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

Anthropometric measures do not explain the 2D:4D ratio sexual dimorphism in 7-year-old children

Objectives

Digit ratio (2D:4D) might reflect prenatal testosterone exposure and has been used as a putative marker for androgen related outcomes. However, such associations might be inflicted by confounders. Application of 2D:4D in epidemiological research motivate identification of biological background determinants.

We examined sex, anthropometric measures, and maternal factors as determinants of 2D:4D in Swedish 7‐year‐old children.

Methods

The study was embedded in the Swedish Environmental, Longitudinal, Mother and Child, Asthma and Allergy (SELMA) pregnancy cohort. A total of 870 pre‐pubertal children, median 7.5 years of age, were studied.

A single assessor performed digit measurements from scanned photocopies using computer software. Child anthropometric measurements investigated were hand size, birthweight, recumbent birth length, standing height, weight, BMI, body fat percentage, and waist/hip circumference. Maternal factors included age, pregnancy length, parity, and education.

Results

We found a significant sexual dimorphism regarding digit lengths and 2D:4D, boys on average presenting a lower 2D:4D than girls also after adjustment for summed finger lengths and body fatness. In crude analyses, maternal age correlated with 2D:4D across the whole population and in females but not in adjusted models. No other study variables were associated with 2D:4D.

Conclusion

Digit ratio showed sexual dimorphism at the age of seven and seems to represent a true sex difference rather than an artifact and bias from hand size, body size or body fat content. Among the rest of our investigated variables, we found no determinants constituting important confounders in future research on 2D:4D ratio.

Eye-tracking analyses of physician face gaze patterns in consultations

Face gaze is a fundamental non-verbal behaviour and can be assessed using eye-tracking glasses. Methodological guidelines are lacking on which measure to use to determine face gaze. To evaluate face gaze patterns we compared three measures: duration, frequency and dwell time. Furthermore, state of the art face gaze analysis requires time and manual effort. We tested if face gaze patterns in the first 30, 60 and 120 s predict face gaze patterns in the remaining interaction. We performed secondary analyses of mobile eye-tracking data of 16 internal medicine physicians in consultation with 100 of their patients. Duration and frequency of face gaze were unrelated. The lack of association between duration and frequency suggests that research may yield different results depending on which measure of face gaze is used. Dwell time correlates both duration and frequency. Face gaze during the first seconds of the consultations predicted face gaze patterns of the remaining consultation time (R² 0.26 to 0.73). Therefore, face gaze during the first minutes of the consultations can be used to predict face gaze patterns over the complete interaction. Researchers interested to study face gaze may use these findings to make optimal methodological choices.

Minipuberty: Why Does it Happen?

Minipuberty describes the transient sex-specific activation of the hypothalamic-pituitary-gonadal (HPG) axis during the first 6 months of life in boys and during the first 2 years in girls. It leads to a rise of luteinizing hormone, follicle-stimulating hormone, estradiol, and testosterone. The existence of minipuberty has been known for >40 years, but we still do not fully understand why it takes place. Current thinking suggests that it is an essential imprinting period for different body functions. Firstly, minipuberty plays an important role in genital organ development; testosterone influences penile growth, the number of Sertoli cells, and spermatogenesis. Secondly, it seems to influence the infant's body composition; testosterone likely has an imprinting effect on BMI and body weight of boys and growth velocity in the first 6 months of life. Thirdly, it affects cognitive functions; testosterone has an impact on language organization in the infant brain and estradiol affects laryngeal sound production and baby babbling. There are inconsistent findings concerning the impact of minipuberty on sex-specific playing behavior. Minipuberty is an interesting field of research, and further studies in this area will teach us more about this exciting period of human development.

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.

The Measurement of Eye Contact in Human Interactions: A Scoping Review

Eye contact is a fundamental aspect of nonverbal communication and therefore important for understanding human interaction. Eye contact has been the subject of research in many disciplines, including communication sciences, social psychology, and psychiatry, and a variety of techniques have been used to measure it. The choice of measurement method has consequences for research outcomes and their interpretation. To ensure that research findings align with study aims and populations, it is essential that methodological choices are well substantiated. Therefore, to enhance the effective examination of eye contact, we performed a literature review of the methods used to study eye contact. We searched Medline, PsycINFO and Web of Science for empirical peer-reviewed articles published in English that described quantitative studies on human eye contact and included a methodological description. The identified studies (N = 109) used two approaches to assess eye contact: direct, i.e., assessing eye contact while it is occurring, and indirect, i.e., assessing eye contact retrospectively (e.g., from video recordings). Within these categories, eight specific techniques were distinguished. Variation was found regarding the reciprocity of eye contact between two individuals, the involvement of an assessor and the behavior of participants while being studied. Measures not involving the interactors in assessment of eye contact and have a higher spatial and temporal resolution, such as eye tracking, have gained popularity. Our results show wide methodological diversity regarding the measurement of eye contact. Although studies often define eye contact as gaze towards an exact location, this may not do justice to the subjective character of eye contact. The various methodologies have hardly ever been compared, limiting the ability to compare findings between studies. Future studies should take notice of the controversy surrounding eye contact measures.

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

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

The association between digit ratio (2D:4D) and overweight or obesity among Chinese children and adolescents: A cross-sectional study

BACKGROUND

The aim of this study was to analyze the relationship between 2D:4D and childhood overweight or obesity, which might indirectly show the role of prenatal hormone in childhood overweight or obesity.

METHODS

Using stratified cluster sampling approach, a school-based cross-sectional investigation was undertaken among 687 children and adolescents aged 8-15 years. Each participant's index finger (2D) and ring finger (4D) length, height, weight, testosterone, and estradiol levels were measured, and their dietary behaviors, physical and sedentary activities were also surveyed.

RESULTS

The prevalence of overweight or obesity among children and adolescents aged 8- and 12-15 years was 36.0% and 25.9%, respectively (P < 0.05). There were no associations between 2D:4D and overweight or obesity in both boys and girls (P > 0.05). However, 2D and multiplying index of digit ratio and length (MIDRL) associated directly with overweight or obesity among boys (OR (95%CI) were 1.45 (1.00, 2.14), 1.46 (1.01, 2.11), respectively), and was indirectly related to overweight or obesity by testosterone (OR were 1.30, 1.26, respectively).

CONCLUSIONS

In boys, 2D and MIDRL were directly related to overweight or obesity, and indirectly associated with overweight or obesity by testosterone, which suggested that prenatal hormone exposure might associate with overweight or obesity among boys.

Associations Between the 2nd to 4th Digit Ratio and Autism Spectrum Disorder in Population-Based Samples of Boys and Girls: Findings from the Study to Explore Early Development

The ratio of the index (2nd) finger to ring (4th) finger lengths (2D:4D) is a proxy for fetal testosterone and estradiol. Studies suggesting 2D:4D is inversely associated with autism spectrum disorder (ASD) in males were limited by lack of confounder and subgroup assessments. Studies of females are sparse. We examined associations between ASD and 2D:4D among children in the Study to Explore Early Development; we considered case subgroups and numerous potential demographic and maternal-perinatal health confounders. We observed a modest inverse association between ASD and right-hand 2D:4D in males; subgroup analyses indicated associations were limited to ASD cases with birth defects/genetic syndromes or dysmorphic features. We observed a positive association between ASD and left-hand 2D:4D in females, overall and within most case subgroups.

Digit ratio (2D:4D) and handgrip strength are correlated in women (but not in men) in Hui ethnicity

INTRODUCTION

Digit ratio, especially second-to-fourth digit ratio (2D:4D) is used as a biomarker of prenatal testosterone exposure and was found associated with physical ability, such as handgrip strength (HGS). Recently, this association has been discussed in several ethnic groups.

AIMS

To investigate correlations between 2D:4D and HGS in Chinese population of Ningxia Hui ethnicity.

METHODS

Here we firstly present the evidence on digit ratio and HGS of 346 college students (119 males and 227 females) by collecting photographs and HGS of both hands at Yinchuan city, in the Ningxia province of China.

RESULTS

Females have higher mean values of digit ratio than males; significant variances of 3D:5D (left and right: P<0.05), 2D:3D, 3D:4D (left and right: P<0.01) and 2D:4D, 2D:5D (left and right: P<0.001) were found between different sex. Males showed significantly greater HGS than females on both hands. 2D:4D of both hands were significantly negative correlated with HGS in females (but not in males).

CONCLUSIONS

2D:4D ratio is negatively correlated with HGS in a Chinese population (especially in females) of Ningxia Hui ethnicity.

Digit ratio (2D:4D) and handgrip strength in a Chinese population of Han ethnicity

BACKGROUND

In humans, the relative lengths of the index finger to the ring finger (2D:4D) is a sexually dimorphic trait which correlated with prenatal sex steroids and has been increasingly used as a promising tool to evaluate the impact of prenatal hormone exposure in some traits, such as physical performance. Handgrip strength (HGS) is one potent index of physical ability and its relationship with 2D:4D ratio has been discussed in several ethnic groups.

AIMS

To investigate whether there is a correlation between 2D:4D ratio and HGS in Chinese college students of Ningxia Han ethnicity.

METHODS

608 students (211 males and 397 females) of Han ethnicity were recruited from Ningxia medical university. Photocopies and HGS of both hands were collected at Yinchuan city, in the Ningxia province of China.

RESULTS

Sexual dimorphism of 2D:4D and HGS were found, males had significantly lower 2D:4D and greater HGS than females. 2D:4D in both hands were significantly negative correlated with HGS in females and not in males.

CONCLUSIONS

2D:4D ratio is negative correlated with HGS in a Chinese population of Ningxia Han ethnicity and this association should be considered on the anthropological research within an evolutionary concept in the future.

Interpreting digit ratio (2D:4D)-behavior correlations: 2D:4D sex difference, stability, and behavioral correlates and their replicability in young children

The popularity of using the ratio of the second to the fourth digit (2D:4D) to study influences of early androgen exposure on human behavior relies, in part, on a report that the ratio is sex-dimorphic and stable from age 2 years (Manning etal., 1998). However, subsequent research has rarely replicated this finding. Moreover, although 2D:4D has been correlated with many behaviors, these correlations are often inconsistent. Young children's 2D:4D-behavior correlations may be more consistent than those of older individuals, because young children have experienced fewer postnatal influences. To evaluate the usefulness of 2D:4D as a biomarker of prenatal androgen exposure in studies of 2D:4D-behavior correlations, we assessed its sex difference, temporal stability, and behavioral correlates over a 6- to 8-month period in 126, 2- to 3-year-old children, providing a rare same-sample replicability test. We found a moderate sex difference on both hands and high temporal stability. However, between-sex overlap and within-sex variability were also large. Only 3 of 24 correlations with sex-typed behaviors-scores on the Preschool Activities Inventory (PSAI), preference for a boy-typical toy, preference for a girl-typical toy, were significant and in the predicted direction, all of which involved the PSAI, partially confirming findings from another study. Correlation coefficients were larger for behaviors that showed larger sex differences. But, as in older samples, the overall pattern showed inconsistency across time, sex, and hand. Therefore, although sex-dimorphic and stable, 2D:4D-behavior correlations are no more consistent for young children than for older samples. Theoretical and methodological implications are discussed.

Sleep in infancy predicts gender specific social-emotional problems in toddlers

Despite strong evidence linking sleep to developmental outcomes, the longitudinal relationship between sleep and emotional well-being remains largely unknown. To address this gap in our knowledge, the current study examined sleep in infancy, measured via actigraphy, as a predictor of social-emotional problems in toddlers. A total of 47 children (29 males) were included in this longitudinal study. At time one, actigraphy measures of sleep were obtained from 3- to 4-month-old infants. At time two, parents rated their 18- to 24-month-old toddler's social-emotional well-being using the Brief Infant Toddler Social Emotional Assessment. Results indicated that boys tended to have higher levels of externalizing behaviors than did girls. Additionally, boys with longer sleep durations also showed lower sleep efficiency. In girls, sleep duration in infancy was a significant predictor of autism spectrum disorder behaviors and approached significance as a predictor of externalizing problems in toddlerhood. Our findings are the first to show a relationship between sleep measured in infancy and autism spectrum disorder symptomatology measured in early childhood. They suggest that the etiology of social-emotional problems may differ between genders and raise the possibility that sleep/wake cycles may be differentially related to autism spectrum disorder symptoms in girls and boys.

Environmental and genetic contributors to salivary testosterone levels in infants

Transient activation of the hypothalamic-pituitary-gonadal axis in early infancy plays an important role in male genital development and sexual differentiation of the brain, but factors contributing to individual variation in testosterone levels during this period are poorly understood. We measured salivary testosterone levels in 222 infants (119 males, 103 females, 108 singletons, 114 twins) between 2.70 and 4.80 months of age. We tested 16 major demographic and medical history variables for effects on inter-individual variation in salivary testosterone. Using the subset of twins, we estimated genetic and environmental contributions to salivary testosterone levels. Finally, we tested single nucleotide polymorphisms (SNPs) within ±5 kb of genes involved in testosterone synthesis, transport, signaling, and metabolism for associations with salivary testosterone using univariate tests and random forest (RF) analysis. We report an association between 5 min APGAR scores and salivary testosterone levels in males. Twin modeling indicated that individual variability in testosterone levels was primarily explained by environmental factors. Regarding genetic variation, univariate tests did not reveal any variants significantly associated with salivary testosterone after adjusting for false discovery rate. The top hit in males was rs10923844, an SNP of unknown function located downstream of HSD3B1 and HSD3B2. The top hits in females were two SNPs located upstream of ESR1 (rs3407085 and rs2295190). RF analysis, which reflects joint and conditional effects of multiple variants, indicated that genes involved in regulation of reproductive function, particularly LHCGR, are related to salivary testosterone levels in male infants, as are genes involved in cholesterol production, transport, and removal, while genes involved in estrogen signaling are related to salivary testosterone levels in female infants.

Postnatal testosterone concentrations and male social development

Converging evidence from over 40 years of behavioral research indicates that higher testicular androgens in prenatal life and at puberty contribute to the masculinization of human behavior. However, the behavioral significance of the transient activation of the hypothalamic-pituitary-gonadal (HPG) axis in early postnatal life remains largely unknown. Although early research on non-human primates indicated that suppression of the postnatal surge in testicular androgens had no measurable effects on the later expression of the male behavioral phenotype, recent research from our laboratory suggests that postnatal testosterone concentrations influence male infant preferences for larger social groups and temperament characteristics associated with the later development of aggression. In later assessment of gender-linked behavior in the second year of life, concentrations of testosterone at 3-4 months of age were unrelated to toy choices and activity levels during toy play. However, higher concentrations of testosterone predicted less vocalization in toddlers and higher parental ratings on an established screening measure for autism spectrum disorder. These findings suggest a role of the transient activation of the HPG axis in the development of typical and atypical male social relations and suggest that it may be useful in future research on the exaggerated rise in testosterone secretion in preterm infants or exposure to hormone disruptors in early postnatal life to include assessment of gender-relevant behavioral outcomes, including childhood disorders with sex-biased prevalence rates.