Fewer retirement savers are keeping all - or none - of their 401(k) accounts in stocks

Fewer investors are going all-in with their retirement savings.

Less than 10 percent of 401(k) accounts administered by Fidelity Investments were invested entirely in stocks last quarter. It's the latest step in a yearslong march toward more balanced nest eggs. Fidelity keeps records for 13.1 million 401(k) participants, and its figures reach back to 2001, when the dot-com bubble was deflating and 33 percent of 401(k) plans were entirely in stocks. The percentage has dropped every year since then.

Putting all your retirement savings into stocks can be tempting. Stocks have the potential for much bigger returns than bonds or cash, particularly when bonds and money-market funds are offering such low interest rates. Last year, the Standard & Poor's 500 index returned 32.4 percent, catnip for aggressive investors looking to maximize their savings.

But anyone keeping all their portfolio in stocks needs to have the willpower to hold steady when stocks fall, which can happen suddenly and often. The S&P 500 lost 37 percent in 2008, for example, and plunging 401(k) values caused much heartache. Anyone whose fright led them to sell then would have missed out on the big ensuing gains. A more balanced portfolio, including a mix of stocks and bonds, would have offered a smoother ride.

Conservative savers, meanwhile, can suffer if they take the opposite approach: not owning any stocks. They may avoid the ups-and-downs of the stock market, but they also lose out on the growth potential that stocks provide. That makes it tougher to keep up with inflation.

Only 5 percent of 401(k) accounts had zero stocks last quarter, according to Fidelity. That's down from 5.9 percent a year earlier and from 12 percent in 2001.

More investors have better balanced accounts as a result of the increased popularity of target-date mutual funds. These funds automatically shift the assets in a portfolio, staying heavy on stocks when the planned retirement date is far away and shifting toward bonds as the date approaches.

Nearly three quarters of all dollars in target-date funds were controlled by just three fund companies last year, and none of them had any funds that were 100 percent in stocks, according to Morningstar.