What a nifty way to prove it:

Schilt and Wiswall found that women who become men (known as FTMs) do significantly better than men who become women (MTFs). MTFs in the study earned, on average, 32% less after they transitioned from male to female, even after the authors controlled for factors like education levels. FTMs earned an average of 1.5% more. The study was just published in the Berkeley Electronic Press' peer-reviewed Journal of Economic Analysis & Policy.