Zucman: Very poorly. Their average pretax income today is $18,000 per adult. In 1980, adjusted for inflation, it was $18,000. So half of the population has had basically zero economic growth over more than a generation. At the same time, income at the very top has been increasing enormously.

Q: Along with your book, you launched a website this week that lets users assess the effects of various tax plans. Under Bernie Sander’s tax proposals, billionaires could face effective rates of 97.5%, versus 62% under Warren’s policies and 23% now under Trump’s tax reform. What about the argument that by putting these super high taxes on the wealthy, you’re putting them on so-called “job creators” and you’re going to hurt the economy?

Zucman: You have to look at the data. How was the economy performing in the 1950s and 1960s, when top tax rates were very high? And how has it been performing more recently? What you see is average income per adult grew 2.2% from 1950 to 1980. And since 1980, it’s grown 1.2% a year on average. So it’s been almost divided by two.

The way that today’s rich countries have become prosperous and wealthy is not by granting low tax rates to billionaires. It’s by investing in the education and the health and the well-being of everybody. Also, extreme wealth concentration means an extreme concentration of power. It means the ability to influence policy making and markets, to buy new entrants, to fight the IRS, to solidify established monopoly positions. If you reduce the wealth of the wealthy, you reduce their influence on policy making and you protect democracy. That’s a very old argument that’s always been at the center of tax debates in the U.S., which thought of itself very much in reaction against the highly unequal European countries in the 19th century and early 20th century.

Q: Why do we need a wealth tax? Aren’t there other ways to fight inequality?

Zucman: Yes, there’s no magic bullet. You need a combination of tools, like fixing the corporate tax and more progressive income taxation. But the wealth tax is necessary to properly tax the super-rich, because many of them as we discussed earn a lot of economic income, but little taxable income.

Q: There are practical concerns about a wealth tax, about its constitutionality and also about how you value fortunes. A good example is WeWork, which seems to have a wildly different value from week to week. How would you tax a stake in WeWork under a wealth tax?

Zucman: First of all, most of the wealth of the top 0.1% is in listed securities and bonds and other forms of wealth that have clear market values. The only difficult thing to value is shares in unlisted or private or closely held businesses. The proposal we are making in the book is the IRS would try its best to come up with a valuation by looking at comparable listed firms. If people disagree, they would always have the opportunity to pay in kind, with shares. The point is not for the government to control businesses. The point is to create a market that’s missing and then to auction the shares to generate tax revenue.

Q: Trump’s campaign seems to be gearing up to make the 2020 elections a battle over “socialism.” Sanders calls himself a socialist, while Warren says she’s a capitalist. How do think about this debate?

Zucman: These debates are mostly about semantics. On many policy areas, Warren and Sanders are pretty similar. A market economy is a good way to produce goods and services, but it needs to be regulated, and in particular through the tax system. Otherwise it tends to generate extreme levels of inequality that then at some point conflict with democracy and our meritocratic ideals.