It's worth bearing in mind that the cost of education in Sweden (the area they got their sample from) is substantially less expensive, specially for research PhD students... were it's literally free.

The money input variable is already determining the possible outputs before the person even goes to school in many cases, and this will likely increase variability in results depending on the costs of pursuing education in the area studied.

Just to go to a basic state college for a BA I had to serve for years in the military to afford the tuition in the United States, as an example. The only thing stopping me from pursuing even further academic accomplishments was a lack of further funds.

The different environments will result in different outcomes.