There is already ample evidence that people at the bottom suffer a range of health problems. More controversially, unequal societies appear to have higher levels of social ills, from teenage pregnancy to violence and obesity, that affect quality of life across the board.

Now there is another reason to decry growing inequality. Greater wealth correlates with selfishness and lack of empathy, which might help explain why the divide persists and the rich seem so reluctant to close it.

Apologists for inequality argue that it is harmless, or even a powerful motivator. The evidence suggests otherwise. A huge gap between have and have-nots is bad both for individuals and for society. It is in everybody's interests that we narrow it.