If all the countries have a 70% rate is OK, but if a single country has this rate, the millionaire will move to other countries, and so, they will not pay the 70% anyway.
In the immediate post WWII time, all the developed countries had very high top marginal income tax rate and they had high gdp growth rates
Can't speak for other countries, but hardly anyone in the United States in, say, the 1950s was paying the tax rates we conveniently throw around to "prove" that the tax burden was higher on the wealthy then.
And let's not forget that taxes are only 1 part of the equation. Ceteris paribus ("all things unchanged") is a thing in (economic) research. Perhaps tax rates negatively impacted growth in the 1950s while other variables that positively impacted economic growth were strong. To know the true causality between two variables you have to look at the whole picture and spend hours in STATA/other econometric software making up linear regression models with endless variables. It's too simplistic to say that high taxes don't harm the economy because taxes were high during a boom period (and it's also simplistic to say that tax cuts positively affect economic growth because the economy grew after taxes were reduced). What about other things that may affect the economy? A country with extremely high taxes but a strong natural workforce growth may perform better economically than a country with extremely low taxes and an ageing workforce, but that doesn't say extremely high taxes are great.
Anyway, personally I think the top marginal rate shouldn't exceed 50%, but that's more because of ideological reasons. I don't think top rates really affect the economy once they're below a certain level. I've read a study that claimed that the revenue maximizing tax rate (idk what the threshold was) depends on the income concentration at the top. The Dutch revenue maximizing top rate would be 49%, so I guess we have a practical reason to cut our top rate
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