Going back to the banking bit a biologist has suggested that (among the upper classes at least) to be considered successful in medieval Europe involved among other things being successful in war, whereas in the Jewish upper classes they were excluded from such things and being successful involved banking/trading/urban type jobs.
As being successful meant you could afford more and better food, more of the children of the upper classes lived till adulthood, inheriting intelligence while their Christian peers inherited physical strength.
Economist article
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