% risk reduction (odds ratio (OR) 0.67, 95% confidence interval (CI) 0.57-0.80). Risk reduction was even greater (87%) in age group <60 (OR 0.13, 95% CI 0.09-0.21).

CONCLUSION

Pattern of finger lengths may be a simple marker of prostate cancer risk, with length of 2D greater than 4D suggestive of lower risk.

Second to fourth digit ratios, male genital development and reproductive health: a clinical study among fertile men and testis cancer patients

Second- to fourth-digit length ratio, 2D:4D, is a marker of testosterone level during foetal life that was found associated with sperm concentration or testosterone levels in some studies, but not in others, a difference possibly related to the way the ratio is assessed. In this study, 2D:4D was assessed in 122 men partners of pregnant women and in 71 testicular cancer patients using a new method based on direct measurements of finger lengths. In addition, we investigated the association between 2D:4D, birth weight, testicular volume, semen quality and time to pregnancy. A validation study of the method demonstrated high reliability and reproducibility. Neither digit lengths nor 2D:4D significantly differed in both groups of men. We found a significant negative association between 2D:4D and birth weight in testicular cancer patients. In fertile men, 2D:4D was associated with testicular volume (r=-0.36, p<0.001), total sperm number (r=-0.18, p=0.04) and time to pregnancy (r=0.24, p<0.02). In addition, participants with a history of epididymal cyst had a significantly higher 2D:4D than those without cysts. In conclusion, all significant findings indicate that the human male reproductive function is negatively related to 2D:4D. However, 2D:4D for testicular cancer patients does not point to a hormonal imbalance during foetal life as the common cause for developing germ-cell cancer. Such results obtained, thanks to an easy, direct and reliable method for measuring finger lengths, suggest the usefulness of this new tool in fertility studies as well as for studying men with developmental disorders of the reproductive tract.

Index to ring digit ratio in Saudi Arabia at Almadinah Almonawarah province: a direct and indirect measurement study

The digit ratio, or the relative lengths, of the 2nd and 4th digits (2D :4D) shows a sex difference, with males tending to have lower values in comparison with females. This sex differences arises early in the fetus and may result from the effects of prenatal testosterone and estrogen on the relative growth rate of the 2nd and 4th digits. This study aimed to estimate finger lengths and the 2D:4D ratios for the first time in Saudi Arabian subjects using direct and indirect measurements, and to evaluate the correlations between both indirect and direct 2D:4D with adult testosterone and various sexually dimorphic physical traits. The results revealed the following: (i) mean 2D:4D in Saudi Arabian samples varied from 0.96 to 0.99; (ii) mean 2D:4D was lower for indirect compared to direct 2D:4D; (iii) sex differences in indirect 2D:4D were higher than in direct 2D:4D measurements; (iv) there were no significant correlations between indirect or direct 2D:4D and testosterone level; (v) there were four significant correlations between direct 2D:4D and body size traits but no significant correlations between indirect 2D:4D and body size.