Which matters more: actual levels of inequality, or the levels that people perceive to exist? There is now clear evidence that perceptions of inequality have a profound impact on societies all over the world — and that people aren’t very good at judging the reality for themselves.

The trouble is, almost all research on inequality focuses on actual levels, which means our understanding of how people respond to inequality has been based on the assumption that people’s perceptions align with reality. And this, according to a unique survey of Indonesian people by the World Bank, is just not true.

The lack of credibility of this assumption suggests a range of research on inequality needs to be revisited, including Thomas Piketty’s famous theory that the actual level of social mobility in a country is related to people’s support for redistribution; the Indonesia results...