For a couple of generations now people have been pushing the idea that the "root causes" of crime are social problems like poverty, income inequality, discrimination, and so on.
I'm of the opinion that this is all nonsense and always has been, but it's a difficult proposition to refute - we just don't get natural experiments that allow one to test this.
But now we have Katrina. Katrina increased poverty by wrecking employers, increased income inequality by eliminating the income of people with few assets while having less effect on their wealthier and better diversified neighbors. And ... I think it's reasonable to conclude that is has increased discrimination, either by flame-fanning from Jesse Jackson and his ilk, or the spread of malicious urban myths by the legacy media.
Yet crime is down in New Orleans. Way down. Way, way, way down. Something like 10-20% of former residents are back in New Orleans, but the rate of violent crimes is down much more. Murder is down literally 100% - there has not been a recorded murder in New Orleans in going on 3 months.
The way you explain something like this is not with root causes. No, it takes something a little more mathematically complex. Not much more mind you, just enough to confuse the folks who think with their emotions.
It works like this. There are a certain number of folks inclined towards crime. Call it p. There are a certain number of victims in the wrong place at the wrong time. Call that q. Crimes require the intersection of the p criminals and the q victims, leading to a number of crimes of p times q. What has happened in New Orleans is that the number of criminals has been reduced - say to p/10, and the number of potential victims to q/10. Now the number of intersections of those two groups is smaller by an order of magnitude at pq/100.
There are a lot of details missing from this argument, but the fact that the percentage decline in crime exceeds the percentage decline in population is a telltale sign that the natural experiment going on in New Orleans strongly supports the "intersection" model of crime.
Alternatively, the root causes story of crime suggests that while the total number of crimes should be smaller in New Orleans after the storm, their rate should be higher. Again, this is a natural experiment that strongly suggests that the root causes approach - a common one in social sciences over the last century - is nothing short of nonsense.
Read up on it in the paper of record.