Diversification Beyond Stocks and Bonds

Diversification is often described as the only free lunch in investing. By combining assets that do not all move together, investors can reduce the overall volatility of a portfolio without necessarily giving up much expected return. For decades, the standard approach has been a blend of stocks for growth and high-quality bonds for stability.

That approach remains a sensible foundation. But periods such as 2022, when stocks and bonds fell at the same time, reminded many investors that the relationship between the two is not guaranteed. This article from the RTIBK editorial team looks at the other building blocks available, how they have tended to behave, and the trade-offs that come with each. If you are new to the topic, our article on asset allocation is a good starting point.

Understanding correlation

Correlation measures how closely two investments move together, on a scale from +1 to −1. A correlation of +1 means they move in lockstep; 0 means their movements are unrelated; −1 means they move in opposite directions. The lower the correlation between holdings, the more diversification benefit they tend to provide.

Three points about correlation are worth keeping in mind:

• It changes over time. The correlation between stocks and high-quality government bonds was often negative during the low-inflation decades before 2020, but it has historically been positive during periods of high or rising inflation.
• It can rise in a crisis. During sharp sell-offs, many risky assets tend to fall together as investors seek cash and safety. Diversification across risky assets can disappoint precisely when it is most wanted.
• Low correlation is not the same as low risk. An asset can be uncorrelated with stocks and still be highly volatile on its own. What matters is its effect on the whole portfolio.

The goal, then, is not to find assets that always rise when stocks fall, which is rare, but to hold a mix whose drivers are different enough that the portfolio is not dependent on a single economic scenario.

Real estate and REITs

Real estate is one of the largest asset classes in the world, and many households already have significant exposure through their homes. For investment portfolios, the most accessible route is usually real estate investment trusts (REITs), companies that own and often operate income-producing property such as offices, apartments, warehouses, data centers and shopping centers. In many jurisdictions, REITs must distribute most of their taxable income to shareholders, which makes them popular with income-oriented investors.

Real estate can offer rental income that may rise with inflation over time, plus exposure to property values. However, listed REITs trade on stock exchanges and, over short periods, often behave much like equities, with added sensitivity to interest rates because they rely on borrowing and compete with bonds for income-seeking investors. Their diversification benefit tends to be more visible over long horizons than month to month.

Direct property ownership and private real estate funds can appear less volatile, partly because they are not priced daily. That smoothness can be misleading: the underlying values still fluctuate, and these investments are typically far less liquid, with higher costs and concentrated exposure to specific properties or regions.

Commodities

Commodities include energy (such as crude oil and natural gas), industrial metals (such as copper and aluminum) and agricultural products (such as wheat, corn and coffee). Their prices are driven by supply and demand for physical goods, which is a different set of forces from those that move corporate earnings or bond yields.

Historically, broad commodity exposure has sometimes performed well during periods of unexpected inflation, including the 1970s and parts of 2021 and 2022, when stocks and bonds struggled. That makes commodities a potential hedge against supply shocks and inflation surprises.

The trade-offs are significant:

• High volatility. Commodity prices can swing dramatically, and long stretches of poor returns have occurred.
• No income. Physical commodities do not pay interest or dividends. Returns depend on price changes and, for futures-based funds, on "roll yield" and interest on collateral.
• Structural complexity. Most investors access commodities through funds that hold futures contracts. When future prices are higher than current prices (a condition called contango), rolling contracts can erode returns even if spot prices are stable.
• Equity alternatives are not the same. Shares of mining or energy companies are influenced by commodity prices but also by broader stock market movements, management decisions and company debt.

Gold

Gold occupies a special place among commodities. It has limited industrial use relative to its value and is widely held by central banks and individuals as a store of value. Investors often turn to it during periods of financial stress, currency weakness or geopolitical uncertainty, and it has at times held up well when both stocks and bonds were under pressure.

Yet gold is not a reliable short-term inflation hedge. Its price has gone through long periods of decline or stagnation, including roughly two decades after its 1980 peak, when it lost substantial value in real terms. Like other commodities, it produces no income, so its opportunity cost rises when real interest rates are high. Gold can be held through exchange-traded funds backed by physical metal, which avoids storage and insurance issues of holding bars or coins directly, though fund fees still apply.

Many diversified portfolios that include gold hold it as a modest allocation rather than a core position, recognizing its value as insurance rather than a growth engine.

Inflation-linked bonds

Inflation-linked bonds, such as U.S. Treasury Inflation-Protected Securities (TIPS) and UK index-linked gilts, adjust their principal in line with a consumer price index. As a result, their coupon payments and final repayment rise with inflation, offering a more direct inflation hedge than most other assets.

These bonds are quoted in terms of a "real" yield, meaning the return above inflation. They protect against inflation, but they are still bonds: if real yields rise, their prices fall. Long-dated inflation-linked bonds lost considerable value during the 2022 rate increases despite high inflation, because real yields rose from very low levels. Shorter-maturity inflation-linked bonds tend to track inflation more closely with less price sensitivity.

Tax treatment can also be a consideration. In some jurisdictions, the inflation adjustment to principal may be taxable each year even though it is not received until maturity, which is why some investors prefer to hold these bonds in tax-advantaged accounts.

International exposure

Investors everywhere tend to overweight their home market, a pattern known as home bias. Yet no single country has consistently delivered the best stock market returns. Leadership has rotated between regions over the decades, and different economies have different sector mixes, demographics, currencies and policy cycles.

International diversification can include developed markets, such as Europe, Japan and Australia, and emerging markets, which offer higher growth potential alongside higher political, currency and governance risks. International bonds, often hedged back to the investor's home currency to reduce volatility, can add further breadth.

Global markets have become more correlated as trade and capital flows have increased, so international holdings may not help much during a worldwide sell-off. Their main benefit is reducing dependence on any one country's economy over long periods. Currency movements add another source of return and risk, which can help or hurt depending on the period.

The role of cash

Cash and cash equivalents, such as savings accounts, money market funds and short-term Treasury bills, are often overlooked as a diversifier. Their value in nominal terms is highly stable, and they provide liquidity to meet expenses or take advantage of opportunities without selling other assets at a bad time.

The drawback is that cash has historically delivered returns only slightly above inflation over long periods, and sometimes below it, eroding purchasing power. Holding too much for too long can leave long-term goals underfunded. A common approach is to hold an emergency reserve plus any money needed in the near term in cash, and invest the rest according to a plan such as those discussed in our article on building a long-term wealth plan.

Comparing asset classes at a glance

The table below summarizes how these asset classes have tended to behave. The descriptions are qualitative and based on long-run historical patterns, which may not repeat.

Costs and liquidity trade-offs

Every additional asset class adds something to a portfolio, but it can also add costs and complexity. Before expanding beyond stocks and bonds, consider the following:

Taxes also matter. Different assets are taxed differently depending on jurisdiction, and some are better held in particular account types. Rebalancing across more asset classes can also generate more transactions and, potentially, more taxable gains.

Finally, there is behavioral cost. Diversifying assets will, by design, sometimes lag the best-performing part of the portfolio. Investors who abandon a diversifier after a few years of underperformance may lose the benefit just before it is needed. A written plan with target ranges and a rebalancing schedule can help. You can model how different return assumptions affect long-term outcomes with our investment calculator.