THE inmates of America’s overcrowded prisons can be a fearsome bunch, from murderers covered in tattoos to drug dealers wielding homemade knives. They readily beat each other up over minor disagreements. They murder fellow inmates, at times apparently without any reason. To outsiders, they may seem like mindless, violent thugs. In fact, such violence is often strictly governed by the gangs which so many prisoners join. Why do prisoners become members of such violent groups? Prison gangs did not exist until the 1950s. Prisons were run according to the unwritten tenets of the "convict code", as David Skarbek describes in his book, "The Social Order of the Underworld: How Prison Gangs Govern the American Penal System". At its most basic, it dictated that inmates should refrain from helping prison or government officials in matters of discipline and from giving them information, especially that which could be used against fellow convicts. This worked while prisons were small: terrifying reputations, ostracism and assault kept a lid on violence. But with the massive growth of the prison population in America from the middle of the 20th century, inmates could no longer rely on the code to keep them safe. First-time offenders who were ignorant of the code and its edicts became more common. And as the number of violent offenders increased, prisons also became more dangerous. As a result prisoners joined gangs for more organised protection. Race and tattoos became useful ways of identifying genuine comrades: no-one wants free-riders claiming to be a gang member to benefit from its protection while contributing nothing.

Gangs, then, provide protection in prisons where officials often fail to do so. But bigger groups also enable prisoners to trade illicit substances—such as drugs—far more efficiently. Consider an inmate working alone. He faces substantial challenges: other sellers can threaten to kill him if he lures their customers away. If, as is common, he allows people to buy drugs on credit, he faces the risk that they will not pay him back, especially if he is not sufficiently frightening to make his threats seem credible. Customers can simply clear their debts through murderous acts. But gangs, by contrast, are altogether more effective. Fail to pay a prison gang for your drugs and they have many more members who can kill you. Murder a dealer instead of paying him and his fellow gangsters will retaliate. Gangs can steal customers from individual dealers without worrying about revenge. And members can help facilitate trade from the outside after their release.

Partly because of this, gangs have spread throughout the American prison system. But their existence also has wider policy implications. Indeed, the emergence of gangs offers some insights into how to constrain them. Improve the way prisons are governed and those locked up inside them would become less dependent on the mob-rule provided by gangs. Make prisons less crowded and dangerous, and inmates would require less group protection. Gangs are a logical response to prison conditions; to reduce them, the authorities must improve the state of incarceration.

Dig deeper:

America is waking up to the cost of mass incarceration (Aug 2013)

Overcrowded and understaffed, prisons in England and Wales are deteriorating (Aug 2014)

Technology is making it easier to trade drugs in British prisons (Dec 2013)