Where did the myth that Jews are invariably rich and good at making money come from?
Capital needs freedom to move around. It doesn’t thrive when there are constraints on its movement. In the Middle Ages, the Jewish community was a diasporic one. Most Jewish families had relatives or friends in other countries, regions, or even continents. As a result, it was easier for Jews to transfer money and help capital move around.
Today, anybody can move money easily, yet the myth remains in existence
Clarissa's Blog 
The explanation I’ve always heard: in medieval times in Europe, Christians were forbidden from lending money with interest, and this profession was relegated to the Jews.
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Yes, there were excommunications and all kinds of persecution for usury among Christians. But I like to look at the story not only from the Christian point of view (Jews had to do it because Christians didn’t want to) but from the Jewish point of view, as well (we were good at this because we had a certain way of life that made this easier or us.)
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