Nashville's lower murder rate not alone; part of a national trend with uncertain causes

The local news media is playing up crime stats that show murder rates have dropped to historic lows in Nashville, and Metro government is taking credit. They are also interpreting the reasons:

For the second year in a row, murders hit a historic low in Nashville. There were just 41 criminal homicides in Nashville in 2014. That’s the smallest figure since the county and city governments consolidated in 1963, when police tracked 45 murders.

The highest year for murders in Nashville was 1997 when 112 people were killed.

Police chief Steve Anderson gives some credit to a recent focus on curtailing domestic violence. Just four of the murders from 2014 involved intimate partners. In 2013, there were nine.

Not so fast. Before revving up pats on backs, don't look at the Nashville numbers in a vacuum. Local stats should be placed in a national context, which lends a different perspective, namely one of uncertainty and caution:

Criminologists say the decrease is linked to several factors, some of which are the product of smart policing, others completely out of authorities’ control. But they also say the lack of a consensus on what’s gone right has them convinced that crime rates could spike once again.

“I don’t think anyone has a perfect handle on why violence has declined,” said Harold Pollack, the co-director of the University of Chicago Crime Lab. “So everyone is a bit nervous that things could turn around.”

Until we factor out the causes of trends nationally out of control of local authorities, we cannot assume that Metro government is doing something unique to prevent these murders. Likewise, if crime spikes it may not be because officials are doing something wrong (unless they choose to be consistent and accept the blame during spikes as well as the credit during lulls).