Digit ratio (2D:4D), salivary testosterone, and handedness

Association between the second- and fourth-digit ratio and oral squamous cell carcinoma

OBJECTIVES

Using an established proxy measure of intra-utero testosterone and estrogen levels-the ratio of second- and fourth-digit lengths-we estimated its association with the oral cancer risk among a population from Southern India.

MATERIAL AND METHODS

In a hospital-based case-control study, incident oral cancer cases (N = 350) and non-cancer controls (N = 371), frequency-matched by age and sex, were recruited from two major referral hospitals in Kerala, India. Structured interviews collected information on several domains of exposure via detailed life course questionnaires. Digit lengths were measured using a ruler in a standardized manner. Unconditional logistic regression was performed to estimate the odds ratios and 95% confidence intervals.

RESULTS

Second- and fourth-digit ratio lower than 1, which indicates relatively higher intra-utero level of testosterone and lower intra-utero level of estrogen, was associated with higher oral cancer risk (OR = 1.60, 95% CI: 1.02-2.52), after accounting for several confounders.

CONCLUSION

Our findings suggest that intra-utero hormonal levels measured by second- and fourth-digit ratio are associated with oral cancer risk. Further studies in different population should confirm these results.

In support of 2D:4D: More data exploring its conflicting results on handedness, sexual orientation and sex differences

In the last few years, several studies have questioned the value of the second-to-fourth digit ratio (2D:4D) as a measure of exposure to sex hormones before birth. Controversy has also extended to the 2D:4D association with individual features previously related to this exposure such as handedness and sexual orientation. Given that it has been argued that sex differences in 2D:4D could be a consequence of body-size differences, we have tested in a large sample the allometric relationship between finger lengths and body size. Our results show that the association is either allometric or isometric, depending on the analyses performed. In any case, the deviation from isometry is not large enough to explain the typically observed sex difference in this trait. We have also tested the association between sexual orientation and 2D:4D, finding a relationship between 2D:4D and sexual orientation in men but not in women. We attribute this discordance with previously published meta-analysis to differences in genetic background, a variable that has gained relevance in recent years in studies involving 2D:4D. Finally, we did not find any relationship between 2D:4D and handedness, evaluated through self-reported preference and hand performance. Our main conclusion is that 2D:4D shows differences between sexes beyond their disparity in body size. In our opinion, 2D:4D can be used cautiously as an indicator of intrauterine exposure to sex hormones taking into account some considerations, such as analysing a very large sample and taking careful measurements of the ethnicity of the sample.

Sex-moderated relationship between the 2D:4D ratio and circulating hormones in an adult Ghanaian population

The second-to-fourth digit ratio (2D:4D) is the putative marker of prenatal hormone exposure. The 2D:4D ratio or the right-left difference (Dr-l) are said to be negative and positive correlates, respectively, of circulating testosterone and estrogen in both adult males and females. However, previous studies on the subject have reported mixed results. This study aimed to determine the sex-moderated relationship between the 2D:4D ratio and adult circulating testosterone, estradiol, testosterone-to-estradiol ratio and the free androgen index. This was a cross-sectional study from January to June 2021 at the University for Development Studies, Ghana. The study involved 62 participants (Female = 28; Male = 34), aged between 20 and 26 years. The right (2D:4DR), the left (2D:4DL), and their difference (Dr-l) were measured by computer-assisted analysis. Fasting venous samples were assayed for total testosterone (T), estradiol (E2 ), and sex hormone-binding globulin (SHBG) using ELISA. The free androgen index (FAI) was then calculated (T/SHBG) and the data were analyzed using moderated and/or weighted regression. Males had significantly higher T and FAI than females while females had significantly higher E2 than males, which were independent of age and body mass index (p < 0.001). There was a significant SEX*Dr-l interaction on FAI (p = 0.007). The Dr-l correlated negatively with FAI in males but positively in females and accounted for about 94.0% of the variability of FAI in males (adjR2  = 0.940) and only 0.2% in females (adjR2  = 0.002). The 2D:4D ratio, a putative marker of prenatal hormone exposure, may have an impact on sex differences in adult free androgen index.

Associations between bio-motor ability, endocrine markers and hand-specific anthropometrics in elite female futsal players: a pilot study

BACKGROUND

The second-to-fourth digit ratio (2D:4D) has been calculated for individual athletes and sports, but it has not been investigated in futsal performance. Therefore, the aim of this study was to investigate any relationships between 2D:4D of the dominant and non-dominant hands and physical capacity performances, selected functional variables and hormone concentrations in elite female futsal players.

METHODS

Twenty-four elite female futsal players were measured for 2D:4D in the dominant (2D:4D_D) and non-dominant (2D:4D_ND) hand. The futsal specific performance test (FSPT), hand-grip strength (HGS) and aerobic power were also assessed. In addition, selected circulatory hormones were measured (estradiol, cortisol, growth hormone and insulin like growth factor-1). Pearson's correlation test was used to identify correlational relationships.

RESULTS

Total test-time and performance time (total time + penalty time) for the FSPT showed a significant correlation with 2D:4D_D (r = 0.53, p = 0.005 and r = 0.55, p = 0.003, respectively). HGS_ND also displayed a significant correlation with the 2D:4D_D (r = 0.59, p = 0.002). Aerobic power and time spent running on the treadmill also showed a significant relationship with 2D:4D_ND (both, r = 0.54, p = 0.006). Cortisol showed a significant correlation with 2D:4D_D (- 0.58, p = 0.003) and 2D:4D_ND(- 0.52, p = 0.008).

CONCLUSIONS

The measurement of 2D:4D ratio could be an important factor in determining potential performance attributes and talent identification of elite female futsal players. Further studies are needed in this area to further examine the results presented here.

No association between adult sex steroids and hand preference in humans

BACKGROUND

There is ongoing debate about the effects of hormones on the lateralization of the developing brain. In humans, there are conflicting theories of how testosterone during development should affect lateralization. Empirical studies linking prenatal and postnatal testosterone levels to hand preference (a proxy for lateralization) are similarly mixed. Links between hand preference and health may also suggest a mediating role of steroid hormones such as testosterone and estradiol. Studies to date of adult steroid hormones and handedness have been hindered by samples that contain small numbers of non-right-handers.

RESULTS

In the largest study of the phenomenon to date, I find that the testosterone (n = 7290) and estradiol (n = 3700) levels of left- and mixed-handed adults are no different to those of right-handers. All Bayesian 95% highest density intervals contained 0.

DISCUSSION AND CONCLUSIONS

The results have implications for studies that show elevated risk of hormonal-related mental and physical disorders in left-handed individuals.

Digit ratio in the common toad Bufo bufo: the effects of reduced fingers and of age dependency

INTRODUCTION

Despite the growing number of studies describing digit ratio patterns in tetrapods, knowledge concerning certain basic issues is still scarce. In lower vertebrates such as tailless amphibians (Anura), the numbering of individual fingers on the forelimbs and their homology with the fingers of other vertebrates pose an unsolved problem. Based on reviewed data on anuran limb development, we argue that the correct finger numbering scheme should be based on the assumption that the first finger, not the fifth finger, was reduced on the forelimbs. We analyzed the digit ratio in the common toad (Bufo bufo, Bufonidae), a species characterized by well-developed sexual dimorphism whereby females are larger than males, using both numbering schemes present in the literature.

RESULTS

We found that the digit ratio on hindlimbs differed significantly between the sexes only in the cases of left 2D:3D, with lower digit ratios in females, and of left 3D:4D, with lower digit ratios in males. We found that sex was the only significant variable for forelimbs, differentiating 2D:3D on the left forelimb, with lower digit ratios in females; 2D:4D on the right forelimb, with lower digit ratios in males; and 3D:4D on both forelimbs, with lower digit ratios in males. These results relate to variant II reflecting the hypothesis that the first digit was reduced during phylogenesis. There was no relationship between the body size (SVL) of individuals and any digit ratio, excluding 2D:4D on the right forelimbs in models with age variables. Additionally, for a subset of data where individual age was known, the models indicated that age was linked to significant differences in 2D:4D and 3D:4D on the left hindlimbs, while age, SVL, and sex influenced 2D:4D on the right forelimbs.

CONCLUSION

We emphasize the importance of the problem of the correct numbering of forelimb digits in Anura and, under the assumption that it was the fifth digit that was reduced, argue that earlier results on digit ratio in this group should be interpreted with caution. The detected relationship between digit ratio and age in amphibians expands our knowledge, indicating that the age of individuals should be included in future digit ratio studies. This relationship may also apply to studies using digit ratio as a noninvasive indicator of endocrine disruption in amphibians.

An examination of the influence of prenatal sex hormones on handedness: Literature review and amniotic fluid data

Competing theories have posited roles for foetal androgen exposure in the development of human handedness. However, due to practical and ethical considerations, few studies have used hormonal measures to examine this possibility. The current paper reviews this literature and reveals a generally inconsistent pattern of results. We also present data from a longitudinal study of prenatal sex hormone exposure and subsequent handedness. More specifically, we examine correlations between testosterone and estradiol measured from second trimester amniotic fluid and hand preference (Dutch language version of the Edinburgh Handedness Inventory) and hand skill asymmetry (pegboard task) measured at 15 years of age. Prenatal sex hormone exposure was not associated with the direction of hand preference in either males or females. However, in females, high levels of prenatal testosterone were associated with weaker lateralisation of hand skill, and high levels of prenatal estradiol were associated with weaker hand preference. In addition, high levels of prenatal testosterone were associated with increased task duration (i.e., slow hand speed) for the right and left hands of males. The pattern of results observed here is not entirely consistent with any of the main theories linking sex hormones with handedness, suggesting that an association between these variables may be more complex than initially thought.

The effects of hand preference and sex on right-left asymmetry in dorsal digit lengths among adults and children

BACKGROUND

Right-hand preference is related to stronger right-directional asymmetry in the length of proximal upper-limb bones, although the relationships of hand preference with directional asymmetry in phalangeal bone lengths are not known. Furthermore, dorsal digit length is an easy-to-measure, faithful proxy of X-rayed phalangeal bone length (which is costly and difficult to measure).

AIM

To study the effects of hand preference, sex, and age on right-left (R-L) asymmetry in dorsal digit lengths.

METHODS

We measured all dorsal digit lengths (except the thumb) in comparable numbers of left-handers and right-handers in samples of adults (N = 151, age: M = 22.6 years, SD = 3.3) and children (N = 65, age: M = 5.0 years, SD = 1.0).

RESULTS

Right-handers and adults had stronger right-directional asymmetry in digit lengths than left-handers and children. A Bayesian analysis yielded an 'extremely strong likelihood' of no sex differences in the R-L asymmetry of dorsal digit lengths 2 and 4.

CONCLUSIONS

The effects of hand preference, sex, and age on R-L asymmetry appear to be similar for phalangeal bone length and other (proximal) upper-limb bone lengths. Two distinct biologic mechanisms (i.e., a general right-directional asymmetry mechanism and a handedness-related directional asymmetry mechanism) may contribute to observed R-L asymmetry in limbs. Fingertip fat and bone digit length do not seem to contribute to sex differences in the R-L asymmetry (Dr-l) of the widely studied second-to-fourth digit ratio (2D:4D).

Digit ratio (2D:4D) and handedness: A meta-analysis of the available literature

The Geschwind-Behan-Galaburda and sexual differentiation models predict an association between elevated foetal androgen exposure and left-handedness whereas the callosal hypothesis predicts the opposite. We present a meta-analysis of correlations between handedness and digit ratio (2D:4D), a putative marker of prenatal testosterone. Left-handedness predicted low (male-typical) right-hand digit ratio (R2D:4D), high (female-typical) left-hand digit ratio (L2D:4D), and low R2D:4D-L2D:4D directional asymmetry (D_[R-L]). Effect sizes were extremely small and not moderated by sex or method of measuring handedness or 2D:4D. The same general pattern was observed after excluding the very large study (110,329 males, 90,412 females) of Manning and Peters ([2009]. Digit ratio (2D:4D) and hand preference for writing in the BBC Internet Study. Laterality: Asymmetries of Body, Brain and Cognition, 14(5), 528-540. doi:10.1080/13576500802637872); however, no significant effects for R2D:4D were observed once these samples were removed. The results do not confirm any theory linking prenatal androgens with handedness, so we speculate they instead reflect the mechanical action of writing causing subtle changes in the musculature and/or fat pads of the fingers. Gripping a pen/pencil might cause an increase in 2D relative to 4D (and/or decrease in 4D relative to 2D) resulting in higher ratios on the writing-hand; furthermore, this could differ between left- and right-handers due to writing in the left-to-right direction (as in English) having asymmetrical effects depending on which hand is used.

Testosterone and the Brain: From Cognition to Autism

Sex and gender matter in all aspects of life. Humans exhibit sexual dimorphism in anatomy, physiology, but also pathology. Many of the differences are due to sex chromosomes and, thus, genetics, other due to endocrine factors such as sex hormones, some are of social origin. Over the past decades, huge number of scientific studies have revealed striking sex differences of the human brain with remarkable behavioral and cognitive consequences. Prenatal and postnatal testosterone influence brain structures and functions, respectively. Cognitive sex differences include especially certain spatial and language tasks, but they also affect many other aspects of the neurotypical brain. Sex differences of the brain are also relevant for the pathogenesis of neuropsychiatric disorders such as autism spectrum disorders, which are much more prevalent in the male population. Structural dimorphism in the human brain was well-described, but recent controversies now question its importance. On the other hand, solid evidence exists regarding gender differences in several brain functions. This review tries to summarize the current understanding of the complexity of the effects of testosterone on brain with special focus on their role in the known sex differences in healthy individuals and people in the autism spectrum.

No credible evidence for links between 2D:4D and COVID-19 outcomes: A probabilistic perspective on digit ratio, ACE variants, and national case fatalities

Research into COVID-19 susceptibility and outcomes are critical, but claims must be carefully evaluated to inform policy decisions. In a recent series of articles, Manning and Fink [1-3] use national-level data to describe associations between case-fatality ratios and male and female finger ratios (2D:4D), a suggested measure of prenatal androgen exposure, as well as angiotensin-converting enzyme (ACE) allele and genotype frequencies. The authors suggest that 2D:4D is linked with ACE variant prevalence, and that higher male 2D:4D is associated with higher case fatality ratios, and point to 2D:4D as a useful prognostic measure for COVID-19 susceptibility. A critical review and robust Bayesian analysis of the hypothesis is described here, finding no conclusive evidence of COVID-19 mortality and 2D:4D, nor associations between 2D:4D and ACE1 allele or ACE2 genotype frequency. This absence of evidence is present for data taken from the second wave of COVID-19 in October 2020. Problematic theoretical grounding, individual-level conclusions drawn from national-level data, and issues with statistical inference in the original articles are discussed. Taken together, the current data offer no clear utility of 2D:4D in determining COVID-19 outcomes.

Effects of hand preference on digit lengths and digit ratios among children and adults

BACKGROUND

Prenatal sex hormones may not exclusively determine effects of hand preference on digit ratios. Genetic determination is an alternative possibility.

AIM

To study the likelihood of similar effects of hand preference on digit lengths and digit ratios.

METHODS

We selected similar numbers of left-handers and right-handers in samples of kindergarten children (N = 101, age range: 3.5-7 years) and adults (N = 189, age range: 17-28 years) and measured digit lengths (excluding the thumb) directly on the palmar hand.

RESULTS

Compared to right-handers, left-handers had longer digits and lower third-to-fourth (3D:4D) digit ratios among children, whereas an opposite pattern of handedness differences occurred among adults.

CONCLUSIONS

Effects of hand preference on digit lengths and ratios might be genetically/ontogenetically determined. Also discussed are implications of this set of findings for digit ratio research.

Asymmetric Behavior in Ptyodactylus guttatus: Can a Digit Ratio Reflect Brain Laterality?

The digit ratio, an indicator of brain laterality, is the ratio of the second and fourth digits on the left (L24) or right foot (R24). Much of the research on the digit ratio and brain laterality focuses on primates, rather than other species such as reptiles. We tested whether the digit ratio in the gecko Ptyodactylus guttatus was associated with behaviors attributed to brain laterality. We examined risk-taking behavior (time spent under cover), foot preference (which foot was the first to start moving) and the side from which geckos bypassed an obstacle, in relation to the digit ratio. Geckos with longer fourth digits on their left hind foot (higher digit ratio) spent more time under cover. Geckos starting to move with their left leg were much more likely to bypass obstacles from the right side, and vice versa. This is the first evidence of laterality being associated with the digit ratio in reptiles. Comparisons among vertebrates are needed in order to decipher the evolutionary origin of the commonalities and peculiarities of brain asymmetry and disentangle the patterns and drivers of our evolutionary tree.

Hand morphometrics, electrodermal activity, and stone tools haptic perception

OBJECTIVES

Tool use requires integration among sensorial, biomechanical, and cognitive factors. Taking into account the importance of tool use in human evolution, changes associated with the genus Homo are to be expected in all these three aspects. Haptics is based on both tactile and proprioceptive feedbacks, and it is associated with emotional reactions. Previous analyses have suggested a difference between males and females, and during haptic exploration of different typologies of stone tools. Here, we analyze the correlation between electrodermal reactions during stone tool handling and hand morphology to provide evidence of possible allometric factors shared by males and females.

METHODS

Electrodermal analysis was used to investigate some specific parameters involved in these reactions, such as changes in the level of attention and arousal. We analyzed the responses of 46 right-handed adults to 20 distinct stone tools while blindfolded.

RESULTS

Females have smaller hands and a wider range of electrodermal reactions. Within males and females, hand diameters and general hand size do not correlate with the degree of electrodermal level and response.

CONCLUSIONS

Sex differences in electrodemal reaction during stone tool handling are apparently not due to the effect of hand size or proportions. Differences between males and females are better interpreted as real sex differences, either due to a biological or cultural influences. Hand size does not influence the degree of arousal or attention during tool exploration, suggesting that other factors trigger individual reactions. These results add to a general cognitive approach on hand-tool evolution and tool sensing.

How do we love? Romantic love style in men is related to lower testosterone levels

Testosterone has been widely investigated in associations with many aspects of social interactions, emotions and behavior. No research has been conducted on its contribution to the variability of love styles in human. The aim of this paper was to uncover the possible relationship between not only the actual plasma testosterone levels, but also the prenatal testosterone level (expressed as 2D:4D ratio) and the sensitivity of androgen receptor and love typology in young healthy men. There are six love styles which are primary including Eros (passionate romantic love), Ludus (playful) and Storge (friendly) and secondary love consisting of Mania (obsessive), Pragma (practical realistic) and Agape (altruistic). Our results pointed out that low testosterone concentrations are associated with higher score for Eros, Ludus, Pragma, Mania love style. No significant association was proved for other tested parameters of androgenicity (2D:4D, sensitivity of androgen receptor) and love style after correction was applied. Different attitudes and behavior in relationships do have a biological foundation related to endogenous testosterone levels in plasma. Future studies should address questions about the family and social background of participants to differentiate here between moral rules or/and social-conventional rules.

Cerebral laterality for language is related to adult salivary testosterone levels but not digit ratio (2D:4D) in men: A functional transcranial Doppler ultrasound study

The adequacy of three competing theories of hormonal effects on cerebral laterality are compared using functional transcranial Doppler sonography (fTCD). Thirty-three adult males participated in the study (21 left-handers). Cerebral lateralization was measured by fTCD using an extensively validated word generation task. Adult salivary testosterone (T) and cortisol (C) concentrations were measured by luminescence immunoassay and prenatal T exposure was indirectly estimated by the somatic marker of 2nd to 4th digit length ratio (2D:4D). A significant quadratic relationship between degree of cerebral laterality for language and adult T concentrations was observed, with enhanced T levels for strong left hemisphere dominance and strong right hemisphere dominance. No systematic effects on laterality were found for cortisol or 2D:4D. Findings suggest that higher levels of T are associated with a relatively attenuated degree of interhemispheric sharing of linguistic information, providing support for the callosal and the sexual differentiation hypotheses rather than the Geschwind, Behan and Galaburda (GBG) hypothesis.

Second to fourth digit ratio (2D:4D) and coronary heart disease

INTRODUCTION

Sex steroid exposure during human development may influence disease susceptibility. Second to fourth digit ratio (2D:4D) is thought to be a putative biomarker for prenatal hormone level during foetal life. Recently there has been a great deal of interest in 2D:4D and risk of disease in adulthood.

METHODS

We explored the relationship between 2D:4D ratio and coronary heart disease. Photocopies of the two hands of 310 males (controls: 155; patients: 155) were collected at Yin chuan city, in the Ningxia province of China. Left hand, right hand, mean hand and right minus left 2D:4D (Dr-l) were analysed.

RESULTS

The mean values of 2D:4D ratio in patients were higher than those of CSIN controls. The patients showed significantly higher 2D:4D (left hand: P < 0.01; right hand, mean hand: P < 0.05) than controls. In patients, 2D:4D (especially left hand 2D:4D) were inversely associated with age at diagnosis (left hand: P < 0.001; right hand, mean hand: P < 0.05). No association between Dr-l and age or age at presentation of disease was observed in the groups.

CONCLUSION

2D:4D may correlate particularly with risk of coronary heart disease in Chinese men.

The Neurobiology and Psychology of Pedophilia: Recent Advances and Challenges

A pedophilic disorder is recognized for its impairment to the individual and for the harm it may cause to others. Pedophilia is often considered a side issue and research into the nature of pedophilia is delayed in comparison to research into other psychiatric disorders. However, with the increasing use of neuroimaging techniques, such as functional and structural magnetic resonance imaging (sMRI, fMRI), together with neuropsychological studies, we are increasing our knowledge of predisposing and accompanying factors contributing to pedophilia development. At the same time, we are faced with methodological challenges, such as group differences between studies, including age, intelligence, and comorbidities, together with a lack of careful assessment and control of child sexual abuse. Having this in mind, this review highlights the most important studies investigating pedophilia, with a strong emphasis on (neuro-) biological studies, combined with a brief explanation of research into normal human sexuality. We focus on some of the recent theories on the etiology of pedophilia such as the concept of a general neurodevelopmental disorder and/or alterations of structure and function in frontal, temporal, and limbic brain areas. With this approach, we aim to not only provide an update and overview but also a framework for future research and to address one of the most significant questions of how pedophilia may be explained by neurobiological and developmental alterations.

Prenatal testosterone and stuttering

BACKGROUND

The prevalence of stuttering is much higher in males compared to females. The biological underpinnings of this skewed sex-ratio is poorly understood, but it has often been speculated that sex hormones could play an important role.

AIMS

The present study investigated a potential link between prenatal testosterone and stuttering. Here, an indirect indicator of prenatal testosterone levels, the Digit Ratio (2D:4D) of the hand, was used. As numerous studies have shown, hands with more "male" characteristics (putatively representing greater prenatal testosterone levels) are characterized by a longer ring finger compared to the index finger (represented as a lower 2D:4D ratio) in the general population.

STUDY DESIGN, SUBJECTS, OUTCOME MEASURES

We searched for differences in the 2D:4D ratios between 38 persons who stutter and 36 persons who do not stutter. In a second step, we investigated potential links between the 2D:4D ratio and the multifaceted symptomatology of stuttering, as measured by the Overall Assessment of the Speaker's Experience of Stuttering (OASES), in a larger sample of 44 adults who stutter.

RESULTS

In the first step, no significant differences in the 2D:4D were observed between individuals who stutter and individuals who do not stutter. In the second step, 2D:4D correlated negatively with higher scores of the OASES (representing higher negative experiences due to stuttering), and this effect was more pronounced for female persons who stutter.

CONCLUSIONS

The findings indicate for the first time that prenatal testosterone may influence individual differences in psychosocial impact of this speech disorder.

Sociosexual orientation and 2D:4D ratios in women: Relationship to men's desirability ratings as a long-term pair bond

The current study examined whether men's ratings of women's desirability as a long-term pairbond, based on static photographs, were related to the women's second-to-fourth digit (2D:4D) ratio and their sexual attitudes and behavior. The 2D:4D ratio was measured in 164 women and facial photographs were taken of 55 of these women. All women completed the Sociosexual Orientation Inventory (SOI). Male participants (n = 89), masked to this information, rated the 55 female participants on their desirability as a long-term sexual partner, specifically along dimensions of faithfulness, youthfulness, and attractiveness. Ten independent judges rated women's photographed faces on masculinity. Results indicated a significant negative relationship between women's SOI scores and men's faithfulness ratings (more unrestricted sociosexuality was associated with lower faithfulness ratings). There was also a significant positive relationship between right (but not left) 2D:4D ratio and faithfulness ratings (women with female-like ratios were rated as being more faithful). The SOI scores of the women were not related to 2D:4D ratios. These results suggest that the potential for sexual infidelity can be gleaned from static facial cues.

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.

Relationship of 2D:4D finger ratio with androgen receptor CAG and GGN repeat polymorphism

OBJECTIVES

The relative length of the second-to-fourth digits (2D:4D) has been linked with prenatal androgen in humans. A recent study shows that the 2D:4D ratio in mice is controlled by the balance of androgen to estrogen signaling during a narrow window of digit development. Androgen receptor (AR) activity is higher in digit 4 than in digit 2, and inactivation of AR decreases growth of digit 4, which causes a higher 2D:4D ratio. At the molecular level, the effect of androgens is mediated through the activation of AR. The CAG/GGN repeat polymorphisms of the AR gene are associated with AR activity. Here, we investigate the effect of CAG/GGN repeat polymorphisms in AR on 2D:4D in Chinese.

METHODS

Digit lengths of the second and fourth fingers were measured from photocopies of the ventral surface of the hand and by actual finger measurements. We genotyped AR polymorphisms by ABI 3730 DNA analyzer.

RESULTS

We found that left hand 2D:4D ratio was longer than that of the right hand both in males and in females. We failed to find any relationship between CAG / GGN alleles and the left hand, right hand, right minus left-hand or mean hand 2D:4D ratios (all, r < 0.20, P > 0.05).

CONCLUSIONS

In this study, we first found that the left hand 2D:4D ratio was longer than that of the right hand in both males and females. However, we found that both CAG and GGN alleles were not associated with the left hand, right hand, right minus left-hand or mean hand 2D:4D ratios.

Early-to-mid gestation fetal testosterone increases right hand 2D:4D finger length ratio in polycystic ovary syndrome-like monkeys

A smaller length ratio for the second relative to the fourth finger (2D:4D) is repeatedly associated with fetal male-typical testosterone (T) and is implicated as a biomarker for a variety of traits and susceptibility to a number of diseases, but no experimental human studies have been performed. The present study utilizes the rhesus monkey, a close relative of humans, and employs discrete gestational exposure of female monkeys to fetal male-typical T levels for 15-35 days during early-to-mid (40-76 days; n = 7) or late (94-139 days; n = 7) gestation (term: 165 days) by daily subcutaneous injection of their dams with 10 mg T propionate. Such gestational exposures are known to enhance male-typical behavior. In this study, compared to control females (n = 19), only early-to-mid gestation T exposure virilizes female external genitalia while increasing 2D:4D ratio in the right hand (RH) by male-like elongation of RH2D. RH2D length and 2D:4D positively correlate with androgen-dependent anogenital distance (AG), and RH2D and AG positively correlate with duration of early-to-mid gestation T exposure. Male monkeys (n = 9) exhibit a sexually dimorphic 2D:4D in the right foot, but this trait is not emulated by early-to-mid or late gestation T exposed females. X-ray determined phalanx measurements indicate elongated finger and toe phalanx length in males, but no other phalanx-related differences. Discrete T exposure during early-to-mid gestation in female rhesus monkeys thus appears to increase RH2D:4D through right-side biased, non-skeletal tissue growth. As variation in timing and duration of gestational T exposure alter male-like dimensions of RH2D independently of RH4D, postnatal RH2D:4D provides a complex biomarker for fetal T exposure.

Second-to-fourth digit ratio and facial shape in boys: the lower the digit ratio, the more robust the face

During human ontogeny, testosterone has powerful organizational and activational effects on the male organism. This has led to the hypothesis that the prenatal environment (as studied through the second-to-fourth digit ratio, 2D : 4D) is not only associated with robust adult male faces that are perceived as dominant and masculine, but also that there is an activational step during puberty. To test the latter, we collected digit ratios and frontal photographs of right-handed Caucasian boys (aged 4-11 years) along with age, body height and body weight. Using geometric morphometrics, we show a significant relationship between facial shape and 2D : 4D before the onset of puberty (explaining 14.5% of shape variation; p = 0.014 after 10 000 permutations, n = 17). Regression analyses depict the same shape patterns as in adults, namely that the lower the 2D : 4D, the smaller and shorter the forehead, the thicker the eyebrows, the wider and shorter the nose, and the larger the lower face. Our findings add to previous evidence that certain adult male facial characteristics that elicit attributions of masculinity and dominance are determined very early in ontogeny. This has implications for future studies in various fields ranging from social perception to life-history strategies.

Brain asymmetry, immunity, handedness

The principle of symmetry-asymmetry is widely presented in the structural and functional organization of the nonliving and living nature. One of the most complex manifestations of this principle is the left-right asymmetry of the human brain. The present review summarizes previous and contemporary literary data regarding the role of brain asymmetry in neuroimmunomodulation. Some handedness-related peculiarities are outlined additionally. Brain asymmetry is considered to be imprinted in the formation and regulation of the individual’s responses and relationships at an immunological level with the external and internal environment. The assumptions that the hemispheres modulate immune response in an asymmetric manner have been confirmed in experiments on animals. Some authors assume that the right hemisphere plays an indirect role in neuroimmunomodulation, controlling and suppressing the left hemispheric inductive signals.

Right hand, left brain: genetic and evolutionary bases of cerebral asymmetries for language and manual action

Most people are right-handed and left-cerebrally dominant for language. This pattern of asymmetry, as well as departures from it, have been reasonably accommodated in terms of a postulated gene with two alleles, one disposing to this common pattern and the other leaving the direction of handedness and language asymmetry to chance. There are some leads as to the location of the gene or genes concerned, but no clear resolution; one possibility is that the chance factor is achieved by epigenetic cancelling of the lateralizing gene rather than through a chance allele. Neurological evidence suggests that the neural basis of manual praxis, including pantomime and tool use, is more closely associated with cerebral asymmetry for language than with handedness, and is homologous with the so-called "mirror system" in the primate brain, which is specialized for manual grasping. The evidence reviewed supports the theory that language itself evolved within the praxic system, and became lateralized in humans, and perhaps to a lesser extent in our common ancestry with the great apes. WIREs Cogn Sci 2012, 3:1-17. doi: 10.1002/wcs.158 For further resources related to this article, please visit the WIREs website.

Low digit ratio 2D:4D in alcohol dependent patients

The ratio of the lengths of the second and fourth finger (2D∶4D) has been described as reflecting the degree of prenatal androgen exposure in humans. 2D∶4D is smaller for males than females and is associated with traits such as left-handedness, physical aggression, attention-deficit-hyperactivity disorder and a genetic polymorphism of the androgen receptor. All of these traits are known to be correlated to the vulnerability for alcohol dependency. We therefore hypothesized low 2D∶4D in patients with alcohol dependency. In the present study on 131 patients suffering from alcohol dependency and 185 healthy volunteers, we found that alcohol dependent patients had smaller 2D∶4D ratios compared to controls with preserved sexual dimorphism but with reduced right-left differences. The detection of alcohol dependency based on 2D∶4D ratios was most accurate using the right hand of males (ROC-analysis: AUC 0.725, sensitivity 0.667, specificity 0.723). These findings provide novel insights into the role of prenatal androgen exposure in the development of alcohol dependency and for the use of 2D∶4D as a possible trait marker in identifying patients with alcohol dependency.

Decreased prevalence of left-handedness among females with male co-twins: evidence suggesting prenatal testosterone transfer in humans?

Studies of singletons suggest that right-handed individuals may have higher levels of testosterone than do left-handed individuals. Prenatal testosterone levels are hypothesised to be especially related to handedness formation. In humans, female members from opposite-sex twin pairs may experience elevated level of prenatal exposure to testosterone in their intrauterine environment shared with a male. We tested for differences in rates of left-handedness/right-handedness in female twins from same-sex and opposite-sex twin pairs. Our sample consisted of 4736 subjects, about 70% of all Finnish twins born in 1983-1987, with information on measured pregnancy and birth related factors. Circulating testosterone and estradiol levels at age 14 were available on 771 and 744 of these twins, respectively. We found significantly (p=.006) lower prevalence of left-handedness in females from opposite-sex pairs (5.3%) compared to females from same-sex pairs (8.6%). The circulating levels of neither testosterone nor estradiol related to handedness in either females or males. Nor were there differences in circulating testosterone or estradiol levels between females from opposite-sex and same-sex twin pairs. Birth and pregnancy related factors for which we had information were unrelated to handedness. Our results are difficult to fully explain by postnatal factors, but they offer support to theory that relates testosterone to formation of handedness, and in a population-based sample, are suggestive of effects of prenatal testosterone transfer.