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NEW YORK (CNNMoney.com) -- Poor and middle-class families are entering the recession in a precarious situation due in part to declining or stagnant income growth, a study released Wednesday has found.

Incomes, on average, have declined by 2.5% among the bottom fifth of families since the late 1990s, while inching up by just 1.3% for those in the middle fifth of households, according to an analysis by the Center on Budget and Policy Priorities and the Economic Policy Institute, two liberal think tanks.

The wealthiest slice of Americans, however, saw their incomes rise by 9%.

The average income of the bottom fifth of families was $18,116; the middle fifth, $50,434; and the wealthiest fifth, $132,131.

Unlike what happened during the economic boom of the 1990s, lower- and middle-class families did not share in the prosperity of recent years, the report found. In fact, the United States has had its longest jobless recovery and slowest rate of payroll growth during this decade.

"We're worried about the impact of the downturn on the families whose incomes haven't recovered from the last recession," said Jared Bernstein, Economic Policy Institute senior economist and co-author of the report.

Wages have not kept up with inflation, families have loaded up on debt and homeowners have seen the value of their largest asset decline, he said. The situation will only get worse during the economic downturn.

"Families are uniquely economically exposed to the costs of recession," he continued. "As we head into a recession, their incomes will take a further hit."

The income gap between the rich and the rest of the population is widening. In 22 states, the top fifth of families made more than seven times what the poorest fifth took home, according to the report. In the late 1980s, only one state - Louisiana - had such a spread. Meanwhile, in more than two-thirds of the country, the wealthiest saw their income grow more than twice as fast as the middle-class over the past two decades.

State governments, however, can step in and help mitigate this growing inequality and insecurity, said Elizabeth McNichol, a senior fellow at the Center on Budget and Policy Priorities and the report's co-author. For instance, they can extend the amount of time workers receive benefits during an economic downturn. Also, they can offer or improve support services, such as child care, health insurance and transportation services.

Governments can also put resources into public services and infrastructure projects, which will help create jobs and stimulate the economy, said James Galbraith, professor at the University of Texas at Austin and income gap specialist. The federal government may also have to funnel more money to the states so they can maintain services at a time when tax revenues may decline.

The study is based on U.S. Census Bureau income data that have been adjusted for inflation, the impact of federal taxes and cash value of government subsidies. It does not factor in capital gains or losses. It compares data from 2004 to 2006 with that of 1987 to 1989 and 1998 to 2000.