The success of your retirement plan is highly dependent on your assumption about the rate of return your 401(k) and individual retirement account (IRA) will earn. If you’re using a retirement calculator that fills in the expected return based on long-term historical averages, you may be overestimating how big your nest egg will grow.

“Compared to historical averages, today we are in a world of lower expected returns,” said a recently published warning by AQR Capital Management, which manages almost $160 billion in assets. “We estimate 4 percent prospective real return for U.S. stocks and near zero for 10-year Treasuries for the coming 5 to 10 years. These estimates are near the lowest levels seen in the past century for both asset classes.”

Other investment pros are also cautioning that seven years into this bull market, returns will probably be lower over the next decade. Research Affiliates, an investment firm whose strategies are the backbone of mutual funds, exchange-traded funds, and other portfolios, says that in the next 10 years it expects U.S. large-cap stocks to return just 1.1 percent annualized after inflation, and core U.S. bonds less than 1 percent. (Inflation is currently struggling to get back above 2 percent, and the long-term norm is about 3 percent.)

GMO, another investment firm with more than $91 billion under management, says it expects U.S. stocks and bonds to post negative annualized returns over the next seven years after adjusting for inflation. The McKinsey Global Institute, an arm of the consultancy company, says that if global economies continue on their current slow growth pace, U.S. stocks may return 4 percent to 5 percent annualized over the next 20 years after adjusting for inflation. That’s well below the 7.9 percent of the past 30 years. Schwab and BlackRock are also on record as saying that the road ahead is not going to be as profitable as we have come to expect.

All of this suggests that one of the smartest decisions you can make for your retirement plan is to stress-test your return assumptions.

Luke Delorme, director of financial planning at American Investment Services, uses a “conservative” assumption of 4 percent annualized returns (that’s before inflation) for clients who have a classic portfolio that is 60 percent stocks and 40 percent bonds. “Plug that into a calculator and see how that might impact your retirement plan,” he says.

If You're Not Saving Enough

If the results alarm you about whether you'll have enough money for retirement, it’s time to make some timely tweaks:

Sock away more. “It isn’t romantic, and it isn’t what people want to hear, but saving more is the best response,” says Brad McMillan, chief investment officer of Commonwealth Financial Network. Very few individuals contribute the maximum to their 401(k). This year that’s $18,000 if you are younger than 50 and $24,000 if you are 50 or older. The IRA limit is $5,500 ($6,500 if you are at least 50).

Spend less. You’ll free up more cash flow to put toward retirement savings, McMillan says. It can have an even more lasting impact on your retirement plan by reducing the amount of retirement savings you will need to support yourself.

Rebalance and don’t add risk. Some pockets of the global marketplace are expected to produce higher returns. Both Research Affiliates and GMO expect emerging-market stocks to post strong returns. But that slice of the market is usually far more volatile and you may not want to overload your portfolio with more risk. A smarter move right now is to check your mix of stocks and bonds and compare it with your long-term diversification goals. Chances are that after seven years of investing in equities during a bull market, your exposure to stocks is higher than you wanted. “Take some profits from your winners and rebalance your portfolio," McMillan says. "That’s always the best way to buy low and sell high.”