Is this round two?

I haven't really looked at studies on the death penalty so I can't comment on how accurate Friedman's history of that debate is. However, I should note that no study can prove that it is not a deterrent, they can only prove that the deterrent effect, if any, is smaller than some bound. Certainly, if it did have a large deterrent effect then even a very simple study would have found it.

I did find a study on criminologists' views on the death penalty which found that even 30 years after Ehrlich's work, criminologists remain unconvinced. Whether that is a reflection on them, or on Ehrlich's work, I cannot say.

Friedman's description of the history of the debate about the alleged deterrent effects of privately held guns is rather inaccurate. The pattern has been for some pro-gun person to argue for deterrence by using a simple comparison of a crime rate in a before year with a crime rate in an after year (for example, Kennesaw in the year before it passed its law to make gun ownership compulsory compared with Kennesaw in some suitable chosen year after the law) and then for a criminologist to perform a more sophisticated analysis using more data and find no effect. (In the Kennesaw example, an interrupted time series analysis using about 20 years of data found that the law had no effect.)

In the case of concealed carry, pro-gunners argued that such laws caused a reduction in the homicide rate based on simple two point comparisons. Some criminologists did a study using interrupted time series analysis in five representative cities in three states and found no effect. I'm not sure why Friedman claims that the study found ``that laws permitting concealed carry increase the murder rate''.