Inequality used to be only about money affecting our politics. Nevertheless, inequality will be a hot topic as we gear up for the 2020 presidential campaign, at least in the Democratic Party.

Inequality, at the level experienced in our country, is corrosive to our democracy in a specific way. Very wealthy people use their wealth to alter the political system to their advantage. The results are policy and law changes that "tilt the playing field" so that the wealthy few have exaggerated influence in our democracy compared to the majority of us. (Look no further than the Trump "tax cut" to see this influence in action.)

Of course, such talk will be widely panned by the Republican Party as "the politics of envy" and dismissed as a topic unworthy of discussion. (That is another mistake this party will make.)

However, inequality turns out to be more corrosive to societies than rich people gaining unfair political influence. Consider The Index of Health and Social Problems used in more developed democracies.

This Index measures life expectancy, trust, mental illness (including alcohol and drug addiction), obesity, infant mortality, children's math and literacy scores, imprisonment rates, homicide rates, teenage births and social mobility.

In their 2009 book "The Spirit Level," epidemiologists Richard Wilkerson and Kate Pickett used this index to demonstrate that these health and social problems are more common in more unequal countries. The United States carries the dubious distinction of being the country with the most exaggerated difference between the incomes of rich vs. poor, while our population suffers a wide range of these health and social problems. This pattern is not only shown in well-off democracies but also in our 50 states. The greater the inequality, the greater are these problems.

Wilkerson and Pickett's new book "The Inner Level" explores this phenomenon further to give more detail and understanding.

When inequality is the subject of political debate, pay attention. It's important.

Steve Miller, Nauvoo, Ill.