So why is inequality so harmful? “The Spirit Level” suggests that inequality undermines social trust and community life, corroding societies as a whole. It also suggests that humans, as social beings, become stressed when they find themselves at the bottom of a hierarchy.

That stress leads to biological changes, such as the release of the hormone cortisol, and to the accumulation of abdominal fat (perhaps an evolutionary adaptation in preparation for starvation ahead?). The result is physical ailments like heart disease, and social ailments like violent crime, mutual distrust, self-destructive behaviors and persistent poverty. Another result is the establishment of alternative systems in which one can win respect and acquire self-esteem, such as gangs.

Granted, humans are not all equal in ability: There will always be some who are more wealthy — and others who constitute the bottom. But inequality does not have to be as harsh, oppressive and polarized as it is in America today. Germany and Japan have attained modern, efficient economies with far less inequality than we have — and far fewer social problems. Likewise, the gap between rich and poor fell during the Clinton administration, according to data cited in “The Spirit Level,” even though that was a period of economic vigor.