IT HAS been a bloody year in Baltimore, Maryland’s largest city. On November 14th the police department reported the city’s 300th homicide in 2015, a total not seen since 1999. The surge in killings in the majority-black city of roughly 623,000 began after the death on April 19th of Freddie Gray, a 25-year-old black man who was fatally injured while in police custody. Since Mr Gray’s death the city has recorded 244 homicides, a 78% increase over the same period in 2014, representing more than 100 additional deaths (see chart). Criminologists and city officials disagree as to the causes. Some...