White flight

White flight is a term coined to describe the gradual movement of typically white working-class and middle-class Americans out of neighborhoods which African-American families are moving into. Although white flight continues to this day in the United States and many other places, the term often refers specifically to its mid and late 20th century nationwide occurrence in the US. The presumption was that African-Americans would bring with them the crime, drug use, and gang activity of the "ghettos" from which they allegedly originated. The fact that many of the blacks moving into these neighborhoods were themselves stable working-class and middle-class citizens, and often younger, college-educated married couples did little to alleviate white fears. The same phenomenon has been seen where one ethnic group replaces another in a small geographic area.

What caused it?
In the United States, the mid-twentieth century was a period of racial turbulence and socioeconomic upheaval. It's generally accepted that a number of causes factored into this phenomenon:
 * Racism
 * Desegregation of public schools (as well as desegregation in general)
 * The rise of suburbia. Many wealthier white families preferred living in the suburbs because they offered larger and more private housing stock. They were also less likely to have to interact with someone with darker skin (god forbid) since fewer African Americans were able to afford a home in the suburbs.
 * When a realtor sells property in a primarily white neighborhood to a black family, neighboring white families fear that their property value will go down and rush to sell their homes. Meanwhile, the realtor is rolling in cash.

Effects

 * Lower tax base in cities, essential services understaffed/funded
 * ...which led to the increased crime, fear of which caused the flight
 * De facto segregation of schools between white rich and black poor
 * Long commutes, which theoretically harm worker productivity; every hour stuck in traffic is an hour not working
 * Massive amounts of pollution due to excessive commutes, as well as a massive trade deficit from oil imports
 * Arguably extends the institution of racism as suburban children seldom come into contact with "outsiders" (until college), privilege blindness, etc.

Africa
Since the end of apartheid in South Africa, people of all colors (but mostly whites) are reported to be leaving the state. This has been chalked up to a number of causes, including changes in political stability and economic opportunity, and stimulated by a violent crime epidemic, serious political upheaval, and economic globalization. Most prevalent of these is the crime rate; South Africa hosts one of the highest murder rates in the world. While some emigrants are reportedly returning, we can expect to see a shortage of skilled workers as well as agricultural collapse in the state if the current trend continues.

Zimbabwe
Since Robert Mugabe took control of Zimbabwe, white people have been fleeing his cronies and their murderous rampages which targeted white farmers. Most of the white people who were there when the country achieved independence are long gone, and most of the few (around 50,000) who remain are generally target practice for Mugabe's thugs.

White return Gentrification
As cities like New York managed to get crime under control and younger generations of white people realized they'd rather live near black people than spend 4 hours a day commuting, a number of young professionals have begun moving back into the cities rather than the suburbs. While this generally raises the tax base of a community and helps businesses, the poor people in the community find that the rents skyrocket and are forced out.