Do skin and hair pigmentation in prepubertal and early pubertal stages correlate with 2D:4D?

Effects of caste, birth season, and family income on digit ratios

OBJECTIVES

The contributions of latitude and ethnicity in the determination of cross-society differences in digit ratios are unclear. In India, different castes (endogamous groups) have been living in the same areas (villages or towns) for the last 1500 years and, therefore, these groups may have different gene pools without a latitude-related difference component. Thus, in the present study, we studied the effect of caste on digit ratios. We also studied the effects of sex, birth season, and family income on digit ratios.

METHODS

We selected a sample of 301 college students (age: M = 19.9 years, SD = 2.63) in Muzaffarnagar city of western Uttar Pradesh, India, and asked participants for information regarding their birth month, religion, caste, and monthly family income. We measured participants' dorsal and palmar digit lengths (of all fingers, except the thumb, in both hands) using vernier calipers of 0.01 mm accuracy.

RESULTS

Other backward castes (intermediate castes) had longer digit lengths than general castes (upper castes), scheduled castes (lower castes), and Muslims. However, there was no difference in digit ratios of caste groups (scheduled castes vs. other backward castes vs. general castes vs. Muslims) or specific castes (Chamar-Jatav vs. Jat vs. Pandit-Tyagi). Winter-born women had lower left dorsal 2D:4D and 3D:4D ratios than summer-born women. Family income was related to higher dorsal 2D:4D and 3D:4D ratios among women. Moreover, in dorsal digit ratios, sex difference (men < women) occurred in digit ratios constituting digit 5, whereas, in palmar digit ratios, sex difference occurred in digit ratios constituting digit 2.

CONCLUSIONS

The present study suggests that endogamy-led genetic difference in ethnic/caste groups is not a determinant, whereas birth season (i.e., the exposure to sunlight) and family income might be determinants of digit ratios. In addition, compared to palmar digit ratios, dorsal digit ratios are better markers of sexual dimorphism.

Maternal Age and Behavior during Pregnancy Affect the 2D:4D Digit Ratio in Polish Children Aged 6–13 Years

Background: The length of the second and fourth finger calculated as a ratio (2D:4D) is a marker of prenatal exposure to sex hormones. Higher exposure to testosterone is related to a lower 2D:4D digit ratio, and inversely, higher exposure to estrogen is related to a higher 2D:4D. The digit ratio in humans (prenatally determined by sex hormone levels and androgen receptor activity) is associated with multiple biological, cognitive, and behavioral traits, as well as health in later life. The aim of this study was to verify if maternal traits during pregnancy are associated with 2D:4D among their children. Methods: We investigated 537 boys, 646 girls, and their mothers. The investigation consisted of a questionnaire and a measurement part. In the questionnaire, we included questions about maternal traits during pregnancy such as: illnesses, active and passive smoking, work activity, psychological trauma (death or serious illness of a loved one, divorce, job loss), and age. We performed length measurements of the second and fourth fingers on both hands for both study groups. Results: The GLM analysis showed that children of smoking mothers were characterized by a higher 2D:4D R as compared to their peers whose mothers did not smoke (β = 0.10, p = 0.0008). In turn, the offspring of women who worked during pregnancy exhibited lower 2D:4D R values than the children of women who did not work (β = −0.07, p = 0.0233). It should be noted, however, that the effects of those maternal factors were small, as each of them explained less than 1% of 2D:4D R in the population, adjusted for child age and sex. Pearson’s linear correlation revealed that maternal age was negatively correlated with 2D:4D R in daughters (r = −0.11, p = 0.0137), but not in sons (r = 0.02, p = 0.6908). The negative correlation of 2D:4D R with maternal age indicates that the daughters of older mothers exhibited lower values of that ratio relative to the same-age daughters of younger mothers. Conclusions: For both study groups, the right-hand digit ratio was positively correlated with maternal smoking and negatively with maternal work during pregnancy. Maternal age during pregnancy was negatively related to right hand 2D:4D only among girls.

Thyroid diseases and second to fourth digit ratio in Polish adults

The association between second to fourth finger ratio and thyroid diseases is unexplained. There is a possible interaction between prenatal exposition to sex hormone and thyroid functions in the adulthood. The study included 175 adults investigated in Łódź in the central Poland. It consisted of two main parts: a survey including questions about occurrence of thyroid gland dysfunction and anthropometric measurements (body mass and height and length of the second and fourth finger, waist and hip circumferences). The women who had thyroid disease had higher 2D:4D digit ratio (left hand) (mean = 1.004; SD = 0.036) than healthy ones (mean = 0.989; SD = 0.030) (t = − 2105; p = 0.038; d = 0.707). The association between thyroid diseases occurrence and prenatal steroid hormone exposition is noticed. Only females who had thyroid diseases tend to have higher 2D:4D digit ratio, for left hand.

Association patterns between 2D:4D ratio and age at menarche: the impact of methodology

The ratio of index finger to ring finger length (2D:4D) is a sexually dimorphic feature and widely used as an indicator of prenatal androgen-estrogen exposure. Several studies have investigated the relationship between the 2D:4D ratio and the onset of the first menstrual period (menarche) in women.

The present study tested the association patterns between 2D:4D ratio and age at menarche. Furthermore, the impact of methods of measuring finger lengths was considered.

Two samples were used to conduct the study. One sample consisted of 110 women who self-measured their finger lengths. The finger length measurements in the second sample (88 women) were taken from one trained observer using the caliper-based technique. Age at menarche was determined using a retrospective method.

Women from the first sample reported an average age for the onset of the first menstrual bleeding at 12.9 (SD = 1.4) years of age. There was no significant association between the left and right 2D:4D ratio and the age at menarche. The second sample showed a mean age at menarche at 12.8 (SD = 1.3) years of age. Here, the right hand 2D:4D ratio and age at menarche were significantly correlated (p<0.001).

A more feminine 2D:4D ratio was significantly associated with an earlier menarcheal age only in the sample with direct finger length measurements by a trained investigator. The sample using self-measurements yielded no significant associations between menarcheal age and 2D:4D. The use of a reliable and well-founded methodology is essential for obtaining meaningful results.