One possible cause of the 2020 murder increase: More guns

The year 2020 saw the largest recorded increase in homicides in United States history — an increase likely propelled by a complex mix of factors, from more guns to stresses of the pandemic to fewer police officers on the streets to a crisis in relations between police and citizens.

But one persistent theory is that a change in policing last summer primarily drove increased gun violence. This is an especially popular explanation among law enforcement figures. Former Baltimore Police Department Deputy Commissioner Jason Johnson recently argued that the real driver of last year’s murder rise was...