What Is Diversification? How to Diversify Your Portfolio

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Diversification is the practice of spreading money across investments that respond differently to the same events, so that a loss in one holding is likely to be offset, at least partly, by steadier or better results in others. In investing, diversifying your portfolio means owning a mix of asset classes, and different securities within each class, so that no single company, sector, country or issuer can do outsized damage. Diversification reduces risk only to the extent that holdings do not move in lockstep. Owning many things that rise and fall together is not diversification.

What Is Diversification?

More precisely, a diversified portfolio is one in which the risks of individual holdings partly cancel each other out, so the portfolio as a whole is less volatile than the average of its parts.

Two levels of diversification usually work together:

• Across asset classes: holding stocks, bonds, cash equivalents, real estate, commodities or other assets, which have different risk and return drivers.
• Within an asset class: holding many stocks rather than one, or bonds from many issuers and maturities rather than a single bond. Owning large-cap and small-cap stocks, for example, is diversification inside equities.

The goal is not to maximize return. It is to get a given level of expected return with less risk, or a higher expected return for the same risk, by removing the kinds of risk an investor is not paid to carry. A single stock can fall 80% because of a failed product, a fraud or a lawsuit; in a portfolio of 50 unrelated stocks, that damage is limited to the company’s share of the portfolio.

How Diversification Works

Correlation Drives the Benefit

The mechanism behind diversification is correlation, measured by the correlation coefficient (ρ), which ranges from -1 to +1:

• +1: the two assets move in perfect step. Combining them gives no risk reduction.
• 0: their movements are unrelated. Combining them reduces risk meaningfully.
• -1: they move in exactly opposite directions. In theory, the right mix eliminates volatility entirely.

Real assets almost never sit at the extremes. Most pairs of stocks are positively correlated because they share exposure to the same economy, interest rates and investor sentiment. Diversification still helps as long as the correlation is below +1.

The Two-Asset Formula

The volatility (standard deviation) of a two-asset portfolio is:

σp² = w₁²σ₁² + w₂²σ₂² + 2·w₁·w₂·ρ·σ₁·σ₂

where w is each asset’s weight, σ is each asset’s standard deviation, and ρ is the correlation between them.

The first two terms reflect each asset’s own risk. The third term is where diversification happens: the lower the correlation, the smaller that term, and the lower the portfolio’s risk.

Hypothetical example. Two assets each have an annual standard deviation of 20%. You hold 50% in each. The expected return of the portfolio is simply the weighted average of the two expected returns, but its risk depends on correlation:

Assumptions: equal weights, identical 20% volatility for both assets, correlation constant over the period. With zero correlation, the portfolio carries about 14.1% volatility even though each holding carries 20%, with no change in expected return.

Why More Holdings Help Less and Less

The same logic extends to many assets. For an equally weighted portfolio of N assets that each have variance σ² and the same pairwise correlation ρ:

σp² = σ²/N + (1 − 1/N)·ρ·σ²

As N grows, the first term shrinks toward zero, but the second term approaches ρ·σ². That floor is the risk that diversification cannot remove.

Hypothetical example. Each stock has 30% volatility and every pair has a correlation of 0.3:

Going from one stock to ten removes most of the reducible risk; going from 30 to 100 removes very little. A frequently cited rule of thumb holds that around 25 to 30 stocks achieves most of the cost-effective risk reduction in an equity portfolio. It is not a law: a 2021 study in the Journal of Risk and Financial Management concluded that, given how many variables are involved, an optimal number of stocks for a well-diversified portfolio does not exist. Correlations, weights, sector mix and the investor’s goals all change the answer.

Diversifiable vs. Non-Diversifiable Risk

That floor is systematic risk. Portfolio risk splits into two types:

Unsystematic risk (also called diversifiable, specific or idiosyncratic risk) belongs to an individual company, issuer or industry. Examples include:

• Business risk: the risk tied to what a company does and how it competes.
• Financial risk: the risk tied to a company’s debt, liquidity and solvency.
• Operational risk: breakdowns in manufacturing, supply chains or distribution.
• Regulatory risk: legislation or rule changes that hit a particular asset or sector.

These risks can be reduced, and in a broad portfolio largely neutralized, because bad news at one company is unrelated to bad news at most others.

Systematic risk (also called market or non-diversifiable risk) affects nearly everything at once: recessions, interest rate shifts, inflation, wars, pandemics. In the 2020 COVID-19 shock, businesses across almost every industry shut down or cut back, consumer spending fell broadly, and then nearly every sector was supported by the same government stimulus and monetary easing. Owning more stocks did not escape that risk, because the market itself was moving.

Financial theory holds that investors are compensated for bearing systematic risk but not for unsystematic risk, since the latter can be diversified away at little cost. Holding a concentrated position therefore means taking risk that, on average, does not come with extra expected return.

Ways to Diversify a Portfolio

Many of the methods below can be combined. A portfolio might be diversified across asset classes, and within its stock allocation across sectors, sizes and countries, and within its bond allocation across issuers and maturities.

Asset Classes

Each asset class has its own risks and return drivers. Common classes include:

• Stocks: ownership shares in public companies.
• Bonds: fixed-income debt issued by governments and corporations.
• Real estate: land and buildings, and in broader definitions natural resources, farmland and mineral rights.
• Commodities: raw goods such as energy, metals and agricultural products.
• Cash and cash equivalents: Treasury bills, certificates of deposit, money market instruments and other short-term, low-risk holdings.
• Alternatives: private equity, venture capital, hedge funds, cryptocurrency and other assets outside the traditional categories.

Funds such as ETFs and mutual funds are not asset classes in themselves; they are wrappers that can hold any of these.

The rationale is that a single event can push asset classes in different directions. Rising interest rates push existing bond prices down, because older bonds’ yields must rise to compete with new issues, yet the same environment may coincide with rising rents for real estate owners or higher commodity prices. Cash, meanwhile, loses purchasing power whenever inflation runs above its return.

Industries and Sectors

Industries respond differently to the same news. A law, a technology shift or a commodity price swing can hit one sector hard and barely touch another: the CHIPS and Science Act of 2022 directly affected semiconductor manufacturers, while financial services felt it only indirectly.

Investors can also pair industries that tend to react in opposite ways. Higher oil prices generally help energy producers and hurt airlines, whose fuel costs rise; lower oil prices tend to do the reverse. Holding both does not remove exposure to the economy, but it reduces exposure to the direction of oil prices specifically.

Sector concentration can creep in through index funds as well. Cap-weighted indexes such as the S&P 500 give the largest companies the largest weights, so a fund tracking one can be heavily tilted toward whichever sectors dominate the top of the index at the time.

Growth vs. Value

Stocks are often split into two styles:

• Growth stocks belong to companies expected to grow revenue or profit faster than their industry average.
• Value stocks belong to companies that appear to trade at a discount relative to their current fundamentals.

Growth stocks carry the risk that expected growth never arrives, and they are sensitive to financing conditions: when the Federal Reserve tightens monetary policy, capital becomes scarcer and more expensive for companies that depend on future expansion. Successful growth companies, though, can exceed expectations by a wide margin. Value companies are typically more established and stable, with less room for dramatic gains but often less downside. Holding both styles spreads the bet between future potential and present fundamentals.

Market Capitalization

Smaller companies generally have more room to expand but are more vulnerable to setbacks; larger companies tend to be steadier. Mixing sizes captures some of the growth potential of smaller firms without making the whole equity allocation depend on them.

Risk Profile and Credit Quality

Within almost any asset class, holdings carry different levels of risk. Ten-year bonds from a top-rated government and from a struggling company raising emergency funds share a maturity but differ sharply in risk, depending on the issuer’s credit rating, debt load and outlook. Likewise, a speculative real estate development carries more risk and upside than a fully leased operating property, and a small, newly launched token carries different risk than a widely adopted cryptocurrency.

Diversification by risk profile means not letting the whole portfolio sit at one end of that range.

Maturity Lengths

For bonds, maturity shapes risk. The longer a bond’s maturity, the more its price moves when interest rates change. Short-term bonds usually pay less but are less exposed to uncertainty about future rates; long-term bonds usually pay more in exchange for that exposure. Spreading holdings across maturities, sometimes in a ladder where bonds come due at regular intervals, balances income against rate risk.

Geography: Domestic vs. Foreign

Economies do not move in perfect step: forces weighing on the U.S. economy may affect Japan or Europe differently, so foreign holdings can cushion a downturn at home. Investing abroad also adds currency exposure, because a foreign stock’s return to a U.S. investor depends on both its local price and the exchange rate.

Emerging and frontier markets offer higher potential growth with higher risk, including political, liquidity and governance risks.

Tangible vs. Financial Assets

Stocks and bonds are financial claims. Land, buildings, farmland, precious metals and other commodities are tangible assets that can be used, consumed, rented or developed, which gives them different return drivers. They also carry risks financial assets do not: physical damage, theft, natural disasters, obsolescence, and ongoing costs for storage, insurance and security.

Platforms and Custodians

Diversification also applies to where assets are held, not just what they are. The asset allocation can be identical while the risk differs.

Example with U.S. bank deposits. Someone holds $400,000 in cash. FDIC insurance covers $250,000 per depositor, per insured bank, per ownership category.

• Two banks, $200,000 each: each deposit falls under the limit, so both are fully insured (assuming no other deposits in the same ownership category at either bank).
• One bank, $400,000: under a single ownership category, $150,000 sits above the insured limit, and if the bank failed, access to funds could be delayed.
• $400,000 in physical cash at home: immediately accessible, but it earns nothing and can be lost to theft, fire or misplacement.

Protections at brokers and crypto platforms differ and can be much weaker. When crypto lender Celsius Network filed for bankruptcy in July 2022, customers could not withdraw or transfer their holdings; those who had spread their crypto across several platforms had less of it frozen. Mixing forms of the same asset works similarly: someone who owns both physical gold and a gold ETF keeps some gold exposure whether the bars are stolen or a financial intermediary fails.

Diversification for Individual Investors

Building a diversified portfolio from individual securities takes money and time. Each purchase may carry a commission, many positions are needed, and the investor has to track weights as prices move. This is a major reason pooled funds are popular with individual investors.

• Mutual funds pool many investors’ money into a professionally managed portfolio, giving each shareholder a slice of every holding.
• ETFs trade on exchanges like stocks and can track broad indexes, single sectors, countries, bond segments or commodities, including markets that would otherwise be hard for an individual to reach.

A single broad index fund can hold hundreds or thousands of securities, which removes most company-specific risk in one purchase. The trade-off is that the investor gives up choosing individual companies, and pays the fund’s expense ratio.

Funds can create an illusion of diversification. Three large-cap U.S. stock funds from different providers may hold mostly the same companies. A useful check is to look at each fund’s top holdings and sector weights and see how much they overlap, rather than counting the number of funds.

A target-date fund goes a step further by holding a mix of stock and bond funds and shifting toward bonds as a chosen retirement year approaches, so the diversification across asset classes is handled within one product.

Measuring Diversification

There is no single number that captures how diversified a portfolio is; too many variables interact across too many assets. Analysts use several rough measures together.

Correlation Coefficient

Values near +1 indicate little diversification between two assets, values near 0 moderate diversification, and values near -1 strong diversification. A correlation matrix shows every pair in a portfolio at once. Because correlations are calculated from historical data, they can shift from one period to the next.

Standard Deviation

Standard deviation measures how widely returns scatter around their average. Two investments may both average 5% a year, but the one with higher standard deviation has produced returns far from 5% in both directions. A portfolio of high-volatility assets may have strong return potential, but those assets may share the same shocks. Comparing portfolio standard deviation with the weighted average of the holdings’ standard deviations shows how much diversification is doing.

Diversification Ratio

That comparison can be expressed directly as a diversification ratio:

Diversification ratio = (Σ wᵢ·σᵢ) / σp

The numerator is the weighted average volatility of the holdings; the denominator is the portfolio’s actual volatility. A ratio of 1.0 means the holdings move together so closely that combining them reduced no risk. Higher values mean more risk is being canceled out. In the earlier two-asset example with zero correlation, the ratio is 20% ÷ 14.1%, or about 1.41.

Risk-Adjusted Return

Because diversification aims to improve the trade-off between return and risk, measures such as the Sharpe ratio, which divides excess return by volatility, show whether a more diversified portfolio is delivering better return per unit of risk than a concentrated one.

Count and Weighting

The simplest measures count holdings or calculate their weights. An investor might find that 15 of their 20 stocks are technology companies, which is 75% of the stock positions by count. Weights by value can tell a different story: if those 15 tech positions are small and the other five are large, the dollar concentration is lower, and vice versa.

Weights can be condensed into an effective number of holdings, calculated as 1 divided by the sum of the squared weights. Hypothetical example: a portfolio has ten positions, one at 55% and nine at 5% each. The sum of squared weights is 0.3025 + (9 × 0.0025) = 0.325, so the effective number is about 3.1. Despite ten line items, the portfolio behaves more like one with three equal positions.

Smart Beta

Smart beta funds track indexes built on rules other than market capitalization, such as equal weighting or screens for value, quality or low volatility. Because they do not automatically give the most weight to the largest companies, they can reduce cap-weighted concentration. Their aim is often to beat the cap-weighted index, so they pursue alpha as much as diversification.

Example of Diversification

Hypothetical example. An investor has $120,000 and compares two approaches over one year.

Concentrated: all $120,000 in one stock.

Diversified: $20,000 in each of six stocks from different sectors.

Suppose one of the six stocks doubles while the others are flat.

• The concentrated investor who happened to pick that stock ends with $240,000, a 100% gain.
• The diversified investor’s $20,000 stake becomes $40,000, so the portfolio is worth $140,000, a gain of about 16.7%.

Now suppose instead that one stock goes to zero while the others are flat.

• The concentrated investor who picked that stock loses everything.
• The diversified investor loses $20,000, about 16.7% of the portfolio.

The diversified investor gives up the chance of the extreme outcome in both directions. Since nobody knows in advance which stock will double and which will fail, diversification swaps a wide range of possible results for a narrower one. Over many years, avoiding large drawdowns also helps compounding, because a portfolio that falls 50% needs a 100% gain just to recover.

A broader version: a more aggressive investor might combine a Japanese equity ETF, an Australian government bond ETF and a cotton commodity exchange-traded note. Driven by different economies, interest rate regimes and supply factors, these holdings are likely less correlated with each other than three U.S. stock funds would be, which lowers the portfolio’s volatility relative to its parts.

Limitations of Diversification

It Does Not Prevent Losses

Diversification lowers specific risk; it does not protect against a falling market. When systematic risk dominates, a fully diversified stock portfolio falls along with the market.

Correlations Can Rise When They Matter Most

Correlations measured in calm periods often rise during broad sell-offs. During the 2008 financial crisis and the March 2020 crash, many asset classes fell together as investors sold whatever they could, offering less protection than historical data suggested. High-quality government bonds and cash have often held up better in such episodes, but no asset’s correlation is fixed.

Diminishing Returns and “Diworsification”

Beyond a certain point, adding holdings barely lowers risk but still adds cost and complexity. Investor Peter Lynch popularized the term “diworsification” for companies that expanded into unrelated businesses and diluted their strengths; investors apply it to portfolios stuffed with overlapping funds or marginal positions, which look spread out but add fees without adding meaningful diversification.

Diversification Is Not Always the Goal

Some investors deliberately concentrate. An all-in bet on one high-risk asset, sometimes called a YOLO strategy, offers the highest chance of both an outsized gain and a large loss, and entrepreneurs and early employees often hold concentrated stakes in their own companies. Concentration is a choice to accept unsystematic risk in exchange for upside; it is not diversified investing.

Drift Undoes Diversification Over Time

A portfolio’s mix changes as prices move. Hypothetical example: a 60/40 stock and bond portfolio in which stocks rise sharply while bonds stay flat could drift toward 70/30, leaving the investor with more equity risk than intended. Rebalancing, selling some of what has risen and buying what has lagged, restores the target mix.

Diversification vs. Related Concepts

Diversification vs. asset allocation. Asset allocation is the decision of how much to put in each asset class, such as 60% stocks and 40% bonds. Diversification is the broader principle of spreading risk, and it also operates within each class. A portfolio can have a sensible allocation yet poor diversification, for example 60% in stocks held in only two companies.

Diversification vs. hedging. A hedge is a position taken specifically to offset a known risk, such as buying a put option against a stock holding or using a currency forward on a foreign investment. Hedging targets one exposure and often has a direct cost. Diversification reduces many unrelated risks at once by combining assets with imperfect correlations.

Diversification vs. rebalancing. Rebalancing is the maintenance step that keeps a diversified portfolio at its intended weights. Diversification sets the structure; rebalancing preserves it.

Diversified vs. many holdings. A portfolio is diversified by how differently its holdings behave, not by how many it has. Fifty bank stocks are less diversified than ten companies spread across unrelated industries and countries.

Diversifying income vs. diversifying investments. Diversifying income means relying on more than one source of earnings, such as a salary plus rental or dividend income. The logic is the same, reducing dependence on one source, but it applies to household cash flow rather than portfolio holdings. A related point: an employee who also holds most of their savings in their employer’s stock has concentrated both job and investment risk on the same company.

How to Diversify Your Portfolio

1. Set an asset allocation based on goals, time horizon and tolerance for losses, deciding the split among stocks, bonds, cash and any alternatives.
2. Diversify within stocks across sectors, company sizes, growth and value styles, and domestic and foreign markets. A broad index fund or a few complementary funds can do most of this.
3. Diversify within bonds across issuers, credit quality and maturities.
4. Check fund overlap by comparing top holdings and sector weights.
5. Spread custody, keeping insured deposits within coverage limits and not holding everything with one custodian.
6. Count outside concentration, such as employer stock or a large real estate holding.
7. Rebalance periodically or when weights drift meaningfully from targets.
8. Keep costs in view, since fees and trading costs reduce the benefit.

FAQs

The Bottom Line

Diversification matters to any investor who wants a given expected return with less risk, especially individuals relying on funds or holding employer stock. It reduces company and sector risk but cannot stop losses in a falling market, and correlations can rise in sell-offs. A sound approach sets an allocation, spreads holdings within each class, checks fund overlap, spreads custody, and rebalances as weights drift.