Evaluating anti-gun control arguments #10: Right-to-carry laws deter crime

Example: 
"Using cross-sectional time-series data for U.S. counties from 1977 to 1992, we find that allowing citizens to carry concealed weapons deters violent crimes and it appears to produce no increase in accidental deaths." -Lott & Mustard 1997 (original working paper from 1996, final 1997 version)
"Studies have shown that cities with high crime rates have seen a drop in that crime rate when concealed-carry laws are passed." -Letter to the editor of the Denver Post

Response:
The claim that right-to-carry (RTC) laws (aka shall-issue laws) deter violent crime originated in a now-famous (or, more accurately, infamous) 1997 study by Lott & Mustard, which was heavily publicized even before it was published. However, it has been shown to suffer from several serious flaws that invalidate its conclusions (Webster et al. 1997). Lott's subsequent book on the same topic, More Guns, Less Crime, also concludes that RTC laws are a deterrent to crime, though their findings have been criticized as being a simple artifact of reporting error (Martin & Legault 2005) or failing to control for regression to the mean (Grambsch 2008).

Subsequent work on this subject has produced mixed results, from finding evidence of an increase in certain crime rates (Ludwig 1998, Donohue et al. 2017) to no evidence of any significant effect on crime rates (Black & Nagin 1998, Kovandzic & Marvell 2003, Kovandzic et al. 2005) or the number of mass shootings (Duwe et al. 2002). Null findings have been reported by studies looking at both the effect of laws on gun ownership and on crime rates separately (Duggan 2001) and looking at the effect of increases in the number of people w/concealed weapon permits on crime rates (Kovandzic & Marvell 2003). Still others have found that these laws have multiple different effects on crime rates-some positive and some negative (Manski & Pepper 2017, Olson & Maltz 2001) to concluding that these laws do reduce violent crime, as Lott & Mustard originally claimed (Moody 2001, Plassmann & Tideman 2001, Bartley & Cohen 1998). In 2004, the National Research Council concluded that "...with the current evidence it is not possible to determine that there is a causal link between the passage of right-to-carry laws and crime rates" (NRC 2004, p. 150). 

However, more recent research provides a good reason to believe that these laws cannot and do not deter crime. First, a recent study showed that no relationship exists between concealed carry policies and citizens' perceptions of the number of concealed carriers in their neighborhood. This finding suggests that concealed carry laws cannot reduce crime, because if they did, it would require potential criminals to know how many people in their neighborhood are carrying concealed firearms (Fortunato 2015). Also, two recent studies provide evidence that RTC laws are associated with higher violent crime and homicide rates (Donohue et al. 2017, Siegel et al. 2017). Another recent study found that shall-issue laws are no better than may-issue laws at deterring crime (Barati 2017).

Sources:
Barati 2017
Bartley & Cohen 1998
Donohue et al. 2017
Duggan 2001
Duwe et al. 2002
Fortunato 2015
Grambsch 2008
Kovandzic et al. 2005
Kovandzic & Marvell 2003
Martin & Legault 2005
Moody 2001
Manski & Pepper 2017
NRC 2004
Olson & Maltz 2001
Plassmann & Tideman 2001
Siegel et al. 2017
Webster et al. 1997