The odds of moving up — or down — the income ladder in the United States have not changed appreciably in the last 20 years, according to a large new academic study that contradicts politicians in both parties who have claimed that income mobility is falling.

Both President Obama and leading Republicans, like Representative Paul Ryan, have argued recently that the odds of climbing the income ladder are lower today than in previous decades. The new study, based on tens of millions of anonymous tax records, finds that the mobility rate has held largely steady in recent decades, although it remains lower than in Canada and in much of Western Europe, where the odds of escaping poverty are higher.

Raj Chetty, a professor of economics at Harvard and one of the authors, said in an interview that he and his colleagues still believed that a lack of mobility was a significant problem in the United States. Despite less discrimination of various kinds and a larger safety net than in previous decades, the odds of escaping the station of one’s birth are no higher today than they were decades ago.

The results suggested that other forces — including sharply rising incomes at the top of the ladder, which allows well-off families to invest far more in their children — were holding back talented people, the authors said.