Tax cuts for the wealthy didn't boost the economies of the U.S. and 17 other countries — but they did worsen income inequality.

"Based on our research, we would argue that the economic rationale for keeping taxes on the rich low is weak," Julian Limberg, a co-author of the study and a lecturer in public policy at King's College London, said in an email to CBS MoneyWatch. "In fact, if we look back into history, the period with the highest taxes on the rich — the postwar period — was also a period with high economic growth and low unemployment."

Already, Mr. Trump's tax cuts have lifted the fortunes of the ultra-rich, according to 2019 research from two prominent economists, Emmanuel Saez and Gabriel Zucman of the University of California at Berkeley. For the first time in a century, the 400 richest American families paid lower taxes in 2018 than people in the middle class, the economists found.

Only about half the U.S. population is invested in the stock market through their retirement and savings accounts, and even then more than 80% of all stocks are owned by the richest 10%.