Brief Communication: Latent toxoplasmosis and salivary testosterone concentration--important confounding factors in second to fourth digit ratio studies.

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.

Flegr J ，Lindová J ，Pivoñková V ，Havlícek J ... -  《-》

Second to fourth digit ratio, body mass index, waist-to-hip ratio, and waist-to-chest ratio: their relationships in heterosexual men and women.

Fink B ，Neave N ，Manning JT 《ANNALS OF HUMAN BIOLOGY》

Epidemiologic study of the sexually dimorphic second to fourth digit ratio (2D:4D) and other finger ratios in Greek population.

Kyriakidis I ，Papaioannidou P 《-》

Digit length ratios predict reactive aggression in women, but not in men.

Benderlioglu Z ，Nelson RJ 《HORMONES AND BEHAVIOR》

2nd to 4th digit ratio (2D:4D) and number of sex partners: evidence for effects of prenatal testosterone in men.

Hönekopp J ，Voracek M ，Manning JT 《PSYCHONEUROENDOCRINOLOGY》