This is not 'clear' at all. What is clear is the correlation. You imply that the cause is 'good genes and habit'.

An equally, if not more valid cause is that having money makes it much easier to get good condition/tutors etc for preparing for the exams.

There are studies that show how heritable intelligence is. Very.

It is quite common people say, that something is only a correlation and not causation. But if you can point to a common denominator, that has been shown multiple times, to have a massive effect, it's not likely to be just a coincidence. Genes are this common denominator. Society and habits (for example, protestants vs catholics) are another.

Things that consistently impact the whole population are not just a random process that picks: "You will be clever, ugly.", "You will be pretty, sporty, but will be dumb." It's always genes, society and habits.