I’ve observed an overcrowding effect on people just in an office. When everyone is in it’s a less pleasing environment. Take a few out for the day and it’s more palatable.
New York City, in particular, at least certain parts, is incredibly crowded. And everyone moves fast. It can be very overwhelming. But I have been there when people stop to help someone in need. It’s not all awful.
That said, as someone who lived many years in NYC and loves it (well, love/hates it), there are many citizen heroes in the metro area. A glance at 9/11 reminds us of that.
Overcrowding and the stress it exerts seem to raise the resistance threshold of the willingness to get involved. Living in NYC is like living inside a giant machine. You realize that you are a much smaller part of the whole...and, as such, you have much less individual power there as opposed to a small town. Hence, there is a greater sense of apprehension...and an increased unwillingness to take dangerous action (because all action appears to be more dangerous).
IMHO.