Since 1926, if you invested in the S&P 500 for any 16 year rolling time frame, you experienced a positive return 100% of the time. For all 10 year rolling time frames since 1926, you experienced positive returns 94% of the time. This shows that, historically speaking, if you invest for long enough the possibility of permanent loss is ZERO, AKA zero risk.
The results of research which studies investor behavior and analyzes investor market returns, consistently shows that the average investor earns below-average returns.
For the twenty years ending 12/31/2015, the S&P 500 Index averaged 9.85% a year. A pretty attractive historical return. The average equity fund investor earned a market return of only 5.19%.