Finger ridge‐counts correlate with the second to fourth digit ratio (2d:4d)

OBJECTIVES: This study examines the relationship of finger ridge-counts to second to fourth digit ratio, which has not yet been definitively demonstrated. The related question of sex dimorphism in finger ridge-counts is further elucidated. METHODS: A sample of Germans, including 1134 males and 1031 females, was examined for sex dimorphism in the finger ridge-counts. Second and fourth digit lengths were measured in a sub-sample of 91 males and 100 females to compute second to fourth digit ratio. Principal component scores were obtained to investigate sex dimorphism and the correlation between ridge-counts and digit ratio. Regression and analysis of covariance were used to investigate relationships. RESULTS: Males generally have higher ridge-counts than females but subtle dimorphic features emerge from the principal components, such as a contrast between digits 2 and 4, suggesting a ratio analogous to the digit ratio. The most dimorphic feature is digit 1 directional asymmetry, males exhibiting a stronger right bias than females. Digit ratio is significantly related to four principal components, expressing various contrasts among digits. Other relationships involve contrasts between digits 3 and 5 and asymmetry on digit 2. CONCLUSIONS: This paper provides definitive evidence that finger ridge-counts correlate with second to fourth digit ratio. The most important finding is that associations of ridge-counts with digit ratio do not involve commonly used summary counts over all digits. Rather, associations act more locally, in ways paralleling the digit ratio, in others reflecting asymmetry. The results support the idea that prenatal sex hormones affect finger ridge-counts.