Income inequality in America is getting worse. In 2018, it reached the highest level it's ever been since the US Census began tracking it 50 years ago, reported Taylor Telford for The Washington Post, citing the US Census 2018 American Community Survey released this week.

Dynastic wealth, which has been bemoaned by Warren Buffett, may be one of the reasons why.

In October 2018, the left-leaning Institute for Policy Studies' "Billionaire Bonanza" report examined the growing concentration of wealth in the US by looking at 15 dynastically wealthy families from the latest Forbes 400 list (also published in October 2018) and data from the Federal Reserve's Survey of Consumer Finance (which is from 2016 and has been adjusted for inflation to 2018 dollars).

"Each of these family's wealth comes from companies started by an earlier generation, either a parent or more distant ancestor," the report said. "Each of them also represents a wealth dynasty passing generation to generation free from interruption."

Read more: Meet the Mars family, heirs to the Snickers and M&M's candy empire, who spent years avoiding the limelight and are America's third-wealthiest family 'dynasty'

It found that the median American family owns just more than $80,000 in household wealth, while the 15 family dynasties own a combined $618 billion.

Since 1982, the combined wealth of the top three families — the Waltons, the Kochs, and the Marses — has increased by 5,868%, totaling $348.7 billion.

Here's a closer look at how they and the rest of the wealthy dynastic families from the Forbes 400 built their wealth, ranked from lowest net worth to highest net worth. The estimated total wealth for each family is a sum of each individual family member's wealth listed on the Forbes 400 and does not include the wealth of any family members not on the Forbes 400.

Note that the Forbes 400 doesn't encompass all of America's wealthy dynastic families — some individual family members have less than the $2.1 billion in personal wealth needed to make the list.