But many experts question whether a mix of 60 percent stocks and 40 percent bonds is suitable. Over the years, 60/40 has become a rough gauge for how to build adequate retirement savings without taking excessive risk. Typically, it’s promoted as a sort of default portfolio balance for investors in their 40s, or even closer to retirement.

The basic rationale: Keep most of your retirement savings in the stock market, because stocks are likely to provide greater long-term growth than bonds. But when stocks fall, you’ll want a significant percentage of your portfolio in bonds to cushion against steep losses.

That approach is under fire. Many suggest it’s important to have more than 60 percent invested in stocks. That’s because retirement can stretch for several decades, and investors will need to increasingly rely on stocks to limit the risk of outliving savings. Also, the long-term outlook for bonds is poor. Their yields are near all-time lows and interest rates are certain to eventually climb.

Another criticism: 60/40 is too narrow an approach to build a truly diversified portfolio because it fails to consider alternative assets classes. Think of investments in commodities such as oil, precious metals or real estate investment trusts. Alternatives may also include using complex strategies that hedge funds pioneered to protect against losses when stocks plunge or inflation spikes. These approaches may sound uncommon, but they’re readily available. Hundreds of mutual funds using alternative assets or strategies have been launched in recent years.

The Associated Press interviewed two experts in asset allocation – the mix of stocks, bond and alternatives in a portfolio – to try to get to the heart of the debate.

One is a 60/40 critic: Ben Inker, co-head of asset allocation for GMO, a Boston-based manager of $104 billion for institutional clients such as endowments and pension funds. He also co-manages Wells Fargo Advantage Absolute Return (WARAX), a mutual fund that uses alternative strategies.

Also weighing in is Fran Kinniry, who embraces traditional stock-and-bond portfolio construction. He’s a principal in the investment strategy group at Vanguard, the nation’s largest mutual fund company, managing more than $2 trillion.

Below are edited excerpts of their arguments on whether a 60/40 portfolio remains a good tool for middle-aged investors trying to build up retirement savings:

INKER: One reason I’m skeptical about 60/40 is that it’s probably not aggressive enough, at least for a 40-year-old investor. You need to invest more in assets that are riskier than bonds if you want to meet your investment goals without having to save an extremely large percentage of your income.

If you’re 40, you’ve got around 25 years before retirement. That’s a long time. With significantly more than 60 percent in stocks, you’ll have a much better chance to achieve your retirement savings goals than you would with just 60 percent. Your main investment goal at that point in life should be trying to generate the highest return on your savings.

While an aggressive allocation to stocks makes sense at that age, that doesn’t necessarily mean you can rely on alternative assets to diversify a portfolio. The problem that a 60/40 portfolio presents is that you can’t rely entirely on bonds as a diversifier, either. They have been good diversifiers in recent years because we’ve had low inflation. In fact, higher-quality bonds like Treasury notes have been especially good diversifiers. But if the future risk we face is from rising inflation, bonds aren’t going to help. They’ll be lousy diversifiers, and will hurt your portfolio.

KINNIRY: It’s true that there are challenges now for investors with 60/40 portfolios, because of the risks bond investors face. It will be mathematically impossible to replicate the strong returns that bonds have delivered over the last 30 years.

But I’d warn investors who want to leave traditional bonds for more exotic asset classes. It has not been demonstrated that those assets can diversify a portfolio when stocks are falling. We shouldn’t expect alternative assets to provide a diversification benefit during the next bear market.

Consider the performance of the larger college endowments that invested in alternative assets over the years. Traditional stock-bond portfolios have been killing many of those endowments in terms of performance. That’s been the case whether you go back just one year, or three, five or 15 years. The attempt to get more exotic hasn’t worked.

It’s true that a lot of alternative assets perform out of sync with the stock and bond markets, so that your portfolio will deliver steadier returns if you invest in alternatives. But the time that you really want diversification is when the primary asset in your portfolio – stocks – is falling sharply.

Vanguard has done research examining the performance of various assets, including alternatives, when stock performance has been the worst – the bottom 10 percent of the performance spectrum, historically. Nearly all the assets posted losses at the same time that stocks were plunging. That was the case for everything from foreign stocks to real estate investment trusts to commodities, high-yield bonds, emerging markets bonds, hedge funds and private equity.

There were only two assets that had positive returns during those periods: Treasury bonds, and investment-grade corporate and municipal bonds. That supports the case for maintaining a basic stock-and-bond mix in a portfolio.