Previous studies like these have generally shown that young attractive females make considerably more than male employees with a similar level of education and experience, but as you go up in age the trend reverses with older or less attractive female employees making less. Basically there is a bias, but it isn't so much gender in general as it is being attractive sexually (or not as the case may be). I'm not aware of any study that has tested if that applies to both genders (from the data it very clearly applies to females, but there may be a lesser effect applied to males as well), but that would be something interesting to look at. I suspect it would show there is a similar bias applied to males, but with a significantly smaller effect that's mostly lost in the noise.