I hate to use the word "gentrification" because it tends to mean different things to different people. Instead, I will use the term urban redevelopment because it is more precise; some cities have experienced a growth in population and this has lead to redevelopment. Two notable examples are New York and Washington DC, though Chicago has experienced some of this as well.
As noted previously, the decline of the American city in the postwar era coincides with many factors and it was most certainly a convergence of these factors which lead to their decline. So too then much the renaissance of the city is a convergence of factors. I'd like to explore another factor in a little more detail.
Chicago's homicide rate, like that of much of America, is following a downward trend. It is interesting to compare Chicago's statistics to that of the nation as a whole as well as other cities for some context. Today we shall have a glimpse of homicide rates in New York City from 1900 - 2007:
As you can see in this graph there was a huge surge in homicides in New York in the late 1960s and a steep drop in the mid-1990s. The rates are gradually trending downward. Not surprisingly this looks almost like an inversion of New York's population:
As you can see it is not a perfect correlation, however the dip in population from the 1960s to the 1990s coincides with the spike in homicides. It would be naive of urbanists to ignore the role that crime played in the decline of cities in the mid 20th century and rise in the late 20th century.