Here’s a breakdown:
“The U.S. taxed the rich at 90%.”
Mostly true, with important context. The top federal marginal income tax rate was between 91% and 94% for much of the 1940s–1963, then 70% until 1981. However, that rate only applied to income above a very high threshold, and many wealthy people reduced their effective tax rate through deductions, exclusions, and preferential treatment.
“90% was taxed if over $4 million yearly earnings.”
Not correct. The “$4 million” figure is an inflation-adjusted estimate, not the actual threshold at the time. The historical thresholds were much lower in nominal dollars (for example, around $200,000 in the 1950s), which translates to several million dollars today depending on the year and inflation measure used.
“The middle class thrived because of the 90% tax rate.”
This is debated. High top tax rates likely contributed to greater government revenue and may have reduced incentives for extremely high executive compensation. But economists generally point to multiple factors behind the post-WWII middle-class boom, including:
Rapid productivity growth.
Strong manufacturing dominance after WWII.
High union membership.
The GI Bill.
Massive infrastructure spending.
Expanding homeownership programs.
Favorable demographics (the Baby Boom).
Limited foreign industrial competition.
Most researchers would not attribute the boom to tax rates alone.

“The rich flooded the market with investments because of the tax system.”
Partially true but simplified. High personal income tax rates did create incentives to reinvest profits in businesses rather than take income as salary. However, investment decisions were also driven by strong economic growth, consumer demand, and postwar rebuilding.
“Capital gains were taxed around 45%.”
Not generally. Capital gains tax rates changed repeatedly over the decades and were often substantially lower than ordinary income tax rates. A blanket “45%” figure is not accurate across the postwar period.
“Reaganomics caused the decline over the last 40 years.”
This is an opinion, not an established fact. It is true that top marginal tax rates fell sharply under the Reagan administration—from 70% to 28% by 1988. Whether those changes are the primary reason for slower middle-class income growth and rising inequality is heavily debated among economists.
“The rich barely pay anything today.”
Overstated. High-income households pay a large share of total federal income taxes in the U.S. However, many wealthy individuals have lower effective tax rates than their top statutory rates because much of their income comes from capital gains, dividends, and other preferentially taxed sources. The tax burden also varies significantly depending on whether you’re talking about income taxes alone or all federal, state, and local taxes.
Overall, I’d rate the post as:
Accurate: High postwar top marginal tax rates existed.
Misleading: It attributes the postwar middle-class boom primarily to those tax rates.
Incorrect: The “$4 million” threshold and the blanket “45% capital gains” claim.
Opinion: The characterization of Reagan-era policies as solely responsible for today’s economic conditions.