[right-to-carry] laws are relatively Republican with large National Rifle Association memberships and low but rising rates of violent crime and property crime.” Non-time-varying systematic differences among states are accounted for by the fixed effects, γi, in Models 6.1 and 6.2 in Chapter 6. However, if there are time-varying factors that differ systematically among states with and without right-to-carry laws and that influence the laws’ effects on crime, then the effects of enacting these laws in states that do not have them cannot be predicted from the experience of states that do have them, even if the other problems just described are not present.
The foregoing problems would not arise if the counties that have right-to-carry laws could be selected randomly. Of course, this is not possible, but consideration of the hypothetical situation in which it is possible provides insight into the methods that are used to estimate the effects of real-world right-to-carry laws. If the counties that have right-to-carry laws in year t are selected randomly, then there can be no systematic differences between counties with and without these laws in year t. Consequently, the average value of is the same across counties in year t regardless of whether a right-to-carry law is in effect. Similarly, the average value of is the same across counties. It follows that the average effect on crime of the right-to-carry law is the average value of in counties with the law