The undesirable element is forced to decamp to a less desirable neighborhood nearby. There, it has no choice but to suffer with high levels of crime, but is typically afraid to ask the police for help, having learned from experience that the police are more likely to harass them then to help them, to arrest them for minor offenses and to round them up and deport them if they happen to be illegal immigrants. They also learn to be careful around members of local gangs and drug dealers. Since official jobs in the neighborhood are scarce, they seek informal, cash-based employment, contributing to an underground economy. Seeking safety in numbers, they self-organize along racial and ethnic lines, and, to promote their common interests, form ethnic mafias that strive to dominate one or more forms of illegal or semi-legal activity. Growing up in a dangerous, violent environment, their children become tough at a young age, and, those that survive, develop excellent situational awareness that allows them to steer clear of dangerous situations and to know when to resort to violence.
When the fossil fuel-based national economy shuts down due to the increasingly well understood local ramifications of the global phenomenon of Peak Oil, both of these communities are harmed, but to different extents and in different ways.