Did more guns cause less crime?

Even if there were more guns and less crime, this is a correlation, not causation. Scientists generally accept a replicable controlled experiment as good enough for showing cause and effect for practical purposes. A non-experimental study like Lott's is merely suggestive of causation. When you have a pile of them and well-understood and verified mechanisms (like for the relationship between smoking and cancer) then you can make some causal claims. Although Lott proposes a mechanism; it is not plausible, as we shall see below.

Crime goes up and down, and we do not understand all the reasons for this. Although Lott makes a commendable effort to control for as many factors as he can, there are still significant changes in the crime rate that are not explained by his model. A way to test to see changes in the crime rate were caused by the carry law or one of the other factors is to look for differential effects--we would not expect the carry law to affect all crime rates equally. For example, since juveniles are not eligible for the concealed weapon permits, we would not expect the law to affect the juvenile homicide rate as much as it effects the adult rate. With one somewhat problematic exception, we do not find differential effects.