The frequency of mass shootings is a uniquely American problem.
But be careful when you hear reports that say how often such shootings happen. They are not well-tracked by federal agencies, and the definition of a “mass shooting” varies so widely among public researchers that the numbers can be confusing.
One commonly cited database, run by Mother Jones magazine, says there have been four mass shootings in America this year.
Those four incidents occurred at a church in Charleston, S.C. (nine victims killed); a military recruitment center in...