1) People that earn more money have very mentally demanding jobs, so working all day, their brain is tired at the end of the day, and it goes on holiday.
2) Stupid people breed like rabbits.
3) Add 1+2 = people that earn more money, shag more often.
4) People that shag often, aren't thinking with the wrong body part during work, so can concentrate on their work more.
5) All of the above could also be stated for loners with sock puppets, just like serial killers.
It could also be that more motivated people earn more because they are more motivated, work out the gym more, go trawling for women more often, and are more successful at picking up chick's because they have hot bods.
The article is confusing the cause & correlations again.
Any single study focuses on a single aspect of an issue, and does not take a holistic approach.
All these studies are assuming they understand sand migration around a continent, by studying a few grains of sand under a microscope.