The U.S. stock market selloff ushered in the New Year with such a scare that a record 6 million investors called Fidelity Investments asking what they should do with their money.

On Jan. 4, the first trading day of 2016, clients returned from the holidays to face a 276 point plunge in the Dow Jones industrial average that wiped out a good chunk of their portfolio.

Already, retirement accounts at Fidelity ended 2015 lower than a year ago, with average 401(k) balances dropping to $87,900 from $91,300, the Boston-based brokerage firm's latest analysis showed on Thursday.

For the two-thirds of workers who have at least some savings in target-date funds, geared toward a future retirement date, the lower balances raised doubts about their choices.

Target-date mutual funds are managed by professional advisers who adjust the mix of stocks, bonds and cash equivalents based on the selected time frame.

But it can be hard to fight the urge to do something -- anything -- in the face of economic uncertainty.

"They're worried about what's going on with China, what's going on with the election," said Jonathan Kelley, a partner with Hinds Financial Group, a wealth advisory firm in Lakewood, Colorado.

One client called this week, Kelley said, wanting to move everything in his portfolio into bonds and other alternative investments.

Kelley's job on days like this is to talk his clients out of making emotional decisions. He typically creates a mix of investments for them, sometimes with target-date funds and a custom mix that operates like a target-date fund, and sticks with it.

Most investors do the same.

At Vanguard, more than 99 percent of account holders do not make any trades. In the fourth quarter of 2015, when the market was also bouncing around, only 4.5 percent of Fidelity's retirement account holders put through a trade, which is consistent with most months, said Jeanne Thompson, a vice president at Fidelity.

Even in the fourth quarter of 2008, in the depths of the last recession, only 6 percent of Fidelity account holders changed their allocations, and only 1 percent took everything out of equities and went to cash.

"We followed those people over time, and if they stayed in cash, they didn't fare nearly as well as the people who stayed in equities," said Thompson.

Thompson advises investors to analyze their accounts at least once a year to make sure they are on the right track.

Of particular concern are those who take half-measures: keeping part of their balance in target-date funds and trying their hand at allocating the rest.

"It's designed to be an all-in-one solution," Thompson said. "You're kind of defeating the purpose of it if you don't."

Custom Allocations

If leaving everything to the target-date fund manager makes you too anxious during big market swings, you may customize allocations for additional fees. Thompson said more employers are offering managed account options within 401(k) plans. Private financial advisers also can do this, as can automated online account management services, like Betterment.com.

Rather than dump target-date funds altogether, an investor could adjust a portfolio simply by choosing a more appropriate fund, Thompson said. Do a quick search online to see how your fund stacks up against other options in terms of return and expenses, she advised.

Millennials, in particular, could be in target-date funds that underestimate how long they will work. A study Scottrade released on Wednesday found that millennials aged 18 to 34 on average expect to retire at 55.

If a 30-year-old selects a target-date fund set for 2042, that may be unnecessarily conservative, leaving him or her without enough income to retire then.

Right now, the stock-bond mix in a 2042 target-date fund would probably be similar to one set for 10 years later, Thompson said, but investors would need to review their options as they get older.