Question

The growth of suburbs during the 1950s and thereafter led to

Answer 1

Although only in the sense that urban decay followed after the growth of the suburbs. The suburbs did not CAUSE the decline of the cities -- that was the result of bad government, sick culture, and stupid public policy ideas. But the flight to the suburbs of people who were responsible and energetic was a great loss of human capital to the cities.

It's not unlike the way responsible and energetic Germans fled from East Germany when the Soviets ran it, and as a result, when the two halves of Germany re-united in the 1990s, East Germany was far sicker and poorer than West Germany, even though the reverse had been true before Germany was divided. That is, when Germany was undivided, before the Second World War, East Germany was where the industry, power, wealth and culture was located, while West Germany was more agrarian and poorer. 50 years of bad government and the flight of their best and smartest citizens reversed that. (East Germany actually tried to prevent this loss by building the Berlin Wall in 1961.)

You see some smiliar things with states, too. Michigan, and the Detroit area in particular used to be a massive industrial powerhouse, wealthy, educated. Now it's a dump where the city can't pay for electricity to keep the streetlights on, and where they're thinking of demolishing whole city blocks and turning it back into wilderness or cropland. That's the result of 50 years of bad government and poor policy choices, which have led millions to flee Detroit and Michigan. (After the last Census, Michigan lost a seat in the House due to population stagnation.)