Dalbar, Inc. has been tracking the performance of individual investors for years, and the group’s president says that, with the stock market climbing higher and higher, investors who want to jump aboard the bull train should be cautious.

“I’d say, if you can reliably predict where the market’s going, then jump in feet first — just buy, buy, buy,” Louis S. Harvey tells The New York Times. If not, he says, caution is warranted. “Make sure that you select a reasonably defensive asset allocation strategy first,” he says, adding, “The most important thing, once you have a strategy is to find a way to actually stick with it.”

Dalbar has found that investors have a strong tendency to make bad market timing decisions, jumping into overheated markets and selling when prices are low. “They get excited or they panic, and they hurt themselves,” Harvey says. Dalbar’s research shows that over the past 20 years through December, the average individual stock mutual fund investor earned 4.25% per year, while the S&P 500 returned 8.21%.  The gap is due in part to the fees mutual funds charge, but also to investors making emotional decisions to jump in and out of the market at inopportune times.

Economist and strategist Ed Yardeni, meanwhile, tells the Times that he thinks the market likely has more room to run. “Obviously, it would’ve been better to buy [stocks] in March 2009,” he said. “But buying now still makes sense if you believe we’re in a secular bull market.” He assigns a 60% probability to that outcome, though he says plenty of risks remain. “You want to be diversified,” he said. “You don’t want to just hold stocks. … [But] you do want to hold some stocks. You’re getting nothing for cash, and at some point bond yields will rise and there will be big losses in the bond market.”