Diversification has long been a core principle of portfolio construction, and recent market swings have reinforced its importance. Returns have varied meaningfully across sectors and asset classes, reminding investors that leadership can change quickly and without much warning. But diversification means more than spreading investments across a few sectors. A well-built portfolio should also reflect an investor’s goals, time horizon, liquidity needs, and risk tolerance. Understanding the risks of an undiversified portfolio is an important first step in building a strategy that holds up over time.
Diversification also goes beyond simply owning different stocks. It can include exposure across company size, geographic regions, issuers, and sectors. The goal is not to eliminate risk entirely, but to avoid having too much of a portfolio depending on any one market segment, theme, or economic outcome.
What Diversification Looks Like
Our modern framework of diversification in a portfolio comes from Harry Markowitz, an esteemed economist who developed what we now know as “Modern Portfolio Theory”, or MPT. This theory came from his 1952 paper, Portfolio Selection, which offered insights that eventually became a foundational pillar of modern investment management. The key theme of his article was that investments should not be evaluated in isolation. Rather, investors should view each position and how it interacts with others, and by combining assets that do not always react to the same news, trends, or themes, you can reduce overall portfolio risk without sacrificing a comparable level of expected returns.
One of the clearest portfolio risks is concentration risk. This occurs when too much of a portfolio depends on a narrow group of companies, sectors, or investment themes. In recent years, enthusiasm around artificial intelligence has led many investors toward large technology and growth stocks. By the end of the first quarter of 2026, the NASDAQ Composite had fallen 10.5% for the quarter, while the S&P 500 was down 5.1% in March, with energy being the only S&P 500 sector to post a gain that month. That rotation is a useful reminder that long-term investing is rarely about owning only what has worked most recently. It is about building a portfolio that can participate across different market environments rather than relying too heavily on a single theme or trade.
International investing can also play an important role. International equities can broaden opportunities and reduce dependence on a single domestic market. At the same time, they bring distinct risks, including currency movements, different regulatory regimes, and geopolitical uncertainty. Although U.S. equities have delivered stronger returns over much of the past 20 years, international stocks have periodically taken the lead. Broad international equities, using the Vanguard Total International Stock ETF, outperformed the U.S. market in seven of the 20 calendar years, from 2006 through 2025. This changing market leadership reinforces the value of maintaining exposure across geographic regions rather than relying exclusively on the market that has performed best recently.
Time horizon is another major factor in portfolio design. An investor in the early stages of building wealth will usually have different needs from someone nearing retirement or beginning to draw on savings. Investors with longer time horizons and steady employment income can often accept more short-term volatility in pursuit of growth. For that reason, their portfolios may lean more heavily toward equities.
As investors near retirement, income, capital preservation, and reduced volatility often become more important. Depending on an investor’s circumstances, this may support a greater allocation to bonds and even a larger emphasis on value-oriented stocks. Value stocks generally trade at lower valuation multiples and often represent established, dividend-paying companies. Growth stocks are typically valued based on expectations for faster future growth, which can make their share prices more sensitive to changes in earnings expectations and investor sentiment.
Despite enthusiasm surrounding artificial intelligence and other emerging technologies, value stocks have meaningfully outperformed growth stocks so far in 2026. As of July 30, the Vanguard Value ETF (VTV) had returned approximately 14% year to date, compared with approximately 3% for the Vanguard Growth ETF (VUG). Both are passively managed index ETFs that track large-cap U.S. value and growth stocks, respectively. This reversal serves as a reminder that market leadership changes over time and that portfolios should not depend too heavily on the style of stocks in your portfolio.
How a Diversified Portfolio Can Help During Market Volatility
A common reference point is the 60/40 portfolio, which allocates 60% to stocks and 40% to bonds, though the right mix can vary by an individual investor’s risk tolerance, time horizon, and many other factors. A recent Morningstar analysis by Emelia Fredlick found that over the past 150 years, a 60/40 portfolio experienced less pain than an all-equity portfolio in nearly every major market downturn.
The only exception occurred during the 2022 market decline. With inflation reaching levels not seen since the 1980s, the onset of the Russia-Ukraine war, and the global economy still contending with the aftereffects of the COVID-19 pandemic, the Federal Reserve rapidly increased interest rates to combat inflation. These conditions created an unusual environment in which both stocks and bonds declined simultaneously, reducing the diversification benefits that investors typically expect from fixed income. An important concept to understand is that bond prices and yields are inversely related. As interest rates rise, the value of existing bonds falls because newly issued bonds offer higher yields, making older bonds less attractive to investors. According to Morningstar’s 150-year analysis, 2022 was effectively the worst bond market decline in the study’s history and the only major downturn in which a traditional 60/40 portfolio experienced more pain than an all-equity portfolio.
Morningstar’s “Lost Decade” example, covering 2000 through 2013, illustrates how diversification can help reduce the impact of significant market declines. At the market’s September 2002 low, stocks fell 47.2%, while a diversified portfolio consisting of 60% stocks and 40% bonds had declined by 24.7%.
To look more deeply into this, the period can be divided into two distinct market cycles for a clearer comparison. During the dot-com downturn and subsequent recovery through October 9, 2007, the 60/40 portfolio’s largest decline was 20.47%, compared with a 45.14% decline for the all-equity benchmark. By the end of the period, the 60/40 portfolio had generated a cumulative return of 39.64%, while the all-equity benchmark had returned 21.06%.
The second period, from October 10, 2007, through March 26, 2012, captures the Global Financial Crisis and the all-equity benchmark’s eventual return to approximately the break-even point. During this period, the 60/40 portfolio declined by as much as 35.04%, while the all-equity portfolio fell by as much as 55.18%. By the time the all-equity benchmark recovered its losses, the 60/40 portfolio had produced a cumulative return of 15.49%.
