Historically, the highest Fidelity 401k yields have been achieved with a diversified stock portfolio averaging 4.8 percent per year, according to Fidelity. When combined with annual employer contributions, these accounts have reported average growth rates of 8.75 percent per year.
Besides stocks, 401k assets can be invested in bonds, annuities, certificates of deposit and other fixed income investments with less yield, risk and volatility. Fidelity manages ten different bond funds that return approximately one percent per year, as of 2015.
Returns on fixed-income investments are at historic lows due to low interest rates and no inflation. In 1995, the yield on a one-year CD was approximately five percent, in 2005, it was three percent and in 2015, it was less than one percent.