Sex-Related Biological Variation and Mental Toughness in Soccer: a 2D:4D Digit-Ratio Study of Type a, Type B and Type C Classification Across Playing Positions

Abstract

Background: Sex-related variation in psychological traits is usually studied as a categorical male–female contrast, which conflates a grouping variable with the continuous biology underlying it. The second-to-fourth digit ratio (2D:4D) provides a non-invasive marker of the prenatal androgen environment, but its relationship with mental toughness and playing position in soccer remains poorly understood. This study examined the associations between 2D:4D-derived classification, playing position, sex and mental toughness, testing whether mental toughness tracks a continuous sex-related biological index rather than sex category alone. Methods: In a cross-sectional design, 270 licensed amateur soccer players (241 men, 29 women; aged 13–35 years) from Denizli, Türkiye, were assessed. Mental toughness was measured with the Sports Mental Toughness Questionnaire (SMTQ). Digit lengths were measured directly on both hands with a calibrated digital caliper, and the two-hand mean ratio assigned each player to one of three operational categories at a cut-off of unity (Type A < 1.00, Type C = 1.00, Type B > 1.00). Data were analysed with one-way and two-way ANOVA with Bonferroni post-hoc comparisons. Results: Type B players reported higher mental toughness (M = 47.36) than Type A (43.85) and Type C (44.24) players, F(2,267) = 6.70, p = 0.002, η 2 = 0.048. Defenders (45.34) scored higher than goalkeepers (41.13; p = 0.021), F(3,266) = 3.00, p = 0.031, η 2 = 0.033. In the two-way model classification remained significant, F(2,258) = 6.10, p = 0.003, whereas position, F(3,258) = 2.22, p = 0.086, and the interaction, F(6,258) = 0.81, p = 0.560, did not. Male players scored higher than female players, t(268) = 1.99, p = 0.047. Conclusion: Mental toughness varied with 2D:4D-derived classification and playing position, and the classification effect ran counter to a simple masculinization account, since the less androgenized Type B profile scored highest. A sex difference was also present, but classification and sex operated in opposing directions and neither was reducible to the other. Types A, B and C are operational classifications, not literal brain categories, and should be interpreted with caution.