The reform of Manhattan - how important was abortion?

The New York Times > Opinion > John Tierney: The Miracle That Wasn't

The city of New York (Manhattan, Bronx, etc) went from one of America's most violent cities to one of our safest towns. Why?

Tierney describes a debate on the question between Steven (Freakonomics) Levitt and Malcom (Tipping Point) Gladwell. Alas, Tierney assumes one is pretty familiar with the particulars of the debates. My tentative reconstruction is that during the early 90s it was believed that social policing was a critical factor that "tipped" the murder rate from a persistently high rate to a relatively low rate. More recently some have argued that New York's early legalization of abortion, and the high rate of abortion in Manhattan, played a critical role.

My completely uninformed suspicion is that this is a multifactorial equation (shocking, I know). Any of the significant factors (policing techniques, police numbers, new software, long sentencing, decreasing unemployment, rising home costs, demographics/abortion rates, crack use, abortion rates, social attitudes) could be important, and in isolation might be considered solelyl responsible. Perhaps the question, and maybe this is what the debate are really about, is whether the equations are linear or non-linear. A linear regression means that the crime rate will change in a smooth and continuous fashion (though possibly exponentially), a non-linear model means that the the transitions may be "sticky" -- that crime rates may persist at one "pole" or another. I've historically favored non-linear explanations, but I don't have that strong a bias (and it's completely uninformed anyway -- non-linear is just "sexier".)

The policy questions are:

1. What is the most cost effective and socially acceptable way to replicate this drop? Is there one intervention to begin with (since all the terms of the equation interact)?
2. What does New York need to do to keep murder rates low?
3. How big a factor is abortion? If abortion use falls and birth rates rise, is there any way to keep murder rates low? Could any other form of birth control compensate? (Abortion has been an infamously popular form of birth control in locations where it's easily available -- such as Russia.)