The study aimed to determine 2D:4D digit ratios and their relationships in first-time mothers and their children in Ghana. The study did not find any considerable differences in digit ratios between male and female children and there was no association between digit ratios and age. Also, mothers’ right-hand digit ratio was markedly higher compared to their children but did not determine offspring sex. There was rightward directional bias in mothers and a leftward directional bias in male children with the observation of a positive association in the right-hand digit ratios between mothers and their children. Lastly, increased digit ratio in mothers was associated with increased blood serum ALT and there were no substantial differences between the actual height of mothers and height predicted using regression models based on their digit ratios.
The prove of the effect of prenatal androgen exposure on digit ratio is indicated by the occurrence of sexual dimorphism in digit ratios at an early age, according to Manning, et al. 7. This current study, however, did not find substantial differences in both the left and the right-hand digit ratios between male and female children. This finding was consistent with a previous study by Yamada, et al. 5, who studied 1,045 children aged 1½ years from the JECS-A (the Aichi regional sub-cohort of the Japan Environment and Children's Study) cohort study in Japan. The authors adopted an easy-to-use photographic method to measure digit lengths. They explained that their findings may be due to difficulties in digit measurements at that age or the rapid growth that occurs at that stage of human development that may have neutralized the in utero sexual dimorphism 5. A similar study by Barrett, et al. 18 among 321 children from the Infant Development and the Environment Study (TIDES) in America found no significant differences in digit ratios between male and female children. The children in their study were, however, 4-year-olds and the researchers directly measured digit lengths with Vernier dial callipers. They explained that sex differences in digit ratios in children, tended to be less consistent and the effect sizes, smaller compared to adults. They also suggested that method variation in digit measurements may have accounted for the inconsistencies in the results of previous studies. In contrast to this current study, Ventura, et al. 6 studied 106 newborn babies in a prospective study conducted at the maternity of Dona Estefânia Hospital (HDE) in Lisbon, where they found reduced digit ratio in the left-hand of boys, which was contrary to the popular belief of right-hand sexual dimorphism. The authors' measured digit lengths from photocopies of the ventral palmar surfaces, and observed substantial overlap between male and female babies with only subtle differences. Similar to the study of Ventura, et al. 6, a study by Ertuğrul, et al. 19, involving 225 newborn infants, found reduced digit ratios in both hands of males regardless of whether the infants were inbred or outbred. In their study, however, digit lengths were directly measured with Vernier callipers and not by computer-assisted analysis.
Although considerable differences in digit ratio were not found between male and female children, there was, however, directional asymmetry in mothers and male children. The left-hand 2D:4D digit ratio in male children was markedly higher than the right-hand (leftward bias) but the reverse was true for the mothers (rightward bias). This was consistent with a study by Richards, et al. 20 who found that left-hand 2D:4D digit ratio was considerably higher than the right-hand digit ratio but found no differences in female children. This study used data from a previous study by Ventura, et al. 6 where digit ratios were estimated from photocopies of 106 newborn babies. Another study involving 1,013 participants from 4 countries, found asymmetry in digit lengths. The age range in this study was, however, between 2–90 years and digit lengths were directly measured using Vernier callipers. Further confirmation of the finding in this study can be found in the work of Voracek, et al. 10, who studied digit ratios in a sample of about 3,000 participants and found that leftward bias was more frequent in males while rightward bias was more frequent in females. The study by Voracek, et al. 21, was a replication of the findings of a previous study by Puts, et al. 22 who analysed data from about 500 individuals. Differences in directional asymmetry are a further indication of the effect of prenatal androgen exposure and its influences on digit ratios in males and females 10.
There were no considerable correlations between age and digit ratios in both the mothers and their children in this study. However, Körner, et al. 23 who also used scanned images in a cohort study involving 274 children in Düsseldorf, Germany measured digit ratios in both hands in males and females at ages 5, 9, 20 and 40 months. The authors reported that age did not have an impact on digit ratios from 9–20 months after birth but had an effect from 20–40 months. Their study population was predominantly Caucasian. Manning, et al. 7 earlier on hypothesized that digit ratios were established in utero and only gain stability about 2 years of age after the postnatal testosterone surge. In contrast to Manning, et al. 7, previous longitudinal studies, one from the Jamaican Symmetry Project, have reported that 2D:4D digit ratios increased from infancy to adulthood 8,9. A 2D:4D digit ratio associations studies in Wales by Richards, et al. 24 found a positive correlation between age and digit ratios in children but a negative correlation in adults. In their study, however, digit lengths were measured from hand scans in 585 parent-child pairs who were mostly White European, aged from 5–89 years. Longitudinal variations in digit ratios might have also accounted for the observed higher digit ratios in mothers than their children, consistent with the findings in a study by Ventura, et al. 6.
Correlational analysis in this study found a positive association between mother and child but only in the right-hand digit ratio. In line with our findings, a study involving 673 mother-child pairs, whose hands scans were studied for 2D:4D found positive associations between mother and child digit ratios. However, this study involved children who were 2–5-year-old from the MIREC (Maternal-Infant Research on Environmental Chemicals) cohort study in Canada 12. Also, a study among families of the Chuvasha and Bashkortostan Autonomies of the Russian Federation revealed parent-offspring correlations in 2D:4D digit ratios. Instead of hand scans, the authors' measured digit ratios from X-ray radiographic images in 1,541 people 25. A positive correlation between 2D:4D digit ratio of mothers and newborn daughters have been reported by Ventura, et al. 6 and Richards, et al. 24. Previous familial studies on digit ratios have indicated that the heritability of 2D:4D digit ratio was up to 57% for the right-hand and 48% for the left -hand 11. Although the process of digit ratio heritability is not well understood and will require further studies, genetic factors have been suggested 11,12.
There was no association between mother’s digit ratio and sex of the child in this study. This was, however, contrasted by a study by Ventura, et al. 6 who observed that mothers with reduced 2D:4D ratios bore male children. Previous studies have reported a negative correlation between 2D:4D digit ratios and the proportion of their male offspring which is in line with the hypothesis that the male sex at conception in mammals correlates with high parental testosterone level (low 2D:4D)6,13.
Digit ratios were associated with some phenotypic traits of the mothers. Increased blood serum ALT was positively associated with digit ratio of both the left and the right-hands. Previous studies have drawn some levels of association between digit ratios and blood serum analytes such as cortisol 16 and lipids 17. Although there was no correlation between the observed and predicted height of mothers, there were no significant differences between them. This indicated an agreement between the observed and predicted height such that the height of the mothers in the study population could be accurately predicted from regression models based on 2D:4D digit ratio 26. A previous study by Barut, et al. 15 in a 386 study population, had indicated an association between directly measured 2D:4D digit ratios and height in right-handers. This is a significant finding that will be useful in medico-legal investigations where only fragmented body parts may be the only evidence available for height estimation.
This current study has many strengths. One of them is that this study is among the very few studies on digit ratios and their relationships in a mother-and-child population in Ghana and maybe the first in children under 2 years of age in the northern part of the country. Also, the study adopted a computer-assisted image analysis for digit measurements which has been found, by previous studies, to be more reliable than direct measurements, photocopies or printed scanned images 27,28. Another strength of this study is the use of the Bland-Altman method to estimate the level of agreement between the repeat measurements even after calculating the intraclass correlation coefficients (ICCs). The correlation between two variables only quantifies the degree to which the variables are related but does not usually imply their level of agreement 26. Despite the many strengths of this study, the authors acknowledge some weaknesses. The study may have been underpowered which may not allow for the generalization of the findings. Also, there was under-representation of minority ethnic groups, as the setting of the study was mainly in an area that is dominated by people of the Mole-Dagomba ethnic group. We conclude that the 2D:4D digit ratio was not significantly different between male and female children but exhibited directional asymmetry and familial associations. Also, the 2D:4D digit ratio was associated with blood serum ALT levels. The height of first-time mothers can be predicted from regression models derived from digit ratios. To support our finding, the authors will recommend further studies involving a larger sample size with equitable ethnic representations.