This post originally appeared here; I've updated it slightly and posted it here as a follow-up to this post.

David Friedman has a fascinating book on alternative legal systems. One chapter focuses on prison law - not the nominal rules, but the rules enforced by prisoners themselves.

The unofficial legal system of California prisoners is particularly interesting because it underwent a phase change sometime after the 1960’s.

Prior to the 1960’s, prisoners ran on a decentralized code of conduct - various unwritten rules roughly amounting to “mind your own business and don’t cheat anyone”. Prisoners who kept to the code were afforded some respect by their fellow inmates. Prisoners who violated the code were ostracized, making them fair game for the more predatory inmates. There was no formal enforcement; the code was essentially a reputation system.

Sometime after the 1960’s, that changed. During the code era, California’s total prison population was only about 5000, with about 1000 inmates in a typical prison. That’s quite a bit more than Dunbar’s number, but still low enough for a reputation system to work through second-order connections. By 1970, California’s prison population had ballooned past 25000; today it is over 170000. The number of prisons also grew, but not nearly as quickly as the population, and today’s prisoners frequently move across prisons anyway. In short, a decentralized reputation system became untenable. There were too many other inmates to keep track of.

As the reputation system collapsed, a new legal institution grew to fill the void: prison gangs. Under the gang system, each inmate is expected to affiliate with a gang (though most are not formal gang members). The gang will explain the rules, often in written form, and enforce them on their own affiliates. When conflict arises between affiliates of different gangs, the gang leaders negotiate settlement, with gang leaders enforcing punishments on their own affiliates. (Gang leaders are strongly motivated to avoid gang-level conflicts.) Rather than needing to track reputation of everyone individually, inmates need only pay attention to gangs at a group level.
Of course, inmates need some way to tell who is affiliated with each gang - thus the rise of racial segregation in prison. During the code era, prisoners te...

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