The Talmud (Pesahim 49a) says that “A man should sell all he possesses in order to marry the daughter of a scholar, as well as marry his daughter to a scholar.” In the Jewish community of the Middle Ages, the smartest men often became Rabbis, and these learned men of high status were able to marry the daughters of successful merchants, thus “selecting” in favor of high intelligence. The Jewish woman who was the teacher of the house ensuring that the children were properly and extensively educated, were themselves well learned.
At the same time, Christians were doing just the opposite: priests, monks and nuns of the dominant Roman Catholic Church, also usually among the best and brightest in their communities, were prohibited from marrying, thus “selecting out” through celibacy most of these intellectually superior men and women from the gene pool.
There is another factor that is not so obvious but is a catch 22 situation (I like catch 22's). In most countries that Jews migrated to the people were happy to have the intelligence and cleverness of the Jew but were apprehensive about giving Jews employment for fear of them dominating and taking over businesses. The same people that held back the Jew from gaining inroads in business and even academic endeavors, helped cultivate the business savvy of the Jew. How so? Jews were "on their own" and forced, out of necessity and survival, to develop their clever ways. They became self-employed, self-educated and, in many cases, self-governing. "Necessity is the mother of invention" an expression that is the history of the Jews.
But, since I like to analyze the absolute truth, I would be remiss to not include the will of Hashem. What the Jew is and has been throughout history is, by design, fulfilling Hashem's will. The intelligence of the chosen people was totally necessary to bring the message of Hashem to the world. The correction of the world depended on its agents who were tasked to bring it to fruition. The stereotyping, Jew hating, Jew bashing all very much resulted from the frustration and jealousy of the non-Jewish population. It was also a message from Hashem that when Jacob is studying Torah, Esau can't touch him. A strong message to the Jews throughout history is simply do the job you were charged with – study Torah, and don't assimilate into the gentile society. Hashem made life difficult on purpose when the Jew decided that he wanted to blend into the non-Jewish world. Jews were welcome even invited to many countries as long as they stayed separate from the local population. When the Jews said "I like the way they dress, I like their food, I like their jobs, I like their secular education" Hashem made sure the population of that country turned on them. I lived in Germany for six years, ich habe viel Deutch gelernt, I remember a speech that I heard from of all sources hitler (may his name be stricken – his name is small case intentionally since he was never a capital individual). he said "I don't want you assimilating with the German people." Who was really sending that message? Even in Israel, the absolutely happiest communities are those that are completely observant Jews and no non-Jews or even non-Jewish behavior in their midst. When we adhere to this vital message from Hashem, no harm will come to any Jew and complete happiness is discovered. It worked for me, my family and the entire city I live in. Thank you, Hashem.