The famous research of David Mustard and John Lott in the mid 90’s.

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. If those states which did
not have right-to-carry concealed gun provisions had adopted them in 1992,
approximately 1,570 murders; 4,177 rapes; and over 60,000 aggravate assaults
would have been avoided yearly. On the other hand, consistent with the notion
of criminals responding to incentives, we find criminals substituting into
property crimes involving stealth and where the probabilities of contact between
the criminal and the victim are minimal. The largest population counties where
the deterrence effect on violent crimes is greatest are where the substitution effect
into property crimes is highest. Concealed handguns also have their greatest
deterrent effect in the highest crime counties. Higher arrest and conviction rates
consistently and dramatically reduce the crime rate. Consistent with other recent
work (Lott, 1992b), the results imply that increasing the arrest rate, independent
of the probability of eventual conviction, imposes a significant penalty on
criminals. The estimated annual gain from allowing concealed handguns is at
least $6.214 billion.

read more: https://chicagounbound.uchicago.edu/cgi/viewcontent.cgi?article=1150&context=law_and_economics