Inequality is usually measured by comparing incomes across households within a country. But there is also a different kind of inequality: in the affordability of homes across cities. The impact of this form of inequality is no less worrying.

In many of the world's urban centres, homes are becoming prohibitively expensive for people with moderate incomes. As a city's real-estate prices rise, some inhabitants may feel compelled to leave. Of course, if that inhabitant already owned a house there that they can sell, they may regard the price increase as a windfall that they can claim by departing. If not, however, they may be forced out with no compensation.