Historical Returns in the S&P 500

Many Government Employees have little clue to how the S&P Composite has performed prior to the 1980's.  The Thrift Savings Plan was implemented in the late 1980's when the stock market went through the greatest Bull Market of all time since it's inception in the late 1700's.  The chart below is the Historical Performance by Decade in the S&P Composite since the 1790's.  As you can clearly see the 1980's and 1990's saw the S&P gain 227% and 316% respectively which was way above the Long Term Decade Mean of 63%.

In the first decade of the 21st Century the S&P lost 24% which was the first decade since the 1930's to have a negative return.  Keep in mind there hasn't been two decades in a row where the S&P has had negative returns so that means the period from 2010 through 2019 will have a positive return.  However in 
my humble opinion the period of the 1980's and 1990's will not be repeated for several more decades and overall Returns by Decade going forward will likely revert back to or below the Long Term Mean of 63%.  Notice there have been several periods where the Decade Performance has been Positive but below the Long Term Mean of 63%.  Thus going forward investors may have to deal with Average Yearly Returns of 5% or less in the "C" Fund which tracks the S&P.