Take steps now to insulate your portfolio from the next big stock selloff.

More than five-and-a-half years into a bull market that has produced a 226.9% total return in Standard & Poor’s 500-stock index, it’s time to prepare for the next bear market.

See Also: The Five Best Stock Funds to Own Now

I don’t know when the gut-wrenching market plunge will occur. No one does. But I do know that stocks are no longer cheap. The S&P 500 trades at 18 times earnings for the past 12 months, compared with the long-term average of about 15.5 times earnings. It’s a good time to go over your portfolio with an eye toward how it will withstand the next bear market.

But first: What should you expect in the next bear market? Let’s look at history. The past is hardly a perfect guide to the future, but it’s the best we have. In spite of stunning global developments and technological advances, human behavior remains little changed. The same strain of mass euphoria that infected investors in the late 1990s was widespread in the late 1920s. Likewise, the same variety of pessimism that haunted us in the 1930s was prevalent after 2008, albeit to a lesser degree because the economy wasn’t nearly as decimated.

The good news is that the next bear market probably won’t be nearly as awful as either of the last two bear markets. That’s because the last two bear markets were the worst since the Great Depression. In the 2000-02 bear, the S&P tumbled 47.4%. The index took until late 2006 to recover—only to endure a 55.3% collapse from 2007-09. And we didn’t break even after that massacre until April 2012. (Ned Davis Research supplied the data on bear markets, which are defined as 20%-plus declines in the S&P 500 including reinvested dividends without intervening rallies of at least 30%.)

The last time a bear market hit investors so hard, Franklin D. Roosevelt was president. Stocks plummeted 51.5% in the 1937-38 bear market. That implosion was the worst since 1926, except for the Great Crash of 1929-32, when stocks utterly collapsed during a rapid series of back-to-back-to-back bear markets. Large-company stocks fell 44.4% in two months in late 1929, regained much of that loss in the ensuing months, and then sank again. By the time the ups (and mostly downs) were complete, the market had lost an unimaginable 83.5%. And that catastrophic loss, as well as the other return statistics in this article, includes dividends. A hapless soul who invested his money on Sept. 6, 1929, didn’t break even until December 1944, as the allies were advancing on Germany toward the end of World War II.

A typical bear market, by comparison, looks almost tame—but only by comparison. On average, bear markets have cost investors 36.9%. Since 1926, we’ve suffered through 19 bear markets, or one nearly every five years. On average, stocks have fallen for 11 months before hitting bottom.

Suppose you have the worst timing possible and dump all your money into stocks just as the market peaks ahead of a bear market. On average, it has taken such an investor three-and-a-half years to break even. But the cumulative return five years from the peak has averaged 25.1%—not great but not horrible, either.

Some bear markets barely cause a pinprick. I remember well the S&P 500’s staggering 20.5% crash on Oct. 19, 1987. I feared we’d fall into a depression, or at least a recession. Instead, the crash marked the end of a two-month bear market, the shortest on record, according to Ned Davis, and the economy barely missed a beat. The S&P sputtered and took almost two years to break even, but then it went on to post a parade of new highs in the following decade.