Children as young as six are being influenced by territorial feuds in Nottingham, according to an official report seen by the BBC.
The report was prompted by the killing of Danielle Beccan
Such disputes - in the Meadows, St Ann's and Radford - have been blamed for a number of murders in Nottingham.
The report found growing antagonism between teenagers, with many "brainwashed" into gang involvement.
It was commissioned by Nottingham Stands Together, a scheme launched after the death of Danielle Beccan, 14.
Danielle was murdered by two men from the Meadows who opened fire at random, motivated, their trial heard, by a "hatred" of people from St Ann's.
The report found "territorialism" was stigmatising Radford, the Meadows and St Ann's - which otherwise all have a strong community spirit.
Researchers also found many teenagers were being seduced and sometimes pressurised into joining informal local gangs or "crews", representing neighbourhoods or even streets.
The study discovered most young people were reluctant to get involved but saw it as unavoidable.
It also found some children, aged as young as six, were making negative comments about rival areas and by the age of 10 or 11 were being "groomed" to join gangs.
Drug dealers, who sometimes provided young people with guns, were cultivating the rivalry to exploit their illegal business, researchers found.
This resulted in people not involved in gangs being afraid to go to rival areas, which even included different shopping centres in the city.
The report said this lifestyle often continued until the people involved had children, got a job, or went to prison.
Officials from Nottingham's Crime and Drugs Partnership said they hoped it would start a public debate.
Chief Executive Alan Given said: "They don't necessarily like each other, but if you speak to the young people in there, or even the middle-aged people in there, they don't really know why.
"We need to try and help them to find ways to unpick that, to find out what it is, that makes them perhaps not get on with their own neighbours."