Based on studies I’ve seen over the years (meaning sorry, no cite, because this is a general accumulation of information), crime tends to lead to poverty, not the other way around (as many commonly believe). There are plenty of low-income areas that are devoid of crime (I’ve lived in several), but there aren’t many high-crime areas that aren’t poor.
I think the general explanation is that when a neighborhood starts to go downhill, all the people who can afford to leave can. This leaves lower-income people in the area. Also, crime drives down property values, which means that you’re likely to attract a poorer class of people.
Why do bad neighborhoods turn good when yuppie-types move in? Beats me, I was unaware that happened. By “young yuppie type”, I’m imagining a class of people who aren’t poor, and who are likely to acquire a decent income, if not now, then in the near future. First of all, while it wouldn’t surprise me too much to see a person of this type move into a low-income neighborhood, I would be surprised to see him move into a high-crime neighborhood. People of this type tend to have greater job opportunities, and are more likely to be able to acquire a job where he can live in a safe area. So, assuming our aspiring yuppie buys a cheap house in a poor-but-safe area, as his income climbed, I can see him fixing up the place a little, making his home, at least, look nicer. This would raise property values ever so slightly, and perhaps attract other people of his type. In this way, I can see a neighborhood slowly but surely becoming nicer.
If we’re talking apartment complexes, and areas larger than a street or two, then I would imagine that if people with higher potential incomes would spend more at local stores. This would enrich the store owners, who would improve their stores, which would attract more people with money, which would give the store-owners more money, and so on. In addition, the landlords can charge more from these yuppies, because they can afford it, which allows them to improve their apartments.
Why do good neighborhoods turn bad when minorities move in? Well, good neighborhoods will turn bad when criminals move in. If there’s a correlation in the area between criminals and minorities, this could be your explanation. Alternately - tossing out the racial explanation, though I don’t know if I buy it - if a neighborhood has racist tendencies, then it could consider the influx of minorities as driving down property values, which could start a downslide into poverty. I doubt this is the case very often any more, though I could be wrong, I suppose.
Jeff