Digit ratio (2D:4D) and sex-role orientation: Further evidence and meta-analysis

Three new studies (N = 132, 623, 1112) did not yield hypothesized associations between digit ratio (2D:4D), an assumed negative correlate of prenatal testosterone, and psychometrically measured sex-role orientation (positive association with femininity, negative ones with masculinity and masculinity-femininity difference scores) among heterosexuals of either sex. Meta-analysis of the pertinent literature (N = 6311, 28 studies from 10 countries, 46% unpublished) showed only men’s (but not women’s) left-hand (but not right-hand) 2D:4D reliably related as expected (positively) to femininity. This effect was tiny (0.14% attributable variance) and possibly non-robust (crucially dependent on one large study included). Hence, the cumulative evidence does not support systematic, robust, noteworthy within-sex correlations between 2D:4D and the masculinity/femininity personality dimensions.     

Special issue preamble: Digit ratio (2D:4D) and individual differences research

The sexually differentiated second-to-fourth digit ratio (2D:4D) is currently frequently utilized in individual differences research as a putative indicator of the masculinizing brain organization effects of prenatal testosterone. This preamble introduces the journal special issue on this theme, entitled “Digit ratio (2D:4D) and individual differences research”. Notes regarding the background, history, and recent progress of 2D:4D research are provided, and the background of the special issue and the contents of its 11 research articles are briefly summarized.     

Digit ratio (2D:4D), academic performance in computer science and computer-related anxiety

Being able to use information communication technology (ICT) effectively has become an essential element of participation within an increasingly digital culture. However, there are differences in participation within this digital culture. Prenatal testosterone exposure is thought to influence the development of numeric capabilities and relate to levels of anxiety, both of which contribute towards engagement with ICT. This study examined whether an index of prenatal exposure to testosterone, digit ratio (2D:4D), is related to successful involvement within a computer-technology context - performance in a Java programming course. Three studies (N = 73,75,65) identified a consistent negative correlation between 2D:4D digit ratio and attainment (r ≈ −0.2). A fourth study (N = 119) found that 2D:4D digit ratio positively correlated with two indices of computer-related anxieties, as well as anxiety sensitivity (r = 0.32/0.51). These results suggest that males and females who have been exposed to higher levels of testosterone within the womb perform better upon academic assessments of Java-related programming ability within computer science education, and have lower levels of computer-related anxieties outside computer science education. Thus, the 2D:4D index of prenatal testosterone exposure correlated with the two factors that directly impact upon ICT engagement, which is increasingly essential to effectively participate within educational and occupational environments.     

Digit ratio (2D:4D) and wearing of wedding rings

Digit ratio (2D:4D), a putative marker for prenatal androgen levels, and absolute finger length, a marker for pubertal androgen action, were not associated with wearing of wedding rings in a sample of 150 male and 199 female married individuals. This failure to replicate associations previously reported by Manning was true for both men and women, self-reported and spousal ring-wearing behavior, and one's own and spousal 2D:4D or absolute finger lengths. Supplemental findings included sex differences and age effects in the prevalence of wearing wedding rings (more frequent in women and the younger than in men and the older) and a sex bias when ring-wearing behavior within couples was discordant (men more frequently did not wear their wedding ring when their wives did than vice versa). Discussed are implications of the current findings and directions for future research into possible associations of 2D:4D with other aspects of body ornamentation.     

Investigating digit ratio (2D:4D) in a highly male‐dominated occupation: The case of firefighters

Voracek, M., Pum, U. & Dressler, S. G. (2009). Investigating digit ratio (2D:4D) in a highly male‐dominated occupation: The case of firefighters. Scandinavian Journal of Psychology, 51, 146–156. Second‐to‐fourth digit ratio (2D:4D), a widely studied putative marker for masculinization through prenatal androgen exposure, is lower (more masculinized) in athletes than in general population controls, and athletes with lower 2D:4D have higher sporting success. Occupations differ markedly in perceived masculinity and actual maleness (sex ratios), but these givens have not yet been picked up and utilized in 2D:4D research. Accordingly, this study extended existing accounts on 2D:4D in athletes to a novel approach: 2D:4D and possible relationships to a variety of candidate variables (demographic, fertility‐related, psychological, and other) were investigated in firefighters, a highly male‐dominated occupation. Contrary to expectation, 2D:4D in firefighters (N = 134) was not lower than in local male population controls. Lower 2D:4D corresponded to lower service ranks. Replicating previous findings either unequivocally or partly, lower 2D:4D was associated with larger family size, later sibling position, left‐handedness, and higher scores in the disinhibition component of sensation seeking. Not replicating prior evidence, 2D:4D was unrelated to body‐mass index, offspring sex ratio, and sporting performance level. Novel findings included low 2D:4D in those with low relationship satisfaction and in cigarette smokers, especially among heavy smokers. Absolute finger length, a positive correlate of pubertal‐adolescent androgen levels, was also considered. This marker showed negative associations with relationship consensus and satisfaction and positive ones with perceived quality of relationship alternatives and the experience seeking component of sensation seeking. The merits of this additional marker, relative to 2D:4D, for supplementing studies of possible sex‐hormonal effects on personality and directions for future inquiry along these lines are discussed.     

Domain Specific Relationships of 2D:4D Digit Ratio in Risk Perception and Risk Behavior

The 2D:4D digit ratio (henceforth, 2D:4D) is accepted as a marker of prenatal testosterone, demonstrating gender differences. The present study reveals that 2D:4D shows not only gender differences but also individual differences in personality and risk-taking behavior. We measured individual participants’ 2D:4D, structural personality, and risk-taking attitude scores. The result of the current study replicates the gender difference in 2D:4D in previous researches. However, we found different aspects of the correlations among 2D:4D, personality, and risk-taking attitude. Path analyses indicated that 2D:4D passes through personality factors before reaching the risk-taking attitude, particularly in the financial domain activities of investing and gambling. Also we observed a specific relationship between right-hand 2D:4D and emotionality and between left-hand 2D:4D and agreeableness. Finally, we suggest multiple path models of 2D:4D and personality in risk taking depending on the domain.     

Testosterone and domain-specific risk: Digit ratios (2D:4D and rel2) as predictors of recreational, financial, and social risk-taking behaviors

Prenatal testosterone has important effects on brain organization and future behavior. The second-to-fourth digit length ratio (2D:4D), a proxy of prenatal testosterone exposure, has been linked to a wide variety of sexually differentiated dispositions and behaviors. We examine the relationship between digit length ratios (2D:4D and rel2, the length of the second finger relative to the sum of the lengths of all four fingers) and risk-taking behaviors across five domains: financial, social, recreational, ethical, and health. In a sub-sample of male Caucasians (ethnically homogeneous), lower rel2 was predictive of greater financial, social, and recreational risk-taking, whereas lower 2D:4D was predictive of greater risk-taking in two domains (social and recreational). In the full male sub-sample (ethnically heterogeneous), the only significant correlation was a negative association between 2D:4D and financial risk. A composite measure of risk-taking across all five domains revealed that both rel2 and 2D:4D were negatively correlated with overall risk-taking in both male sub-samples. No significant correlations were found in the female sub-samples. Finally, men were more risk-seeking than women across all five contexts.     

An interactionist perspective on the relation between 2D:4D and behavior: An overview of (moderated) relationships between 2D:4D and economic decision making

The present paper gives an overview of recent research on the association between 2D:4D and different types of economic behavior. Particular attention is given to the importance of context dependence of relationships between 2D:4D and economic as well as other, social, behavior. Patterns of results are discussed from the perspective that 2D:4D might serve as a biological marker of dispositional dominance. This overview hopes to stimulate research from an interactionist perspective which focuses at relationships between 2D:4D and behavior that only appear when specific situational pressures interact with individual differences in 2D:4D.     

Digit ratio (2D:4D) in twins: heritability estimates and evidence for a masculinized trait expression in women from opposite-sex pairs

The second-to-fourth digit ratio (2D:4D) is sexually dimorphic in humans, such that men on average have a lower 2D:4D than women. This somatic trait has been proposed as a biomarker for the organizational (permanent) effects of prenatal testosterone on the brain and behavior. Over the past few years, an accumulating research program has shown 2D:4D to be related to a multitude of sex-dependent, hormonally influenced psychological and behavioral traits. The present study investigated the 2D:4D ratio of 44 men and 70 women from 36 identical and 21 fraternal twin pairs. Both basic and advanced approaches for estimating heritability concordantly suggested that the trait is substantially heritable. The best-fitting structural equation model indicated that the contributions to individual differences in 2D:4D are 81% additively genetic, 19% nonshared environmental, and 0% shared environmental. Supplemental analyses showed that, consistent with a prediction from sex-hormone transfer theory, women from opposite-sex fraternal twin pairs had significantly lower (more male-typical) 2D:4D than women from same-sex fraternal twin pairs. Directions for research are discussed, such as investigating possible influences of the sex chromosomes on the expression of 2D:4D. Further family studies will be needed to test whether the transmission mode of 2D:4D is consistent with X-linked or Y-linked inheritance. The study of sex chromosome aberrations should indicate whether the presence of additional X or Y chromosomes is associated with 2D:4D levels.     

Lack of correlation between digit ratio (2D:4D) and Baron-Cohen’s “Reading the Mind in the Eyes” test, empathy, systemising, and autism-spectrum quotients in a general population sample

The second-to-fourth digit ratio (2D:4D) is sexually differentiated and is a likely biomarker for the organisational (permanent) effects of prenatal testosterone on the human brain. Recent research has highlighted a possible role of prenatal testosterone levels in both the etiology of autism-spectrum disorders and in sex and individual differences in cognitive styles of the normal mind (Baron-Cohen’s Extreme Male Brain Theory of Autism and Empathising/Systemising Theory). Importantly, autistic children present lower (hypermasculinised) 2D:4D than healthy controls. Based on these accounts, we investigated the relation of 2D:4D with Baron-Cohen’s measures of empathising (“Reading the Mind in the Eyes” test, RMET; Empathy Quotient, EQ), systemising (Systemising Quotient, SQ), and autistic-like traits (Autism-Spectrum Quotient, AQ) in the general population (N = 423 Austrian adults). Whereas sex differences into the expected direction and of expected size were obtained for all variables and internal scale consistencies tallied to retrievable reference values, 2D:4D was unrelated to RMET, EQ, SQ, and AQ scores. Candidate explanations for this lack of correlation might be possible developmental timing differences in the expression of 2D:4D and empathising/systemising, qualitative (as opposed to quantitative) functional differences between the normal and the autistic mind, or the suboptimal psychometric properties of the measures.     

Masculinised brain and romantic jealousy: Examining the association between digit ratio (2D:4D) and between- and within-sex differences

We examined the relationship between second-to-fourth digit ratio (2D:4D), a correlate of prenatal testosterone exposure, and distress at sexual versus emotional infidelity in hypothetical scenarios of relationship threat. As predicted, a significant negative association was found between 2D:4D and greater distress at sexual infidelity for the whole sample (N = 179, females = 101). While this novel finding supports the view of romantic jealousy as a sexually-dimorphic adaptation, we explore reasons for the relatively weak association and discuss how underlying differences in brain structure could have influenced sex-specific behavioural capacities in romantic jealousy. We suggest a useful direction for future research will be to develop novel methods that facilitate the investigation of implicit emotional, rather than explicit cognitive, processes in response to pair-bond threat.     

The 2nd to 4th digit ratio (2D:4D) and eating disorder diagnosis in women

Eating disorders are more common in females than in males and are believed to be caused, in part, by biological and hormonal factors. Digit ratio or 2D:4D (the ratio of the 2nd to the 4th digit) is considered to be a proxy for prenatal testosterone (PT) and prenatal oestrogen (PE) exposure. However, how 2D:4D may be related to type of eating pathology is unknown. The relationship between 2D:4D and eating disorder diagnosis was investigated in recovered and currently eating disordered (n = 31) and control (n = 99) women. Mean 2D:4D ratio was significantly lower (indicating higher levels of PT and lower levels of PE) in anorexic (AN) compared to bulimic (BN) women, with controls intermediary. In eating disordered women, 2D:4D was positively and significantly related to current weight, lowest weight and current BMI, with strongest associations for right 2D:4D. Among women, low 2D:4D is related to AN and high 2D:4D to BN, suggesting a differential causal influence of prenatal sex hormones on later eating pathology.     

The presence of aggression cues inverts the relation between digit ratio (2D:4D) and prosocial behaviour in a dictator game

Digit ratio (2D:4D) is a sexually dimorphic trait. Men have relatively shorter index (2D) compared to ring (4D) fingers than women. More masculine ratios are thought to be influenced by higher prenatal testosterone levels. In the present paper, we aim to show the context‐dependency of the relation between 2D:4D and social behaviour. In two studies, we expose participants either to control or to aggression cues. Afterwards, they make a decision in a dictator game. Participants with low 2D:4D showed higher allocation levels (i.e. they were more prosocial) than participants with high 2D:4D in a neutral situation. However, this relationship inverts after exposure to an aggression cue. It turns out that in high 2D:4D people, aggression cues even increase prosocial behaviour. We call for future research which focuses on other plausible interactions between 2D:4D and context cues rather than on linear relations.