Abstract:Stafford (2018) found that female chess players outperform expectations when playing against men, in a study of data from over 5.5 million official games around the world. I examined whether that result could stem from not controlling for the ages of both players, as female players tend to be much younger than male players. Using the same data as Stafford, I was able to replicate his main result only when the opponent s age was ignored. When the ages of both players were included in the analysis, the gender-composition effect was reversed. Further analyses using other data demonstrated the robustness of this pattern, re-establishing that female chess players underperform when playing against men. Prior to Stafford s paper, the leading premise was that women encounter psychological obstacles that prevent them from performing at their normal capacity against men. My commentary continues that line of evidence and is consistent with the stereotype-threat explanation.