Cooperation is a hard behavior to explain. Often it involves making at least a small sacrifice for the benefit of the whole, which superficially seems to be at odds with evolutionary pressures that encourage individuals to maximize their benefits at the expense of their competitors.
Lots of theories exist to explain why cooperation arises in society, but a relatively new one examines how extortionists can lead to complete cooperation in a society. It's an idea I'm sure Mafia Don Vito Corleone of the movie The Godfather would have considered trivially obvious.
In a paper recently published in Physical Review E, Zhi-Xi Wu1(Lanzhou University) and Zhihai Rong (University of Electronic Science and Technology of China) considered a situation called the prisoner's dilemma game. Typically the game involves two or more people who have to choose to cooperate on not based on various rewards.
In the simplest version of the game, imagine two criminals who are being separately questioned by the police. If neither prisoner confesses or implicates the other, they both go to jail for 1 year. If they both implicate the other, they both go to jail for 2 years. But if one remains silent while the other implicates the silent one, the squealer gets off scott free and the loyal one goes to jail for 3 years.
It makes sense, in this basic form of the game, for both prisoners to rat each other out so that they both go to jail for 2 years. The game can be complicated significantly by adding other factors, like requiring the prisoners to play the game many times or changing the way the prisoners are related (colleagues vs. married couples, for example).
The game can also apply to much less sinister situations, with people deciding to cooperate in general (as in business partnerships), or to turn on their associates and reap all the benefits while others suffer.
Wu and Rong attempted to simulate a much more realistic situation than the two-player prisoner's dilemma by considering a large number of people who have to decide whether to cooperate with their nearest neighbors, or to turn on them. If you only have two types of people - those who choose to cooperate (cooperators) and those who choose to take the money and run (defectors) - then cooperation tends to be a fragile thing. Usually, cooperation breaks down.
But if you have some Don Corleones in society who take it upon themselves to make various offers that people can't refuse, then cooperation dominates. In many cases, defectors are eliminated altogether, thanks to the existence of relatively small number of extortionists who threaten potential defectors with various unpleasant punishments if they don't play along.
All things have their limits, of course, and that's true of the simulations Wu and Rong ran. Provided everyone had aspirations below a certain level, cooperation abounds. But when people start to aspire to too much, the situation breaks down and people again start to defect. The physicists don't give any specific examples to say what they mean by elevated aspirations, but I think Don Barzini's ambition to take over the mob activities in New York qualifies. Defection has its benefits, but as Barzini and his cronies in The Godfather learned, sometimes defection has a price too.