(I believe Mungofitch has talked about this and surrounding issues in the past.)

This is a Fed working paper addressing the topic "End of an era: The coming long-run slowdown in corporate profit
growth and stock returns"

The Federal Reserve: PDF document

"I show that the decline in interest rates and corporate tax rates over the past three decades accounts
for the majority of the period's exceptional stock market performance. Lower interest expenses
and corporate tax rates mechanically explain over 40 percent of the real growth in corporate profits
from 1989 to 2019. In addition, the decline in risk-free rates alone accounts for all of the expansion
in price-to-earnings multiples. I argue, however, that the boost to profits and valuations from ever-
declining interest and corporate tax rates is unlikely to continue, indicating significantly lower
profit growth and stock returns in the future."