Dalbar Inc.’s latest Quantitative Analysis of Investor Behavior (QAIB) is out, and –once again– the results show that most investors continue to shoot themselves in the foot.

“The average investor in a blend of equities and fixed-income mutual funds has garnered only a 2.6% net annualized rate of return for the 10-year time period ending Dec. 31, 2013,” Sean Hanlon writes in discussing the study’s findings in a Forbes.com column. By contrast, the S&P 500 has returned 7.4% over that same period. “The same average investor hasn’t fared any better over longer time frames,” adds Hanlon. “The 20-year annualized return comes in at 2.5%, while the 30-year annualized rate is just 1.9%.”

Why do average investors fail so miserably? “Investors may only have themselves to blame. According to Dalbar’s QAIB, investors make poor investment choices that hurt their investment returns,” Hanlon says. “These decisions, including when to buy and sell, are often driven by emotion.” Hanlon offers some examples of how emotional decision making can hurt returns and result in higher taxes paid.

To see Dalbar’s full study, click here.