"Why would the mafia focus on citrus production and not, for example, on the cultivation of wheat or wine?" said the paper. "There are three basic reasons for the special importance of citrus fruit. First, the market value and profitability of citrus fruits was unusually high at the time, certainly much higher than for basic food crops like wheat. Second, the large fixed costs associated with irrigation and the long time before trees matured, made producers sensitive to predation. Third, the technology of predation on citrus fruits was relatively easy and cheap. According to Lupo (2011), a harvest of lemon fruits is very difficult to protect when the fruits are still on the trees. Picking a few hundred ripe lemons from a grove during a dark night should have been much easier for a thief than harvesting olives or grapes, not to mention wheat. As a consequence, lemon groves were more vulnerable to predation, despite the frequent construction of walls and the use of dogs and guards." 

The researchers based this conclusion on data from a Sicilian parliamentary inquiry into the assets and peasant conditions of the island conducted from 1881-1886 and an additional one from 1900. They found that Mafia presence in the 1880s is strongly associated with the presence of citrus cultivation. The more citrus, the more Mafia. This result holds even when controlling for variables like climatic conditions. 

The paper noted that this calls to mind the phenomena of the "resource curse," where the presence of valuable natural resources, paradoxically, depresses economic development due to corruption, patronage and rent-seeking behaviors. It pointed to, for example, Nigeria, where any benefit from oil revenues were cancelled out by political corruption. 

"A recurrent theme in this tradition is that resource windfalls might actually destabilize and deteriorate institutions, if key groups in the society believe that predation is more profitable than production," said the paper.