Sicilian murder rates more than doubled between 1911 and 1921.

Evidence

In Sicily in 1921 there were 17.7 murders for every 100,000 inhabitants, as opposed to 7.3 in the whole of Italy (in itself a high murder rate) and in contrast to Sicily’s own figures of 7.7 ten years earlier (1911) and 3.6 ten years later (1931). — Diego Gambetta, The Sicilian Mafia The Business of…, 866

Categorization

• QUESTION: The spike around 1921 is the post-WWI mafia surge Gambetta’s theory presumably explains — a 230 percent rise in a decade. What mechanism does Gambetta attribute this to?