The 2D:4D magnitude and its sexual dimorphism remained stable throughout the ontogeny; both sex and population origin were permanent reliable predictors of 2D:4D, whereas average digit length was not

Sex, population origin, age and average digit length as predictors of digit ratio in three large world populations. Marina Butovskaya, Valentina Burkova, Yulia Apalkova, Daria Dronova, Victoria Rostovtseva, Dmitriy Karelin, Ruzan Mkrtchyan, Marina Negasheva & Valery Batsevich. Scientific Reports volume 11, Article number: 8157. Apr 14 2021. https://www.nature.com/articles/s41598-021-87394-6

Abstract: Recently, a number of authors have claimed that sexual dimorphism in the second-to-fourth digit ratio (2D:4D) is simply dependent on digit length and is an artifact of allometry. The goal of our study is to verify the validity of these assumptions. The study sample comprised 7,582 individuals (3,802 men and 3,780 women) from three large world populations: Europeans (n = 3043), East Africans (n = 2844), and Central Asians (n = 1695). The lengths of the second and fourth digits on both hands were measured. Digit ratios were computed according to standard procedures. Analyses were conducted separately for each hand for the whole sample and in succession for the three large populations. Additionally, we separately tested four age cohorts (≤ 13, 14–18, 19–30, and 31 ≥ years) to test the effect of developmental allometry. The second and fourth digits showed strong positive linear relationships on both hands, and demonstrated an increase with age; digit length in women from the youngest age cohort was longer or equal to that of men, and shorter than men in older age cohorts. However, the 2D:4D magnitude and its sexual dimorphism remained stable throughout the ontogeny. To test for an allometric effect on 2D:4D, the average digit lengths were calculated. Both sex and population origin were permanent reliable predictors of 2D:4D, whereas average digit length was not. Height was applied as another measure of allometric effect on the limited sample (≤ 30 years) from the European population, along with sex and age. No allometric effect was observed in this case. We conclude that sex differences in 2D:4D are not an artifact of allometry.

Discussion

The main conclusion of our study is that 2D:4D ratios on the right and left hands were sexually dimorphic for the whole sample, as well as separately for all three tested populations. This was not the case with the second and fourth digits and their averages. The effect sizes, Cohen’s d of sex differences for the 2D:4Ds, as well as for the second and fourth digits, and averages for both digits ranged from small to medium. We demonstrate that for the whole sample, as well as for separate populations, every 0.9 cm increase in the second digit was related to a 1.0 cm increase in fourth digit. The digit lengths (second and fourth) increased substantially from childhood to adulthood, and there was a strong positive correlation between second and fourth digits across individuals. These findings are in accordance with those of earlier studies43. Sexual dimorphism in digit lengths, evident in human adults, was not observed in prepubertal children, which is again in line with previously reported data48.

The developmental allometry effects were tested in four age cohorts, with special emphasis on younger subsamples. This was conducted in accordance with the knowledge about the intensive growth of fingers in this period, as well as existing data on changes in growth patterns from childhood to young adulthood. The fourth age cohort (older adults) was substantially underrepresented and contained an insufficient number of individuals of European and Asian origin. Hence, we refrained from drawing specific conclusions for this age cohort.

For the whole sample, Cohen’s d for the 2D:4D ratios and digit lengths were of comparable sizes. In the case of separate prepubertal, pubertal, and young adult age cohorts, the situation was radically different. The data on sex differences in digit lengths in prepubertal and pubertal cohorts were in accordance with general expectations that females reach puberty considerably earlier than males68,69,70. On average, these results resulted in one to two-year differences71. Females in the prepubertal age cohort had significantly longer second digits and significantly higher 2D:4D ratios on both hands than males, whereas no sex differences for the fourth digits on both hands were found. For the prepubertal sample, the effect sizes for 2D:4D (right and left hand) were approximately 0.5 standard deviations, while for separate and average digits, the effect sizes were four times lower at the minimum.

Obvious population differences need to be considered. For prepubertal children in the African population, both the second and fourth digits were significantly longer in females, whereas for Europeans of the same age cohort, this was true only for the second digit; for Asians, no sex differences in digit length were present for prepubertal children. For the second age cohort, both fingers became significantly longer in males for Europeans and Asians, but were of equal lengths in males and females from the African sample. These differences suggest that respondents from African samples matured slower, and developmental processes in this population had different trajectories than their European and Asian peers. In young and older adults, both the second and fourth digits on both hands were significantly longer for males in all three populations. The 2D:4D ratio magnitude of sex differences essentially remained stable throughout ontogeny (in all four age cohorts) and of medium effect size. According to the logic of Lolli et al.41,42, the 2D:4D values would have to decrease with an increase in digit length, particularly in prepubertal and pubertal samples. In reality, the 2D:4Ds were remarkably stable with age, despite the increase of second and fourth digit length during ontogeny and the reversions in finger lengths in males and females that occurred during puberty.

Our results based on cross-sectional data are in line with other cross-sectional data, as well as with longitudinal studies30. Our data simultaneously revealed some population-specific variations in ontogenetic trajectories. In particular, in the African sample, female digits remained longer than male digits until 15–16 years of age (mid-adolescence), while in European and Asian samples, finger lengths were inversed before the age of 14 and remained longer in males than in females in all older ages. The differences obtained for Africans may be caused by specific life history trajectories with slower maturation, resulting from a mixture of environmental and social stressors, including malnutrition, a high risk of infections, and limited access to modern medical assistance in rural African populations72,73,74,75,76.

The GLM ANCOVA tests conducted in our study demonstrated the significant effects of sex and population origin (medium size), and a small effect size for age as predictors of 2D:4D. However, the average finger length was not a significant predictor for the right 2D:4D in accordance with recently reported data for adult Hadza males by other authors77. The effect of height as another potential marker of allometry on 2D:4D has not been detected in a limited sample of respondents until the age of 30; however, the data on height were tested for Europeans only. We do not know if the same peculiarities will be present in other large world populations as well as in older age cohorts. More data in this respect will be needed in the future to confirm these results.

Many previous studies have demonstrated that population/ethnic origin may be an important predictor for the 2D:4D ratio36,37,78,79,80,81,82, and our data support these conclusions. Along with numerous environmental factors, the heritability factor needs to be considered in this respect83,84,85. In particular, twin studies provide an estimate of approximately 60%76. Another support in favour of the inheritance of digit ratios has recently been presented by Chuvashian studies85,86. The clear familial aggregation of 2D:4D ratio variation in the Chuvashians, with significant parent–offspring and sibling correlations, was unrelated to common environmental effects. Hence, along with the various environmental and socio-cultural factors, certain genetic effects also need to be considered and tested with more care in the future.

In this study, we refrained from analysing the right-left difference in 2D:4D (D[R-L]). This was done deliberately, not only to limit the amount of information for analysis, but also for the following reasons: 1. the lack of studies on the validity of this marker; 2. using asymmetry in digits two and/or four may cause biases in the associations between asymmetry and digit ratios87; and 3. currently expressed concerns regarding the utility of D[R-L] as an indicator of prenatal androgen exposure22.

In summary, our data suggest that there is no reason to reject the sexual dimorphism of 2D:4D associated with both prenatal and postnatal factors. Hence, we should not throw the baby out with the bathwater. The sex differences in second and fourth digit lengths were not stable within ontogeny, and even reversed in adulthood, whereas 2D:4D ratios remained unchanged since six years of age. The ontogenetic transformations in finger lengths in boys and girls do not make the sex effect on 2D:4D less statistically significant. The effect size of sex was higher than the average digit length in all cases, and height (in the case of Europeans, less than 30 years of age). The theory according to which the sex difference in 2D:4D has been driven by the sex difference in digit length may have arisen from a misunderstanding and incorrect assumption ignoring the human growth pattern trajectory. The stability of 2D:4D may be an example of homeostasis of form, and our data completely support J. Manning and B. Fink’s idea30. Another important conclusion is that the degree to which androgenisation (2D:4D being a potential proxy) affects particular behaviour or morpho-physiological conditions may be population- and situation-specific and culturally mediated. Our data, along with currently accumulated information from different world populations, call for treating the allometric effect on 2D:4D with caution. It is worth stressing the importance of differentiation between the static and developmental allometry effects, and the necessity of considering as many factors as possible (genetics, particularly population origin, environmental and social factors affecting maturation, urbanisation effects, etc.) while searching for explanations of 2D:4D sex differences30,88. Populations currently undergoing rapid transformations due to global and local changes must be treated with special care. Representatives of different age cohorts may differ in their maturation patterns, particularly the timing and duration of growth morphological changes. Due to the small to medium effect sizes that are usually obtained, studies using 2D:4D may need to consider very large sample sizes in order to be of practical use89. However, 2D:4D remains a useful measure of sexual dimorphism for anthropologists. Along with a set of other sexually dimorphic traits, it may be used in projects dealing with ecological and socio-cultural transformations in growth and development in contemporary representatives from large- and small-scale societies.