Where's the mechanism?

Lott believes that the carry laws reduced violent crime by increasing the probability of the criminal encountering an armed victim and thus the cost of the crime to the criminal.

However, the evidence from the Dade county study [8] mentioned earlier shows that the carry laws did not increase the probability of criminals encountering an armed victim.

Lott reckons that the carry law caused a reduction of 8% in murders, 5% in rapes, 7% in aggravated assaults and 2% in robberies. For Dade county that translates to 1,500 fewer aggravated assaults, 450 fewer robberies, 65 fewer rapes and 30 fewer murders each year, amounting to a total of about 10,000 fewer violent crimes over the five year period. This seems to be far too large an effect to be caused by a mere 12 incidents where a criminal encountered an armed permit holder.

By how much did the carry law increase the cost of crime to criminals? Arrest is 5,000 times more likely than encountering an armed permit holder. If the criminal considers these to be equally bad outcomes, then the carry law increased the cost by no more than 0.02%. It is absurd to expect a 7% decrease in crime from such an insignificant change in the cost.

Anyway, since Lott has estimated the effect of increasing the arrest rate we can do a much more direct calculation. In chapter 4 he reports that concealed carry had an effect of 4.8% reduction in violent crime while a 100 percentage point increase in the arrest rate reduced violent crime by 0.48%. The 12 incidents out of 100,000 violent crimes is equivalent to increasing the arrest rate by 0.01 percentage points. Consequently, Lott's model predicts that this would reduce the violent crime rate by $0.01\times 0.48/100 = 0.000048\%$. That is 100,000 (five orders of magnitude) times smaller than the effect he actually attributes to carry laws.

To be fair, it looks like Lott has made an error in reporting his results and understated the effect of increasing the arrest rate by a factor of 100. (The numbers seem ridiculously small, and the corresponding numbers in the table giving effects when the data is aggregated by state are about 100 times larger.) Even if we allow for this possible error, the effect is still 1,000 times smaller than the effect he actually attributes to carry laws.

Even if (contrary to what criminals said in Wright and Rossi's study of criminals attitudes to firearms [46]) criminals are not afraid of police guns but are afraid of victim's guns, the change in the cost is still insignificant. If you believe Kleck's survey of defensive gun use [21] there were at least 100,000 DGUs (defensive gun uses) in Dade county over the five year period. If you believe the NCVS [40] there were at least 2,500. Either way, the 12 by permit holders makes no significant difference to the total.

If we decide that the only cost that criminals care about is the chance of getting shot by a victim, then the comparison is even starker. Not one criminal was shot by a permit holder during the entire five year period, while probably around 500 criminals were shot by non-permit holders acting in self-defence [21].

Nor is it plausible that there were large number of DGUs by permit holders that were not reported to the police. While Kleck's survey and the NCVS give wildly different estimates for the number of DGUs they agree that about half of DGUs are reported to the police.

Some have argued that the publicity about the passing of the law caused criminals to mistakenly believe that the risk they faced increase. This is possible, but Lott didn't test this model (since he didn't have a variable for the publicity). Nor is this consistent with the results of his trend analysis, which shows that the decrease was small at first and gradually increased as time passed. The publicity about the law would have been greatest at the time the law was passed and lessened as time passed.

Tim Lambert