Sexual dimorphism in the ontogeny of second (2D) and fourth (4D) digit lengths, and digit ratio (2D:4D).
Level 4 - case-series / case-control
Cross-sectional observational study by CEBM design analogy
PubMed 30133905 · doi:10.1002/ajhb.23138
What was done
The authors examined sex differences in second (2D) and fourth (4D) digit lengths and digit ratio (2D:4D) across two cohorts: children aged 2 to 18 years (Sample I, n = 680) and adults aged 18 to 30 years (Sample II, n = 89,246). Digit length was determined by direct experimenter measurement in Sample I and direct self-measurement in Sample II. Data were analyzed using two-factor ANOVAs for sex and year-group.
What was found
The abstract reports statistical directions without exact numbers. In both samples, digit length was positively related to age, showing significant main effects of sex and year-group, as well as a significant interaction effect on digit length. Boys had longer digits than girls only after 13 years of age. Men had longer digits than women, with dimorphism increasing from 18 to 30 years. Sex differences in 2D:4D were significant (males < females), but there was no significant effect of age and no interaction effect of age and sex on 2D:4D in either children or adults.
Why it matters
The findings indicate that while individual 2D and 4D digit lengths and their sexual dimorphism change throughout development, the 2D:4D ratio remains stable and age-independent between ages 2 and 30.
Limits
The study relies on cross-sectional age comparisons rather than a true longitudinal design. Digit measurements in the adult cohort were self-reported rather than taken by trained experimenters. The abstract does not provide numerical values, effect sizes, or confidence intervals.
Cited by
- supports The ratio of the second digit to the fourth digit (2D:4D) tends to be smaller in men than in women, and this sex difference is present in children.