Basically, we don't entirely know why America's urban murder rate has fallen. As Philip Cohen points out, it doesn't seem to have much to do with rates of single motherhood. Beyond that, it could be several or all of dozens of different factors. What's the takeaway message? I'd say there are two of them. First of all, beware of takeaway messages! Lots of things in life, maybe most things, often the most important things, don't have explanations that can be packaged as a simple, coherent thesis. Second, given our inability to explain definitively why the crime rate is falling, we may need some scepticism about the recent push to demand scientifically valid evidence for the effectiveness of social betterment programmes. Random controlled trials might very well have found that the broken-windows strategy doesn't prevent crime, "Project Ceasefire" doesn't prevent crime, reducing rates of single motherhood doesn't prevent crime, family planning doesn't prevent crime, banning lead doesn't prevent crime, and so on and so forth; there might have been no statistically significant difference one could isolate for any of these things. And yet it seems extremely likely to me that most or all of these were good things to do! The drop in violent crime probably has to do with all of them.