"Dead man walking" is an expression used by prison guards as the condemned were led to their execution. This bull market is 76 months old (nearly twice as old as the average bull market), and, against all odds, still alive. Why against all odds? According to many investment pros and celebrities, this bull market was “supposed to” have died many months ago, but it keeps going. Is this bull market a dead man walking? About two years ago, I figured out what keeps this bull market alive. The same force that propelled stocks higher back than is still at work today. Fear Many, if not most, investors fear that a major market top is imminent, and have felt that way for years. Every time stocks decline a few percent, investors think; "That's it. The gig is over," and hit the sell button. This is the market's way of shaking out the weak hands. As soon as the weak hands leave the room, the party continues. The chart below suggests that investors are becoming more fearful, even though the S&P 500 is within 2% of its all-time high. The percentage of bullish retail investors surveyed by the American Association for Individual Investors (AAII) is (and has been) the most bearish (or fearful) since April 2013. I'll be the first one to admit that the AAII survey has a relatively poor track record and shouldn't be used as a timing tool. That's why I look at literally dozens of sentiment gauges. Nevertheless, the AAII poll shows a tendency. This tendency is confirmed by an entirely different sentiment indicator, the CBOE equity put/call ratio. The ratio shows how many put options (bearish bets) are traded for every call option (bullish bets). I use the five-day simple moving average (SMA) to smooth out daily swings. The five-day SMA of the equity put/call ratio is currently at 0.75, on of the highest readings in recent years. As the green circles show, the put/call ratio can still move higher, but the fact that it is already elevated after a mere 2% drop from the all-time high suggests that down side is limited. Is it time to “ fear fear”? I monitor dozens of indicators — including technical analysis, Elliott Wave, sentiment and seasonality — in an effort to get a comprehensive understanding of what's going on. Some indicators allow for a deeper correction, but sentiment — in particular investors' tendency to turn bearish on a dime — suggests that downside is limited. The only sentiment development that would suggest a deeper correction is: Bullish-sentiment extremes (which seems unlikely at this point), orFear morphing into panic and bearish sentiment turning into a self-fulfilling prophecy. This condition is too rare to bet on. More from MarketWatch