If you never change your portfolio, leaving it the way you set it up, over the years you will:

• Own an investment mix that is vastly different from the one you selected.
• Have greater risk than perhaps you realize.
• Be betting that tomorrow’s markets will look a lot like today’s.

Did you ever think that doing nothing could mean so much?

The theme here: Once a year, rebalance your portfolio.

Take a look at the asset allocation you decided on when you set up your portfolio and compare it to what you have now. Do you have too much in bonds and too little in stocks? Or is your cash component smaller than what you planned on? If your investments are out of balance, you buy or sell until the portfolio is back in line with your asset allocation plan.

Why bother?

It’s a good question because just a few years ago in the booming stock market of the late 1990s, you could have ignored rebalancing and still looked like a very smart investor.

However, “The problem is in rising markets you often take on more risk than you’re suited to,” said Bryan Olson, vice president for investment research at Charles Schwab & Co.

He points out that if in the beginning of 1994 you had put together a 60 percent stock, 40 percent bond portfolio (using the S&P 500 and the Lehman Aggregate Bond Index), and you did nothing for five years, the rising stock market would change all that. At the end of 1999 you’d be 80 percent into stocks and have only 20 percent in bonds.

“Just in time for what, the stock market to turn around and come crashing down?” Olson asked in a telephone interview.

So you get smart; you rebalance in 1999 and go back to the 60-40 mix. The bear market pushes stocks down and bonds rise. You again do nothing until the end of 2002. At this point, you’re 40 percent into stocks and 60 percent into bonds, just in time for another market turnaround, Olson said — the current rising stock market and the recently troubled bond market.

Callan Associates, a San Francisco investment consulting firm, looked at the issue in 2000.

During the past 30 years, Callan’s research — and a similar study by Ibbotson Associates for 1977 through 2002 — found that rebalanced portfolios had virtually the same or slightly higher returns than those that were not rebalanced. They did so, generally, while taking on far less risk.

Same money, less risk — who wouldn’t want that?

The picture, it turns out, varies greatly by decade.

In the 1970s and 1980s, when we had choppy markets, Callan found that rebalanced portfolios had better returns and lower risk.

That’s in part because, before 1995, you could not discern a long-term pattern in the market that would have helped you pick the winner. No single asset class was on top for long.

All that changed in 1995, when growth stocks became the clear trend winner. For four years, large growth stocks, the S&P 500, beat anything else. For the fifth year, it was small growth stocks.

The result: If you had bought and held growth stocks in the entire 1990s, you would have come out with much better returns than someone who took money off the table, selling growth stocks at their highs and buying, let’s say, bonds at their lows.

The never-rebalanced investor, however, would have taken on more risk, as stocks became an ever-larger proportion of the portfolio because they were increasing in value. Because stocks tend to outperform bonds over the long term, this always happens.

This portfolio would have been badly hurt in 2000 through 2002, when bonds were the winner but it was stuck in declining growth stocks.

Those results tell you that if you do not rebalance, you are predicting the future. You are saying the uptrends and downtrends we see today are going to continue, and you’re positioned to profit from them.

You also are willing to overlook the chance that there will be a big market change or turnaround.

That’s a lot to overlook.

If there is a choppy market — and this is far more common in our lifetimes than a trending one with an obvious winning asset class — the least risky way to survive, just as in the ’70s and ’80s, is through regular rebalancing.

Rebalancing eliminates the need for decision-making. It forces you to sell the thing that’s gone up and buy the thing that hasn’t, as in sell high and buy low.

Of course, the transaction costs and tax consequences of rebalancing cannot be ignored. In fact, they may offset any extra returns that rebalancing delivers.

To keep them to a minimum, here are two suggestions:

• Direct new investments to that part of the portfolio that needs to grow. Don’t sell and pay taxes. Just shrink that asset’s importance.
• Handle all the adjusting you need to do in your tax-deferred portfolio, such as a 401(k) plan or an IRA. You pay no taxes, only transaction costs.

Finally, how often?

Once a year seems to work best for individual investors, says Ibbotson Associates.

Schwab’s advice is the same, or whenever the market has taken a major turn and your asset mix is out of line by 5 percent or more.