A study released last week by the Pew Charitable Trusts shows that over a generation African-Americans have a more difficult time maintaining middle class status than other groups.

The study, which began in 1968, tracked the economic status of more than 2,300 Americans, 730 of whom were African-American. Forty-five percent of blacks born to middle-class families in 1968 slid down the socioeconomic ladder. Their median family income was $23,100 compared to an inflation-adjusted $55,600 for their parents in 1968.

Only 16 percent of whites born to middle-class families had lower median incomes than their parents.

For lower-income blacks and whites, this disparity also existed.

The study found that 90 percent of whites born into low-income families now earned more than their parents did. In comparison, 75 percent of blacks surpassed the income levels of their parents.

Researchers don’t have an explanation for why the gap exists. But some economists have speculated the increase in single, mostly female headed households in the black community and the difference in education levels between blacks and whites are two factors.