Why are there prison gangs? Do they lead to more violence? David Sharbek studies policentric governance of prison gangs, especially those in the USA. There is less violence and unrest in prisons since the 1950s, the period when the first gangs appeared. So, gangs seems to correlate with a reduction of violence, not an increase. In fact, it is more likely that prison gangs provide an important service, they provide safety. Due to the increase of the number of inmates, with many prisoners not familiar with the norms within prison (first time prisoners) gangs do a lot of peer enforcement. They also set up businesses on smuggling drugs, mobile phones, and regulate these businesses with other gangs. Prison guards are reluctant to provide space for these illegal businesses since they are with too few to regulate the safety themselves, and cooperating with the gangs also will help them to function. David Sharbek is writing up a book on this fascinating topic which will appear next year.