arising from C. Eisenegger, M. Naef, R. Snozzi, M. Heinrichs & E. Fehr Nature 463, 356–359 (2010)10.1038/nature08711; Eisenegger et al. reply

In February 2010, Eisenegger et al. reported increased fair bargaining behaviour after administration of testosterone in an ultimatum game1. However, unfair offers in the ultimatum game typically are rejected; thus, not only the motives for social cooperation but also the threat of financial punishment may have accounted for these effects. Here, using the public goods game (PGG), we unambiguously show increased social cooperation after testosterone administration, but only among subjects with low levels of prenatal testosterone (measured by the right hand’s second-to-fourth-digit ratio (2D:4D)). This finding establishes positive effects of testosterone on social cooperation, with prenatal hormonal priming providing for important individual variability.