Professors Wilkinson and Pickett crunch the numbers and show that the same relationship holds true for a range of social problems. Among rich countries, those that are more unequal appear to have more mental illness, infant mortality, obesity, high school dropouts, teenage births, homicides, and so on.

They find the same thing is true among the 50 American states. More unequal states, like Mississippi and Louisiana, do poorly by these social measures. More equal states, like New Hampshire and Minnesota, do far better.

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.

“Inequality is divisive, and even small differences seem to make an important difference,” Professors Wilkinson and Pickett note. They suggest that it is not just the poor who benefit from the social cohesion that comes with equality, but the entire society.

So as we debate national policy in 2011  from the estate tax to unemployment insurance to early childhood education  let’s push to reduce the stunning levels of inequality in America today. These inequities seem profoundly unhealthy, for us and for our nation’s soul.