Video - War - Slippin into Darkness - Live 1972 Chicago version
Downward mobility is most common among middle-class people who are divorced or separated from their spouses, did not attend college, scored poorly on standardized tests, or used hard drugs, the report says.
"A middle-class upbringing does not guarantee the same status over the course of a lifetime," the report says.
The study focused on people who were middle-class teenagers in 1979 and who were between 39 and 44 years old in 2004 and 2006. It defines people as middle-class if they fall between the 30th and 70th percentiles in income distribution, which for a family of four is between $32,900 and $64,000 a year in 2010 dollars.
People were deemed downwardly mobile if they fell below the 30th percentile in income, if their income rank was 20 or more percentiles below their parents' or if, in absolute terms, they earn at least 20 percent less than their parents. The findings do not cover the difficult times that the nation has endured since 2007.
Pew researchers said the study's structure did not permit an analysis of whether upward mobility has become more difficult through the years. Nonetheless, some economists point to growing income inequality and widely stagnating wages as evidence that the American Dream is slipping out of reach for many people.