[dankohn1]

Here's a great illustration of the falling density in Manhattan from 1910 to today: http://www.vox.com/2014/9/23/6832975/manhattan-population-de...

[nostrademons]

Interesting...based on that illustration, it seems like #2 is the dominant factor. The areas of previously highest density are the Lower East Side and East Village, formerly working-class immigrant enclaves with more tenements than highrises.