This is a great story in Barrons, written by the founder of Dalbar. They have done studies on investor behavior that basically found that investors with advisors do better than DIY investors because of their behavior. A few quotes:
"What we found were some of the folks who performed well in the 2008 crash. We found that people working with fiduciaries, advisors with a legal obligation to put the client first, and whose personal assets were on the line, tended to be among the winners—these clients were well protected."
"Consumer-oriented funds, whose shares are sold to the public like no-load funds and whose shares track some benchmark or follow a grid system, tended not to go into cash, to avoid style drift. Ultimately they suffered the consequences in bigger losses."
"Back in the late 1980s we found that investors tended to turn funds over way too fast and so missed recoveries and advances. When we first published our findings, we encountered absolute disbelief in how little investors were making because the concept of behavioral finance was not even born. The fund companies took the studies as a slam against them.
The underperformance was as much as 10% at the start of the survey. In previous years, we've broken out the do-it-yourself investors and compared their returns with those who used advisors—we found the former tended to underperform. Over time the gap has narrowed between investor results and mutual-fund performance, we think because of investor education, to 5%. There is more work to do. "