Maternal 2nd to 4th digit ratio does not predict lifetime offspring sex ratio at birth

The association between the 2D:4D ratio and offspring sex at birth: A cross-sectional study in Ghana

OBJECTIVES

The 2D:4D ratio is the putative marker of prenatal androgen exposure. Low maternal 2D:4D ratio has been associated with high or male-biased secondary sex ratio. Hitherto, there has not been any study in Ghana regarding the maternal 2D:4D ratio and sex ratio at birth. This study sought to investigate this observation in a Ghanaian population.

METHOD

The study was cross-sectional from December 2020 to April 2021 involving 272 first-time mothers. The mean ± SD age of the mothers was 23.9 ± 3.67 years. The right (2D:4DR), the left (2D:4DL), the mean (M2D:4D) digit ratios and the right-left difference (Dr-l) of the mothers were measured using computer-assisted analysis. The mothers were stratified by their digit ratios, and the cohort sex ratio (CSR) for each stratum was then calculated as the proportion of sons.

RESULTS

The mean ± SD of the 2D:4DR of mothers-with-daughters and mothers-with-sons were 0.941 ± 0.032 and 0.933 ± 0.037, respectively. The mean ± SD of the 2D:4DL of mothers-with-daughters was 0.934 ± 0.034, while that of mothers-with-sons was 0.931 ± 0.039. The offspring sex at birth and the CSR was not associated with either the mother's right, left, mean 2D:4D ratio or their difference (Dr-l). However, mothers-with-daughters showed rightward asymmetry in their 2D:4D ratios as the Dr-l significantly and positively deviated from zero (p < .010).

CONCLUSION

The maternal 2D:4D ratio may not be associated with their offspring sex at birth among first-time mothers. This study adds to the limited data on studies regarding offspring sex at birth and the 2D:4D ratio in Ghana and Sub-Saharan Africa.

Does the Mother or Father Determine the Offspring Sex Ratio? Investigating the Relationship between Maternal Digit Ratio and Offspring Sex Ratio

OBJECTIVE

In mammals, high parental testosterone levels present around the time of conception are thought to skew offspring sex ratio toward sons. The second to fourth digit ratio (digit ratio) is now widely accepted as a negative correlate of prenatal testosterone. Thus, we investigated the association between digit ratio and offspring sex ratio.

METHODS

A total of 508 Korean patients (257 males and 251 females) less than 60 years old who had one or more offspring were prospectively enrolled. The lengths of the 2nd and 4th digits of the right hand were measured by a single investigator using a digital vernier calliper. Next, the patients' lifetime offspring birth sex ratios were investigated.

RESULTS

Maternal (rather than paternal) digit ratio was significantly associated with the number of sons (r = -0.153, p = 0.015), number of daughters (r = 0.130, p = 0.039), and offspring sex ratio (r = -0.171, p = 0.007). And, the maternal digit ratio was a significant factor for predicting offspring sex ratio (B = -1.620, p = 0.008) on multiple linear regression analysis. The female patients with a lower digit ratio (< 0.95) were found to have a higher offspring sex ratio (0.609 versus 0.521, p = 0.046) compared to those with a higher digit ratio (≥ 0.95). Furthermore, females in the low digit ratio group have a probability 1.138 greater of having sons than females in the high digit ratio group.

CONCLUSIONS

Maternal digit ratio was negatively associated with offspring sex ratio. Females with a lower digit ratio were more likely to have more male offspring compared to those with a higher digit ratio. Thus, our results suggest that the sex of offspring might be more influenced by maternal rather than paternal factors.

Hand asymmetry does not relate to key life history traits in post-menopausal contemporary Finnish women

Associations between fluctuating asymmetry (FA, a putative marker of developmental instability, DI) and life history traits have received a great deal of attention in the non-human literature. However, the patterns found are very heterogeneous and generalizations are difficult to make. In humans, only a few studies have related FA to life histories and fitness. In this paper we study such relationships using hand FA and several key life history traits in 209 post-menopausal Finnish women born between 1946 and 1958. Asymmetry measurements were based on scans of the hands and the life histories of these women were collected using questionnaires. No significant associations were detected and trends were opposite to expectations. We did find evidence for directional asymmetry, as traits in the right hand were larger on average. This may be due to handedness, questioning the usefulness of hand FA as a measure of DI. We conclude that future studies in humans should carefully examine the usefulness of traits as measures of DI.

Does 2D:4D predict fitness in a wild mammal?

Environmental pressures present during critical periods in fetal development can have a strong influence on the long-term fitness of an individual. Susceptibility to parasites, immune function, and future reproductive success are all vulnerable to stressful events in utero. The causes and consequences of prenatal environmental stress are often difficult (if not impossible) to evaluate, especially in wild populations. Digit ratio, the ratio of 2nd digit to 4th digit length (2D:4D) has been identified as an index of fetal androgen exposure. Current techniques for assessing digit ratio pose severe limitations to the accuracy of digit measurement of wild animals owing to the constant movement of the feet and inaccessibility of laboratory equipment. Our study attempts a new indirect technique wherein subjects had an imprint taken of each foot that was then photographed and scanned for digital measurement. Using red squirrels ( Tamiasciurus hudsonicus (Erxleben, 1777)) as a model species, we hypothesized that higher degrees of fetal testosterone exposure (assumed from digit ratios) would result in reduced reproductive success and increased susceptibility to parasite infection. Digit ratio was negatively correlated with reproductive output and positively correlated with parasite loads. Although this may indicate that exposure to excess testosterone in utero may jeopardize future fitness, the relationships are inconsistent.

Behavioural and biological determinants of human sex ratio at birth

The human sex ratio SR (proportion male) at birth has been reported to vary with many variables. The explanation of this variation is not established, but I have hypothesized that it is partially caused by the hormonal concentrations of both parents around the time of conception. The present note suggests how this hypothesis might accommodate recent sex ratio findings relating to 'psychosexual restriction', female genital cutting, sexes of prior sibs, finger length ratios, the autism spectrum disorder, parental occupation and maternal eating disorders. Tests of such suggestions are offered, and it is hypothesized that: (a) in women, Manning's R (the ratio of the lengths of the 2nd and 4th digits) is positively correlated with offspring sex ratio (proportion male); (b) women who have undergone female genital cutting (FGC) have high androgen levels; (c) offspring sex ratio correlates positively with 'masculinity' of parental occupation, the correlation being mediated by testosterone levels. It is noted that the lines of evidence for three hypotheses (James', Manning's and Baron-Cohen's) are mutually supportive.

Does second-to-fourth digit length ratio (2D:4D) predict age at menarche in women?

Prenatal steroid levels, estimated as the ratio of second-to-fourth digit length (2D:4D), have been related to reproductive success in women, but direct associations between 2D:4D and physiological measures of fertility remain rare. A recent study reported that lower, masculinized right hand 2D:4D was correlated with delayed age at menarche in women. We addressed this question by investigating whether 2D:4D was associated with recalled age at menarche in 282 post-reproductive Finnish women, using Cox regression model that controlled for a woman's sibling composition, urban or rural residence and temporal trend in menarcheal age. We found no evidence that neither the 2D:4D of the right nor the left hand were related to a woman's age at menarche among these Finnish women.

Scientometric analysis and bibliography of digit ratio (2D:4D) research, 1998-2008

A scientometric analysis of modern research on the second-to-fourth digit ratio (2D:4D), a widely studied putative marker for prenatal androgen action, is presented. In early 2009, this literature totalled more than 300 publications and, since its initiation in 1998, has grown at a rate slightly faster than linear. Key findings included evidence of publication bias and citation bias, incomplete coverage and outdatedness of existing reviews, and a dearth of meta-analyses in this field. 2D:4D research clusters noticeably in terms of researchers, institutions, countries, and journals involved. Although 2D:4D is an anthropometric trait, most of the research has been conducted at psychology departments, not anthropology departments. However, 2D:4D research has not been predominantly published in core and specialized journals of psychology, but rather in more broadly scoped journals of the behavioral sciences, biomedical social sciences, and neurosciences. Total citation numbers of 2D:4D papers for the most part were not larger than their citation counts within 2D:4D research, indicating that until now, only a few 2D:4D studies have attained broader interest outside this specific field. Comparative citation analyses show that 2D:4D research presently is commensurate in size and importance to evolutionary psychological jealousy research, but has grown faster than the latter field. In contrast, it is much smaller and has spread more slowly than research about the Implicit Association Test Fifteen conjectures about anticipated trends in 2D:4D research are outlined, appendixed by a first-time bibliography of the entirety of the published 2D:4D literature.