Reply to Yang et al.: Gender is not a confounding factor of our result

Yang et al (1) argue that sex and age are confounding factors in our study (2). However, by definition, they actually are not. Their sex difference argument does not have any influence on our result from a young Japanese male sample. In our article, we were interested in individual differences in reaction to unfairness observed in the ultimatum game. We were fully aware that sex and age are major factors to account for many brain functions and their disorders (3, 4). We also understand that sex difference in personalities is a major research topic. Therefore, we focused on young male volunteers. In addition, to prevent young females from unnecessary radiation exposure, we recruited only male volunteers as a first step. Sex difference was already addressed as a limitation. Needless to say, it is highly recommended to test this if a similar result could be observed in a female sample as well.

As for mediation analysis, we have also tested a bootstrapping method. We generated 1,000 samples from our data and tested the significance of the indirect effect using the bootstrap bias-corrected and accelerated bootstrap 95% confidence intervals (CI) (5). The indirect effect was considered significant if CI of the indirect effect did not include zero. The analysis revealed that both straightforwardness (CI −0.77 ∼ −0.0027) and trust (CI −0.49 ∼ −0.0046) were significant mediators of the relationship between serotonin transporter binding in the midbrain and rejection to unfair offers in the ultimatum game.