One of the most important investment maxims consists of just one word: diversification. Almost any investment professional will urge you to hold a mix of stocks, bonds and other assets for protection from sudden market swings and the prospect of steadier returns.
But a stubborn subset of investors persists in ignoring that advice. Some 10.2 percent of the savers in a study by the Employee Benefit Research Institute, or EBRI, had no exposure to stocks in their 401(k) account as of 2013, and 11.8 percent had 90 percent or more of their money in equity funds.
A separate analysis for CNBC.com by Federal Reserve analysts, using data from the Survey of Consumer Finances, found that among households of all ages with a 401(k), IRA or both, 18 percent had less than 10 percent of their retirement assets in equities, and 20 percent of households had more than 90 percent in 2013.
Some may have chosen that allocation, while others may have simply drifted into it over time as some asset classes outperformed others.
"It's inertia. People set it and forget it," said Lorie Latham, a director in investment services at Towers Watson. In addition, a retirement plan's structure may induce people to choose a suboptimal investment mix, she said. She consults to large retirement savings plans, and said "they can have upwards of 20 to 30 percent [of plan assets] sitting in stable value" funds, which may curb volatility but generally provide lower returns.
Clearly, diversification can protect a portfolio. T. Rowe Price analyzed the performance of different asset allocations from Dec. 31, 1954, to Dec. 31, 2014, and found that while an all-equity portfolio delivered the highest return for its best year, it also delivered the worst for its worst year. A portfolio invested 80 percent in equities and 20 percent in bonds delivered the highest average annual nominal return, at 10.8 percent. (The all-equity portfolio returned 8.7 percent annually, on average.)
Between Dec. 31, 1985, and Dec. 31, 2014, T. Rowe Price found that a diversified portfolio invested 60 percent in equities, 30 percent in bonds and 10 percent in cash would have delivered 91 percent of the returns generated by 100 percent stock exposure, with about 83 percent of the volatility.
Optimal asset mixes also change as people age. Stuart Ritter, a senior financial planner at T. Rowe Price, frames that concept around inflation.
"The reason you hold stocks or don't hold stocks is that you are balancing short-term market risk against inflation risk," he said. "If I am using my money in two years, inflation isn't going to make that much difference, but market volatility would be a huge risk," so less equity exposure would make sense. "If I'm not using that money for half a century, market volatility doesn't feel good but I need to manage that inflation risk, and that means holding equities."
For that reason, Ritter is especially perturbed by the 9.9 percent of 20-somethings in the EBRI data who reported no equity exposure in their 401(k)s. "You've got an age group that has literally half a century before they use this money. The biggest threat is the increase in asset prices due to inflation," he said. (Tweet This)
Ritter has analyzed the performance of the S&P 500 index relative to inflation, looking at 15-year periods beginning each month from January 1926-December 1940 to January 2000-December 2014, and he found that the index beat inflation 95 percent of the time, by an average of 8 percentage points.
"When your goal is to keep up with inflation, historically, stocks have been the best asset class to do that," he said.
Read More Asset allocation key in retirement plans
To be sure, the EBRI data on 401(k)s only illustrates what people do with one portion of their savings. And while there are similar analyses like that conducted by the Federal Reserve, which examine asset allocation within other savings vehicles such as IRA accounts-many of which also show significant levels of extreme asset allocation-without more comprehensive data on those households it is hard to know just how many people actually have potentially problematic extreme asset allocations.
"It is hard to evaluate the overall balance sheet" of a household, said Sarah Holden, senior director of retirement and investor research at the Investment Company Institute, which has also examined retirement asset allocation. She added that when the institute conducts household surveys, people indicate that they do intend to rebalance their portfolios as they approach retirement.
Jack VanDerhei, research director at the EBRI, agreed that for many people a look at 401(k) allocations alone provides an incomplete picture. But 20-somethings are less likely to have other investment assets, he said, so the 9.9 percent of that cohort who reported having no equity allocation may be a concern.
Some of those people may have seen parents or a family member have an "unfortunate situation" in 2008 or 2009 when the stock market crashed. "There are always going to be some people who, unless you auto-enroll them, are going to be too scared to pull the trigger and go into equities on their own," he added.
VanDerhei believes the rise of automatic enrollment for 401(k) plans should help to reduce extreme allocations in those accounts, at least, as people change jobs.
Extreme allocation "will happen less and less as there is more job turnover," he predicted. "As people become new employees, they get auto-enrolled in new plans and they stick with the default [asset allocation]." And most automatic-enrollment plans use diversified target-date funds in their default enrollment option.
Even if employees stick with the same company and retirement plan, they may end up with a more diversified portfolio by default, Latham said. When employers re-enroll participants in their retirement plans, they will often take the reins with their plans, shifting workers' accounts into an investment option like a target-date fund unless they opt out.
"We should be concerned. At the same time, we should be heartened by how far we have come," said Ritter, noting that the share of 20-somethings with no equity allocation dropped by nearly half between 2005 and 2013 as automatic enrollment and target date funds gained favor with retirement plan administrators. Things are moving in the right direction."
More From CNBC