LyrArc Article Gist
A study by professors at Yale, University of Southern California, and UC Davis, (Geanakoplos, Magill, Quinzii) uses a MY ratio of persons 35-49 years in age to persons 25-34 years in age to predict stock market performance trend. This number is expected to rise in years 2018 to 2035 as more millenials come into an age when they need to start investing in the stock market. This kind of model would show a much better performance for the stock market and S&P 500 for the next decade than for the period 2000-2015, years of the financial crisis and recovery. 

Part of the reason not mentioned here is the gradual completion of the recovery itself from the financial crisis and the controls put in place to prevent a recurrence of past mistakes in financial markets. Needs for infrastructure and defense spending, efforts to ensure more favorable trade relations, efforts to help the middle class, university students in future budgets to increase opportunity, would create more opportunities for equity price growth.