The Democratic Iowa caucuses happened on Monday, and inequality has been one of the biggest issues in the nascent presidential campaign.

Among the frontrunners in the race, Senators Bernie Sanders and Elizabeth Warren have made inequality and related issues central to their campaigns. Both candidates support drastically raising taxes on the wealthiest Americans, along with programs like a Medicare for All single-payer health system and student-debt forgiveness aiming to improve the fortunes of the middle- and working-classes.

Other candidates have taken different approaches to  inequality. Businessman Andrew Yang has proposed a $1,000 per month “Freedom Dividend” as a form of universal basic income, seeking to directly increase Americans’ incomes.

To help see why income and wealth inequality are such perennial issues, we made the following charts to illustrate how the fortunes of the top 1% have outpaced those of the rest of Americans.

One of the most basic ways to look at inequality is the share of income going to the top 1%.

top 1% income share

Business Insider/Andy Kiersz, data from World Inequality Database

The share of total national income going to the top 1% of Americans has changed over the last century. That share, as measured from an analysis of income tax records assembled by the World Inequality Database, dropped steadily from around 20% in 1913, when the IRS began keeping such records, to a little over 10% in the early 1970s.

Since then, the income share for the top 1% has sharply increased, and is now back up to its Gilded-Age-highs around 20%.

After 60 years of the richest Americans getting a smaller share of the national income — and therefore, a larger piece of the pie going to the bottom 99% — that trend has been reversing for four decades.

Read the rest of the article and charts at Business Insider