Income inequality is likely worse than before the Great Depression

Further, the top 1% richest U.S. families own 40 times the average family's wealth.

• "No country (apart from Russia) for which estimates of wealth inequality are available has similarly high recorded levels of wealth inequality," Zucman writes.

Between the lines: Perhaps the most interesting part of Zucman's research may be his point that the top 1% of American households likely hold much more of the nation's and the world's wealth than anyone realizes.

• "It is not enough to study wealth concentration using self-reported survey data or tax return data," Zucman says in the report, estimating that 8% of the world’s household financial wealth is held offshore.

"Because the wealthy have access to many opportunities for tax avoidance and tax evasion—and because the available evidence suggests that the tax planning industry has grown since the 1980s as it became globalized—traditional data sources are likely to under-estimate the level and rise of wealth concentration."

Zucman also notes that data shows the share of total wealth owned by the top 1% has increased by 9 points since 1989 and by 10 points when including the Forbes 400. In capitalized income estimates, it has increased by 11 points.

• "The share of wealth owned by the bottom 90% has collapsed in similar proportions."

Worth mentioning: Zucman is one of the economists behind Elizabeth Warren's wealth tax proposal.