All of the above. Many people left Eastern Europe for fear of religious persecution (this was around the time of WWII). People from Western Europe left because as America became a "land of opportunity" and capitalist venture, they saw many golden chances to make millions of dollars through the heavy industrialization of the northern states, like Michigan. Political freedoms were also a chance for people to have their voice heard and help others through legislation and politics. Finally, many people (even today) leave their countries to raise their kids in a 'safer' environment than their native countries (this is especially true for people immigrating from Central America, where gang violence is widely present) and in order for their kids to recieve a better education.
In the 1940s and 1950s, 'A. many people left the city to live in the suburbs" There was also some movement from the south to the northern urban areas, but this was relatively little.
