While we're in a raging bull stock market now, one day we won't be. And at that point, you'll really want a mutual fund that performs well in a bear market.

Research shows that mutual funds that have the best returns when the market is high don't necessarily outperform the following year.

The Wall Street Journal's Joe Light offers a statistic to measure how a fund performs in bull and bear markets, courtesy of David Vincent, a vice president at Fred Alger Management. It's about capture ratios.

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For starters, say you have a large cap stock fund that loses 8 percent when the Standard & Poor's 500 Index slumps 10 percent. That gives you downside capture ratio of 80 percent.

What you should look at is the ratio of the upside capture ratio over the downside capture ratio. A ratio above 1 will often mean a fund does well in both bullish and bearish markets.

For example, a fund with an upside capture ratio of 110 percent and a downside ratio of 90 percent would have an upside/downside ratio of 1.2.

Of 2,547 stock funds tracked by Morningstar, 59 percent had an upside/downside ratio of less than 1 for the five years through April. Only 9 percent had a ratio greater than 1.1.

Vanguard Group founder John Bogle's solution to this issue would be to just buy low-fee index mutual funds. And if you're looking for a low-cost, simple way to achieve market returns, that's a great way to do it.

If you decide instead to opt for actively managed funds, the upside/downside ratio is one criterion you might use in making your selections.

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