Searching for the next winning investment can take up considerable energy. But long-term success often comes from something less exciting — and far more important: portfolio diversification.
Diversification is one of the most strategic decisions you can make when coordinating a growing portfolio with your broader financial plan.
In this article, we explain what diversification means, why it matters, how you can build a diversified investment portfolio, and the mistakes even experienced investors sometimes make.
What is portfolio diversification?
Portfolio diversification is the practice of spreading investments across different asset classes, sectors, and geographies so that no single position has an outsized impact on your overall results.
It’s often described as “not putting all your eggs in one basket,” but investment diversification goes well beyond that. It’s a structured approach to managing concentration risk — the danger that too much of your wealth depends on a single company, sector or market.
Diversification in plain English
Why diversification matters
Market volatility is unavoidable. No asset class consistently outperforms every year, and concentrated portfolios can expose investors to unnecessary risk — sometimes without their realizing it.
To that point, international equities outperformed U.S. markets in 2025 and provided meaningful ballast during the sharp April 2025 sell-off.1 It was a real-time reminder of how geographic diversification may help stabilize portfolios. A diversified portfolio can help limit the impact of poor performance in any single investment, sector, or asset class.
The goal of a diversification strategy isn’t to maximize returns from a single position. Rather, it may help to promote a smoother long-term outcome potential through changing markets.
The four main layers of diversification
1. Asset class diversification
A diversified investment portfolio typically includes stocks for growth potential, bonds for income and stability, cash for liquidity, and alternative investments for additional portfolio risk management. Different asset classes serve different purposes within the overall portfolio and often respond differently to changing economic environments. Ensuring the portfolio’s risk level is aligned to your financial plan and goals is one of the first steps to building a long-term portfolio.
2. Geographic diversification
Investing globally seeks to reduce reliance on a single economy. A portfolio that includes U.S. markets, developed international markets, and emerging markets captures a broader range of opportunities and helps reduce regional concentration risk.
3. Sector diversification
Economic trends affect industries differently. Technology may lead during innovation cycles; healthcare may hold up during downturns; financials and industrials may benefit from different macroeconomic conditions. Diversifying across sectors seeks to reduce the risk that one industry’s decline materially damages the portfolio.
4. Diversification within asset classes
Even within a single asset class, diversification matters. For equities, this could mean owning large-, mid-, and small-cap stocks, as well as growth and value positions. In fixed income, it may mean balancing government and corporate bonds across varying maturities.[
What diversification does — and doesn’t do
Understanding the limits of diversification is as important as understanding its benefits.
What it can do: Reduce concentration risk, help manage portfolio volatility, and can create a more consistent investment experience across market cycles.
What it cannot do: Guarantee gains, prevent losses, eliminate market risk, or ensure any particular outcome.
Diversification is designed to improve risk-adjusted outcomes, not guarantee positive returns.
Frequent diversification mistakes investors might make
Owning too much company stock
For business owners, executives, and professionals with equity compensation, large concentration in a single company can leave a portfolio exposed. Concentrated positions may limit liquidity, create tax complexity, and magnify volatility. Transitioning to a broader, tax-aware portfolio is one of the most impactful moves an investor can make.
Thinking multiple funds means true diversification
Owning several mutual funds or ETFs doesn’t automatically mean you’re diversified. If those funds hold many of the same underlying securities — particularly in today’s market, where a handful of large-cap technology companies dominate many index funds — you may have more concentration risk than you think.
Chasing recent winners
Investors who shift toward last year’s top performers often buy high and concentrate their portfolio at the wrong time. A disciplined diversification strategy prioritizes balance and long-term objectives over recent performance.
Ignoring international exposure
Overweighting U.S. assets simply because they’re familiar can leave portfolios structurally under diversified. Given how markets fluctuate, global diversification may offer meaningful benefits over time.
Forgetting to rebalance
Diversification isn’t a one-time decision. As markets change, your portfolio’s allocations can shift over time, causing a once-balanced investment strategy to become more concentrated than intended. We recommend rebalancing your portfolio regularly to ensure asset class exposures remain aligned with your objectives.
How often should you rebalance a portfolio?
Rebalancing a portfolio means periodically returning your asset mix to its target allocation. When markets move, particularly in a sustained bull market, equities can grow to represent a much larger share of the portfolio than originally intended.
Example
Two primary approaches exist:
- Calendar-based rebalancing on a fixed schedule (quarterly or annually)
- Threshold-based rebalancing when any allocation drifts beyond a set percentage from its target
The important point is that rebalancing helps maintain the intended risk level of the portfolio, which accounts for tax implications before executing any trades.
Does diversification change as you get closer to retirement?
Yes, and the shift is usually meaningful. As your time horizon to retirement shortens, the sequence of investment returns becomes more important. A significant loss early in retirement can have a lasting impact on a portfolio’s ability to generate income over decades.
- Early career: prioritizing higher equity allocations with a longer time horizon to ride out volatility
- Peak earning years: optimizing accounts and managing concentrated positions from equity compensation
- Preretirement: shifting toward income-producing assets and stress-testing against sequence-of-returns risk
- Retirement: preserving capital, generating income, and maintaining growth potential to address longevity
At Mercer Advisors, we align your strategy with your retirement timeline as an important component of comprehensive planning.
Are alternative investments part of diversification?
For investors whose portfolios warrant it, alternative investments may offer diversification benefits beyond traditional stocks and bonds. These include private equity, venture capital, private credit, real estate, and hedge fund strategies.
Broadly diversified portfolios that include allocations to private markets as appropriate may be better positioned for long-term resilience. However, alternatives introduce additional risks, complexity, and liquidity constraints. Access is typically limited to qualified investors, and these investments can carry higher fees and less transparency than public market equivalents.
Three integrated strategies for protecting and growing your wealth
When managing portfolios, diversification shouldn’t exist in isolation. Ideally, it connects directly to tax planning, investment management, and financial planning decisions.
- Investment management
If a significant portion of your wealth is tied to a single company, a structured transition plan can help reduce concentration risk while managing the tax impact of moving assets into a more diversified portfolio. This can happen through an employer stock, a business sale, or equity compensation. - Tax planning
How you rebalance matters as much as when you rebalance. Tax-loss harvesting, asset location across taxable and tax-advantaged accounts, and careful sequencing of trades can meaningfully reduce the after-tax cost of maintaining a well-diversified investment portfolio. Mercer Advisors has tax planning and preparation specialists who work alongside our wealth advisors to coordinate these decisions. - Financial planning
A diversified portfolio that isn’t aligned with your retirement timeline, income needs, and estate plan may still leave you exposed in notable ways. Comprehensive, coordinated financial planning, including estate planning and insurance, seeks to ensure your portfolio is built not just for performance, but for your life.
A practical diversification checklist
The bottom line
Portfolio diversification remains one of the most effective tools available for managing investment risk. While it cannot eliminate volatility or guarantee positive returns, a thoughtfully diversified portfolio may help you protect what you’ve built and stay focused on long-term goals through changing markets.
Wondering whether your portfolio is well-positioned? A comprehensive portfolio review can help identify hidden concentration risks and opportunities to better align your investments with your goals and your family’s future. At Mercer Advisors our portfolio diversification approach is coordinated with tax planning, financial planning, and estate planning.
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Portfolio diversification is an investment strategy that spreads holdings across different asset classes, sectors, geographies, and investment types. The goal is to reduce the risk that any single position significantly damages the overall portfolio. A diversified portfolio combines investments that may respond differently to the same economic or market event, which seeks to reduce volatility over time.
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Diversification is important because no single investment, sector, or market consistently outperforms every year. By spreading risk, investors seek to reduce the impact of poor performance in any one area and create a more consistent experience across different market cycles. It’s one of the most widely supported principles in long-term investment management.
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There’s no fixed number. What matters more than quantity is breadth — owning investments that behave differently across market conditions. A portfolio spanning multiple asset classes, geographies, sectors, and market capitalizations is generally considered well-diversified, even if the total number of holdings varies.
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No. Diversification seeks to reduce concentration risk and manage portfolio volatility, but it cannot eliminate market risk, prevent losses, or guarantee positive returns. It is designed to improve risk-adjusted outcomes — not to make investing risk-free.
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We generally advise that you review your allocation at least annually or whenever it drifts meaningfully from its target — often defined as a shift of five percentage points or more. Rebalancing a portfolio helps maintain your intended risk level and may be done on a calendar or threshold basis, ideally with attention to tax implications.
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Examples include investing across stocks, bonds, and cash; adding international exposure through developed and emerging markets; diversifying within equities across sectors and market capitalizations; including alternative investments where appropriate; and transitioning from a concentrated stock position into a broader portfolio over time.
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Yes. As investors approach and enter retirement, the emphasis often shifts from long-term growth to capital preservation and income generation. Sequence-of-returns risk — the impact of significant losses early in retirement — becomes more important. Investors can reduce equity exposure gradually and align their diversification strategy with their specific income needs and time horizon.
- “US Stocks Had a Remarkable 2025. But International Markets Did Much Better.” CNN, Jan. 4, 2026.
All expressions of opinion reflect the judgment of the author as of the date of publication and are subject to change. Some of the research and ratings shown in this presentation come from third parties that are not affiliated with Mercer Advisors. The information is believed to be accurate but is not guaranteed or warranted by Mercer Advisors. Content, research, tools and stock or option symbols are for educational and illustrative purposes only and do not imply a recommendation or solicitation to buy or sell a particular security or to engage in any particular investment strategy. Different types of investments involve varying degrees of risk, investments mentioned in this document may not be suitable for all investors. Investments are subject to market risk, including the possible loss of principal. All investment strategies have the potential for profit or loss. Changes in investment strategies, contributions or withdrawals may materially alter the performance and results of your portfolio. Portfolio management strategies such as diversification, asset allocation, and rebalancing do not ensure a profit or guarantee against loss. Investing in private funds is speculative and will entail substantial risks. Hypothetical examples are for illustrative purposes only.
Mercer Advisors is not a law firm and does not provide legal advice to clients. All Estate planning document preparation and other legal advice are provided through select third parties, with which Mercer Advisors has a contractual relationship. Mercer Advisors Tax Services, LLC, does not provide financial audit, assurance, compilations, or forensic accounting services. Insurance products are provided by Mercer Advisors Insurance Services, LLC (MAIS), which places individual life, disability, long term care coverage, and property and casualty coverage through select insurance companies. Trustee services are offered through select third parties with which a client would sign an additional agreement, and additional fees may apply.
Tax preparation and filing services are provided by Mercer Advisors Tax Services, LLC. Clients will sign a separate agreement when engaging Mercer Advisors Tax Services that defines the services provided and any additional fees that may apply.
