Is there really a simple explanation for record-low homicide rate in NYC (or the increase in Chicago)?

The question in the title of this post is prompted by this new piece in today's New York Times headlined "414 Homicides in ’12 Is a Record Low for New York City."  The article includes lots of interesting data and stories concerning homicides in NYC, and here are excerpts:

Murders in New York have dropped to their lowest level in over 40 years, city officials announced on Friday, even as overall crimes increased slightly because of a rise in thefts — a phenomenon based solely on robberies of iPhones and other Apple devices.

There were 414 recorded homicides so far in 2012, compared with 515 for the same period in 2011, city officials said.  That is a striking decline from murder totals in the low-2,000s that were common in the early 1990s, and is also below the record low: 471, set in 2009.  “The essence of civilization is that you can walk down the street without having to look over your shoulder,” Mayor Michael R. Bloomberg said.

Mr. Bloomberg acclaimed the accomplishment during a graduation ceremony for more than 1,000 new police officers at the Barclays Center in Brooklyn.  He attributed the low murder rate to the department’s controversial practice of “stop, question and frisk,” in which people are stopped on the street and questioned by officers, and aggressive hot-spot policing, in which officers are deployed to areas with crime spikes.  Shootings are also down for the year so far. The number of murders is the lowest since 1963, when improvements in the recording of data were made.

The Police Department said thefts of Apple products had risen by 3,890, which was more than the overall increase in “major crimes.”  In the last two decades, trumpeting declines in crime trends has become an annual end-of-the-year event, even when the numbers inched up.

But figures alone do not tell the whole story, and several homicides this year stood out as particularly disturbing, given the age of the victims and the manner of death.  Detectives described the stabbing deaths of two children at the hands of their nanny inside the bathroom of their Manhattan apartment in October as among the most horrific crimes they could recall. “I think those images get embedded in the minds of detectives more than other crime scenes,” said Michael Palladino, president of the Detectives’ Endowment Association, the union that represents detectives, adding, “It certainly makes you rethink the things that you take for granted, which is the safety of children.”

So far this year, the police said, 20 children — ages 9 and younger — were murdered, up from 16 in 2011.  Among the victims was a 4-year-old boy, Lloyd Morgan Jr., who was shot in the head on a Bronx playground during a basketball tournament.  There were also several anomalies in the 2012 homicide tally, including a serial killer who murdered three shopkeepers in Brooklyn....

But overall killings have dropped to such a low level that more New Yorkers now commit suicide than are the victims of homicides. About 475 New Yorkers kill themselves each year, according to the city’s health department.

Mr. Bloomberg praised Police Commissioner Raymond W. Kelly, saying the 19 percent drop in homicides compared with 2011 was achieved despite a shrinking police force and an increasing population.  Mr. Kelly said he believed that relatively new policing strategies, including adding more police officers dedicated to curbing domestic violence, and monitoring social media to thwart gang-related murders, were working. “We’re preventing crimes before someone is killed and before someone else has to go to prison,” the commissioner said.

Of the 414 murders, 14 deaths from previous years were counted as homicides for the first time...  Of the 400 murders in 2012, 223 were .gunshot victims, 84 victims were stabbed to death, 43 died of blunt trauma and 11 died of asphyxiation. 

The majority of the 400 homicides occurred on a Saturday, followed by early Sunday morning. Most occurred at 2 a.m.  People were more likely to be killed outside than in. Nearly 70 percent of the victims had prior criminal arrests, the police said.  Domestic-related homicides dropped to 68, from 94 in 2011.

The likelihood of being killed by a stranger was slight.  The vast majority of the homicides, Mr. Kelly said, grew out of “disputes” between a victim and killer who knew each other.

Though I am sure improved policing practices have played a significant role in the modern crime declines in New York City and elsewhere, I am not confident that this is the whole (or even most) of the story. Police practices in NYC surely did not get even 20% better in 2012 compared to 2011, and reductions in the police force must have diminished a bit the proactive policing potential of the NYC blue line. Consequently, some other (complex?) factors are likely part of the explanatory mix, though I suppose the record-low number may be just a statistical blip in the "usual" homicide numbers.

UPDATE:  Only hours after posting about the record-low number of of homicides in NYC in 2012, I came across this new Chicago Tribune article concerning the inverse homicide trend in Chicago.  The lengthy piece is headlined "In Chicago, killings and questions on the rise: As year's homicides hit 500, causes and solutions still being debated," and it starts this way:

The rising homicide toll — 500 as of Friday, a 17 percent increase in slayings over last year — has been a looming shadow over Chicago, plaguing residents and the city's leadership for much of the year.

Although Chicago had almost twice as many homicides 20 years ago as it did this year, the increase in violent deaths represents a backslide for a city that Mayor Rahm Emanuel has said he wants to move forward. And with Chicago's homicide rate exceeding those in some other major U.S. cities such as Los Angeles and New York, Emanuel, ever mindful of the city and his administration's image, has seen the city's violence attract unwanted national attention.

Since taking the helm last year, Emanuel and his hand-picked police superintendent, Garry McCarthy, have made safer streets a top priority, with McCarthy declaring "the murder rate in this city is way too high."

But a particularly bloody winter in early 2012 has kept both men on the defensive, and residents on edge. As homicides climbed, Emanuel and McCarthy repeatedly have had to defend themselves, making it a point to publicly note short periods when the city goes without a murder or to highlight successful violence-reduction efforts in certain neighborhoods. Meanwhile, neighborhood residents decried the gun and gang violence that claimed the vast majority of this year's homicide victims.

Experts warn not to put too fine a point on year-over-year increases in homicides, but Chicago's tally this year is the highest since 2008. Although everyone agrees the increase in violence is deplorable, what's more difficult to discern is exactly why Chicago's homicides have surged. But experts, police and community leaders have offered myriad possible factors [ranging from gang factions to policing patterns to the weather in early 2012].