Economic opportunity grew more slowly for the average American household than for either the upper or lower end of the income scale over the past two decades, according to an index compiled by Hope Street, a not-for-profit organisation.

The study, led by Andrew Tilton, an economist at Goldman Sachs, tries to estimate how expected lifetime income has evolved for different segments of the US population. It finds that the expected lifetime income of the median US household did increase, from $1.3m to $1.7m (after inflation) from 1985 to 2005, but less rapidly than for other groups.