Figure 1. Figure above: (a) Historical S&P 500 and (b) CPI.  The impact of inflation is clearly significant over the past 2.7 decades, as evidenced by the "adjusted" S&P 500  (S&P 500 historical data and CPI historical data courtesy Yahoo.com).

The "adjusted" S&P 500 is essentially "flat" for the past decade -- which agrees with current assessments regarding average Market performance during this period.  The decline, of course, during the 10/2008 - 3/2009 period is amply evident.

Figure 2. (Figure below: Attempt to fit bi-modal Gaussian PDF to S&P 500. -- Histogram S&P 500 data based on Yahoo.com S&P 500 historical data.)

Figure 3. Figure left: Error surface for fit of various multi-modal Gaussians to adjusted S&P 500.  Results indicate that a bi-modal Gaussian yields the smallest  RMS error.  When running a Monte Carlo based on the Market it is best to use a bi-modal PDF, rather than a single Gaussian, to approximate historical Market performance. (Historical S&P 500 data courtesy Yahoo.com)