Meet the 15 richest family-wealth dynasties in the US, who have a combined net worth of $618 billion

• The fortunes of wealthy family dynasties in the US have been increasing, according to a new report by the Institute for Policy Studies.
• The report looked at the 15 wealthiest family dynasties from the Forbes 400 list and found they have a combined wealth of $618 billion.
• Since 1982, the combined wealth of the top three families has increased by 5,868%, now totaling $348.7 billion.

Income inequality in America is getting worse, and dynastic wealth, which has been bemoaned by Warren Buffett, may be one of the reasons why.

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 Forbes 400 list and data from the Federal Reserve Survey of Consumer Finance.

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

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They found that the median American family owns just over $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 Mars - increased by 5,868%, now 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.