There's also the fact that the effect size is huge — 63%. Most real world population differences are not weird enough to generate that large an effect without b

There's also the fact that the effect size is huge — 63%. Most real world population differences are not weird enough to generate that large an effect without bias. The bigger questions are how much would the effect size change without removing outliers (Uber as pg mentions but presumably others too) and how significant is the result.