The First Allometric Analysis in Newborns Rejects the Allometry Hypothesis for 2D : 4D : Strong Developmental Evidence for Prenatal Hormonal Programming

OBJECTIVE: This study aimed to evaluate the presence of sexual dimorphism in the second-to-fourth digit length ratio (2D:4D) and to systematically investigate the contribution of potential allometric interactions to this variation in a sample of Turkish newborns (N = 225, 125 girls). Investigating newborns provides a unique opportunity to determine whether 2D:4D sexual dimorphism is primarily shaped by prenatal hormonal programming or by postnatal growth-related allometric interactions. MATERIALS AND METHODS: Digit lengths (2D and 4D) were measured for both hands with 0.01 mm precision. Statistical analyses included ANOVA, Kruskal-Wallis, Spearman correlation, and Analysis of Covariance (ANCOVA), utilizing the mean 4D length as a covariate to isolate the size effect. RESULTS: Male newborns exhibited significantly lower right and left 2D:4D ratios compared to females (right d = 0.68; left d = 0.80). Although males had significantly longer 4D lengths, ANCOVA results demonstrated that the sexual dimorphism in the 2D:4D ratio remained significant and independent of the 4D size effect (Right p = 0.003; left p = 0.001). Furthermore, when 4D length was statistically controlled, 2D length itself significantly differed between sexes, suggesting independent hormonal influence on 2D development. CONCLUSION: The findings provide evidence for sexual dimorphism in 2D:4D in Turkish neonates. In this respect, the sex difference appears to be independent of the absolute length of the fourth digit, providing developmental evidence that is inconsistent with the allometry hypothesis and supporting the premise that 2D:4D dimorphism is likely related to prenatal hormonal programming rather than simple differential growth.