A sexually dimorphic characteristic, the second to fourth digit ratio (2D:4D ratio), has been shown to reflect the prenatal concentration of sex steroid hormones and to correlate with many personality, physiological, and life history traits. The correlations are usually stronger for the right than the left hand. Most studies have shown that the 2D:4D ratio does not vary with age or postnatal concentration of sex steroid hormones. Recently, a strong association between left hand 2D:4D ratio and infection with a common human parasite Toxoplasma has been reported. We hypothesized that the confounding effect of Toxoplasma infection on left hand 2D:4D ratio could be responsible for the stronger association between different traits and right hand rather than left hand 2D:4D ratio. This confounding effect of toxoplasmosis could also be responsible for the difficulty in finding an association between 2D:4D ratio and age or postnatal steroid hormone concentration. To test this hypothesis, we analyzed the association between sex and age and 2D:4D ratio in a population of 194 female and 106 male students with and without controlling for the confounding variables of Toxoplasma infection and testosterone concentration. Our results showed that the relationship between age and sex and 2D:4D ratio increased sharply when Toxoplasma infection and testosterone concentration were controlled. These results suggest that left hand 2D:4D ratio is more susceptible to postnatal influences and that the confounding factors of Toxoplasma infection, testosterone concentration and possibly also age, should be controlled in future 2D:4D ratio studies. Because of a stronger 2D:4D dimorphism in Toxoplasma-infected than Toxoplasma-free subjects, we predict that 2D:4D ratio dimorphism as well as right hand/left hand 2D:4D ratio dimorphism will be higher in countries with a high prevalence of Toxoplasma infection than in those with a low prevalence.

The ratio between second and fourth finger (2D:4D) is sexually dimorphic; it is lower in men than in women. Studies using broad personality domains yielded correlations of 2D:4D with neuroticism, extraversion or agreeableness, but the obtained results have been inconsistent. We correlated 2D:4D of 184 women and 101 men with their scores in Cattell's 16 Personality Factor (16PF) Questionnaire. We found women with a higher (more ‘feminine’) right hand 2D:4D to score lower in emotional stability and social boldness and higher in privateness. Mediator analysis showed emotional stability to be probably primarily correlated with 2D:4D and to act as a mediator between 2D:4D and social boldness. Privateness appears to be mediated by an even more complex path. We discuss the usefulness of primary‐level personality questionnaires and mediator analyses in the investigation of psycho‐morphological associations. Copyright © 2007 John Wiley & Sons, Ltd.

Individuals tend to judge personality traits on the basis of physical characteristics, particularly facial traits, although this phenomenon has been mostly studied in relation to the halo effect of attractiveness. However, there are other facial traits which may also have an impact on personality attributions; here, we focused on masculinity. We carried out principal component analysis (PCA) of 15 anthropometric measurements from 71 male faces, resulting in three components: Face Height (C1), Inner Face Breadth (C2), and Cheekbones-Jaw Prominence (C3). The targets' photographs were rated by 210 women and 177 men on scales for masculinity, attractiveness, and nine psychological characteristics (selected Cattell's factors): Warmth, Reasoning, Emotional Stability, Dominance, Liveliness, Rule-Consciousness, Social Boldness, Abstractedness, and Privateness. We found that masculinity correlated positively with ratings of Dominance and Social Boldness and that masculinity rated by men correlated positively with ratings of Emotional Stability and Privateness. We found no relationship between masculinity rated by women and the PCA components, while masculinity rated by men correlated negatively with C2 (possibly related to babyface features) and positively with C3 (which included features developed under the control of testosterone, such as jaw prominence). Our results imply sex differences in masculinity ratings. In particular, men used Cheekbones-Jaw Prominence and Inner Face Breadth as cues for masculinity judgments; on the other hand, women apparently perceive masculinity in a more holistic way.