How will high market concentration and valuations affect stock returns?
Core argument: The S&P 500 is estimated to deliver a 3% annualized nominal total return over the next 10 years — the 7th percentile since 1930 — and roughly 1% in real terms.
We estimate the S&P 500 will deliver an annualized nominal total return of 3% during the next 10 years (7th percentile since 1930) and roughly 1% on a real basis. We model prospective long-term equity returns as a function of five variables: starting absolute valuation, stock market concentration, economic contraction frequency, corporate profitability, and interest rates. The S&P 500 index during the past decade has posted an annualized total return of 13%. The annualized return translates into a cumulative 233% total return across the period. Dividends contributed 54 pp (roughly 1/4) of the total return. Of the remaining 178% of price gain, 127 pp (nearly 3/4) was attributable to increased earnings during the period and 52 pp (approximately 1/4) was attributable to valuation expansion. Increased sales, higher operating margins, and lower tax rates contributed to annualized earnings growth of 7% over the period while the forward P/E multiple expanded from 16x to 22x. The 13% annualized stock market return during the past ten years exceeded the long-term average of 11%.

