Here is a historical chart of the cyclically-adjusted price/earnings ratio (CAPE, inverted, left axis, blue line) vs 10-year annualized forward S&P 500 total returns (right axis, orange bars). 
Here is a similar chart, showing the cyclically-adjusted price/earnings ratio (CAPE, inverted, left axis, blue line) vs 10-year annualized forward outperformance/underperformance of the S&P 500 over gold (right axis, orange bars).
This chart shows my preferred version of the equity risk premium (inverse S&P 500 CAPE ratio minus current 10-year UST yield, blue line) and forward 10-year annualized outperformance/underperformance of the S&P 500 over 10-year UST (orange bars).
And this one shows 10-year t-note yields (blue line) along with the 10-year forward annualized inflation-adjusted returns of those 10-year UST notes (orange bars).
A notable observation is that the S&P 500 has never provided good 10-year returns from points where the CAPE ratio was this high, and yet the S&P 500 has also never underperformed t-notes over a 10-year period starting with an equity risk premium this high. 🧐
