Investors may not be as dumb as one often cited study suggests, according to Jason Zweig, but their hot-stock-chasing ways still keep most from faring well over the long haul.

In a recent Wall Street Journal column, Zweig discusses the latest study from Dalbar, Inc., which finds that the average US stock fund investor has underperformed the S&P 500 by 7.4 percentage points annually over the past 30 years. Part of that is due to fees and expenses, “But the biggest factor is that investors chase returns — jumping aboard after a streak of hot performance and diving over the gunwales after it goes bad,” Zweig writes. “Because of that buy-high, sell-low behavior, investors in the typical fund have a lower average return than the fund itself.”

Zweig does discuss some reasons that Dalbar’s measured performance gap may be overstated. But the broader point — that investors lag the broader market, largely because of bad timing decisions — doesn’t seem to be in question. Zweig cites another study showing that stock investors lagged the stock market itself by 1.3 percentage points annually between 1926 and 2002, and says that ” even pension plans and other ‘sophisticated’ investors earn an average of at least three percentage points less than the hedge funds they buy. And several studies have shown that mutual funds outperform their own investors by between one and two percentage points annually.”

Zweig talks about some of the other reasons for this gap in investor performance and broader market returns. And he offers some advice: “You can do better than your fund if you add money when markets fall and stand pat (or trim a little) when markets are on the rise. You might never be able to beat the market, but you can beat your own fund if you buy when you feel like selling — and vice versa.”