1. The construction of shopping centers such as Roosevelt Field sucked the life out of downtown Hempstead. Landlords had trouble renting the buildings (both the stores and the apartments above them) and started taking section 8/allowing disreputable businesses.
2. The construction of newer suburbs like Levittown and East Meadow made Hempstead less desirable, leading realtors to steer black families moving out of NYC to the one place where there would be little resistance (due to lack of demand from the white majority) to them moving in.
There were attempts to make Hempstead a "succesfully integrated" community during the 1960s and 1970s (a la Montclair or Shaker Heights), but they failed.
The older white folks died off and were replaced by black families. LI is the most segregated are in the country, by choice of all parties. Black LIers choose to invest their home buying dollars in "black" towns. It has been that way since the forties. Gordon Heights in Suffolk was created specifically to be a black Levittown.
Hempstead hit the tipping point in the sixties and the town has deteriorated steadily since then with the Carter years hitting particularly hard.
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