Brief Communication: Latent toxoplasmosis and salivary testosterone concentration—Important confounding factors in second to fourth digit ratio studies

Abstract

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 Relation of Toxoplasma Infection and Sexual Attraction to Fear, Danger, Pain, and Submissiveness

“…Such a decrease was observed in infected male and female mice and in female students (Flegr, Lindová, Pivoňková, & Havlíček, 2008;Kaň-ková et al, 2011). Homosexuality, that is, the sexual attractiveness of same sex but not opposite sex subjects-the second most important dimension obtained by the factor analysis-showed no association with toxoplasmosis.…”

Heritability of digit ratio (2D:4D) in rhesus macaques (Macaca mulatta)

“…Thus, genetic effects appear to have less influence on L2D:4D, which might suggest that environmental factors may be exerting stronger influences on the left hand. For example, it has been shown that human males and females infected with Toxoplasma gondii (a protozoan parasite) have low L2D:4D compared with the R2D:4D (Flegr et al 2008).…”

Influence of latent toxoplasmosis on the phenotype of intermediate hosts

“…infected men are higher, have a lower second to fourth digit length ratio in the left hand (Flegr et al 2005b(Flegr et al , 2008a, i.e. have longer fingers (Kratochvíl and Flegr 2009), and their faces in photographs are rated by female raters as more dominant (Hodková et al 2007b).…”