The economic fates of diverse cities such as San Francisco, New York and Detroit would seem to be vastly different – but they share a common thread.

Over the last half-century, the story of America’s cities is a tale of booms and busts. New York and tech hubs like San Francisco – once cities in financial distress – have transformed into economic powerhouses. At the other extreme, one-time prosperous manufacturing cities like Detroit now find themselves in economic turmoil.

Viewed in isolation, the economic fates of these cities would seem to bear little resemblance to each other. However, they all share a common thread: since 1970 these cities, like nearly every other major American city, have experienced a “hollowing out” of the middle class.

The graphic above shows the change in income distribution in 20 major US cities between 1970 and 2015. In 1970, each of these cities exhibits a near-symmetrical, bell-shaped income distribution – a high concentration of households in the middle, with narrow tails of low and high-income households on either end. By 2015, the distributions have grown more polarised – fewer middle-income households, and more households in the low-income and/or high-income extremes.

The growing polarisation of wealth in US cities is quite stark when viewed geographically. The maps below show the changing distribution of wealth in four US cities between 1970 and 2015. All dollar amounts are adjusted for inflation.