Ah yes, selection bias. In surveys of centenarians (people 100 years or older), a surprising number of them credit their longevity to eating onions. Now, onions are good for you, to be sure, but will they make you live to 100? Yes---if you have good genes, do a bunch of other things right, and are really, really lucky. And so it is with all these "multi-bazillionaires attribute their success to X". Are they smart and savvy, with possibly unusual backgrounds that give them an edge? Sure. But a bunch of other things have to go right as well---and even then I hear that the most successful people also eat lots of onions.

As far as research that shows a causal relationship between X and success, all we have that I know of is YC.

For research on the results of Montessori education, look here:

http://www.montessori-ami.org/research/research.htm

Yes, and then there's always the possibility of a third variable that causes both X and Y, and hence makes it appear as if X causes Y when in reality they are simply correlated. Oh my, statistics is a tricky affair :-)