"The economic boom of the 1990s raised the incomes of the poorest Americans, held the size of the middle class steady and swelled the ranks of those with six-digit incomes, according to census data released yesterday.</br></br>The figures confirm a long-term momentum that has given millions of Americans significantly more income: since 1980, the share of the U.S. population earning more than $100,000 annually has doubled, after adjusting for inflation. That upward income mobility was so broad that, when the census was taken two years ago, 5 out of 6 Americans lived in counties in which the lowest niches -- those with incomes of less than $15,000 -- were shrinking.</br></br>But there were notable exceptions, places where the middle class shrank while the ranks of the rich and poor expanded. They included a swath of major metropolitan areas from Washington through Baltimore, Philadelphia and New York, as well as large parts of Southern California.</br></br>Poverty may have grown in those areas because of high numbers of immigrants from poor countries, experts say, or in some cases, an ailing local economy.</br></br>""It's a positive picture overall,"" said John Logan, a sociology professor at the State University of New York in Albany, who compiled his own study of census data for metropolitan areas."