The following comes from FundAdvice.Com. They used the period 1970 through 2002, which includes two major market meltdowns, one bout of runaway inflation, and a couple minor market crises. You'll have to assess your own risk/reward tolerance, but the value of a fixed income allocation seems obvious.

% Return
Equity/Fixed Income Ratio 60/40 70/30 80/20 S&P 500

Compound Annual Return 11.6 12.1 12.7 10.8

Worst 12 Months -20.6 -24.8 -28.9 -38.9

Worst 60 Months 7.9 4.0 0.1 -11.2