Here you go (from one my favorite books):
How can it be possible that the English ancestry group does not have the highest concentration of millionaire households? After all, they were among the first Europeans to arrive in the New World. They were on the ground floor to take economic advantage in this land of opportunity. In 1790 Colonial America, more than two-thirds of households were headed by a self-employed person. In America, the achievements of the current generation are more a factor in explaining wealth accumulation than what has taken place in the past. Again, most American millionaires today (about 80 percent) are first-generation rich. Typically, the fortunes built by these people will be completely dissipated by the second or third generation. The American economy is a fluid one. There are many people today who are on their way to becoming wealthy. And there are many others who are spending their way out of the affluent category
http://www.nytimes.com/books/first/s/stanley-millionaire.html