Professionally managed retirement accounts were the hot 401(k) trend in 2011, according to Vanguard's "How America Saves 2012" report. It found that 33 percent of all Vanguard 401(k) plan participants invested their entire account balance in either a single target-date fund (TDF) or balanced fund or through a managed account advisory service.

Of those, 24 percent invested in a single TDF; 6 percent invested in a single traditional balanced fund; and 3 percent invested in a managed account advisory program. In 2005, only 9 percent of Vanguard's 401(k) plan participants invested in managed retirement accounts.

According to the report, these options have become more popular because they improve the diversification in a retirement portfolio, which reduces the risks associated with having too much exposure to equities and market volatility or too little. In 2011, 18 percent of Vanguard participants took an extreme position in equities, holding either 100 percent in equities (10 percent) or no equities (8 percent). In contrast, a total of 34 percent of participants held extreme equity positions in 2005.