From the piece:
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. Looking at two aggregate trends is never enough to tell a whole story of social change, of course. However, if two trends going together doesn't prove a causal relationship, the opposite is not quite as true. If two trends do not go together, the theory that one causes the other has a steeper hill to climb. In the case of family breakdown driving crime rates, I don't think the story will make it anymore.