Philip Cohen interprets it:
Violent crime has fallen through the floor (or at least back to the rates of the 1970s) relative to the bad old days. And this is true not just for homicide but also for rape and other assaults. At the same time, the decline of marriage has continued apace.
I've written before about the assumption that the rise in single-parent families was responsible for the violent crime bonanza of the 1980s and 1990s. (Romney and Ryan returned to this theme….) By my reading of the research, it is true that children of single mothers are more likely to commit crimes. But other factors are more important.