Why a Diversified Portfolio Can Still Be a Bad Portfolio
Updated: Sep 9
Why can a diversified portfolio still be concentrated?
A portfolio can hold many funds and still depend on the same companies, sectors, currencies, factors or economic conditions. The number of holdings is not the same as the number of independent risk drivers.
The key mechanism is hidden correlation and overlap. Diversification works when different exposures respond differently to important shocks, not merely when the portfolio contains many product names.
A portfolio can own 40 funds, hold assets across several countries, pay very low fees, and still be badly built.
That sounds strange because diversification has become shorthand for “safe” or “sensible”. In many cases, it is both. Spreading money across assets can reduce the damage from one failed company, one weak sector, or one poor decision.
But diversification is not a guarantee. It can be cosmetic. It can hide repeated risks. It can give investors comfort without giving them real protection.
The harder question is not whether a portfolio looks diversified. It is whether each holding still earns its place.
This article is for information only and does not constitute personal financial advice.
The diversification illusion makes weak portfolios look stronger
Diversification often gets judged by count.
How many funds do you own?
How many regions are represented?
How many asset classes appear on the statement?
Those are useful starting points, but they do not prove much on their own.
An investor might own:
A global equity index fund
A US technology fund
A quality growth fund
A thematic artificial intelligence fund
Shares in several large platform companies
That portfolio has multiple lines. It may even have different fund providers and names. Yet a large part of the outcome can still depend on the same thing: high expectations for large growth companies, often concentrated in the United States.
The illusion becomes stronger when tools display neat pie charts. One slice says Europe. Another says North America. Another says emerging markets. A bond slice sits next to an equity slice. It looks controlled.
But the chart may not show what really drives returns.
A portfolio is not diversified because it contains many items. It is diversified when those items behave differently for sound reasons.
The difference matters most during stress. When markets are calm, many portfolios look balanced. When conditions change, hidden overlaps become visible.
Diversification by name is not the same as diversification by risk
Fund names can create a false sense of separation.
A “global” fund and a “sustainable leaders” fund may hold many of the same companies. A “balanced” fund and a “multi-asset” fund may both rely on equity markets rising and bonds cushioning losses. A “dividend” fund may reduce some growth exposure, but add sensitivity to financials, energy, or interest rates.
The job is to look through the label and ask what risk the asset carries.
Diversification by name | Diversification by risk |
Counts the number of holdings | Studies what drives returns |
Treats different fund names as different exposures | Checks overlap between positions |
Focuses on regions, sectors, or wrappers | Focuses on economic sensitivity |
Feels diversified when the list is long | Looks diversified only when risks differ |
Think of a portfolio as a group of engines. If every engine depends on the same fuel, adding more engines does not solve the problem. It may only make the vehicle larger and harder to steer.
Real diversification considers exposures such as:
Equity market risk
Interest rate risk
Credit risk
Currency risk
Inflation sensitivity
Liquidity risk
Valuation risk
Concentration in a region, sector, or investment style
Some overlap is normal. No portfolio is perfectly clean. The goal is not to eliminate every shared risk. The goal is to know which risks dominate and decide whether they are intentional.
A bad portfolio is often not bad because every asset is weak. It is bad because too many reasonable assets depend on the same assumption.
Correlations change when diversification is needed most
Correlation measures how assets move in relation to each other. If two assets often rise and fall together, they have positive correlation. If they move in opposite directions, they may help offset each other.
The problem is that correlations are not fixed.
During normal periods, two assets may appear to behave differently. During a shock, they can suddenly fall together. Investors often discover this at the worst possible time.
This happens for several reasons.
When markets panic, investors sell what they can, not only what they want to sell. Liquidity becomes valuable. Assets with different stories can suffer because they are held by the same investors or financed by the same risk appetite.
When inflation expectations shift, bonds and equities may both struggle. Bonds can fall because yields rise. Equities can fall because higher discount rates reduce the value of future earnings.
When a currency moves sharply, international assets may not provide the expected protection for a euro-based investor. The investment may perform one way in local terms and another way after currency effects.
That does not make diversification pointless. It means diversification should not rely only on past correlations.
Historical data tells a story, but it does not promise the next chapter.
A stronger review asks:
What type of stress should this asset protect against?
Has it actually helped in that type of stress before?
Could the same condition hurt several assets at once?
Am I relying on a relationship that may break when markets change?
These questions are more useful than simply checking whether two assets had a low correlation number over the last few years.
Rebalancing can preserve a bad thesis
Rebalancing is often described as a disciplined habit. It means bringing a portfolio back to its target weights after markets move.
In many cases, that discipline helps. It prevents winners from taking over the portfolio. It forces a system rather than emotional buying and selling.
But rebalancing can also protect a weak idea from being challenged.
Suppose an investor decides that 20% of a portfolio should sit in a specific sector fund. The sector performs poorly for several years because the original growth story has faded, valuations remain high, and earnings disappoint. A mechanical rebalance keeps topping it back up to 20%.
The process looks disciplined. The thesis may be broken.
Rebalancing answers one question: “How do I return to my chosen allocation?”
It does not answer a better question: “Does this allocation still deserve to exist?”
That distinction matters. A rebalancing rule should not replace judgement. If an asset was bought for a reason, that reason needs review.
For example:
A bond fund bought for stability may now carry more interest rate risk than expected.
A high-dividend fund bought for income may have become concentrated in a few weak sectors.
A private asset fund bought for lower volatility may simply price less often, which can make risk harder to see.
A thematic fund bought for long-term growth may now depend on very optimistic valuations.
Good rebalancing starts after a thesis check, not before it.
If the thesis still holds, rebalancing can support discipline. If the thesis has failed, rebalancing can become a quiet way to add money to an old mistake.
Low cost does not mean low risk
Low fees are valuable. Costs reduce returns with certainty, while returns are uncertain. Paying less for broad exposure can make a big difference over long periods.
Still, low cost is not the same as low risk.
An index fund may be cheap and still heavily exposed to expensive markets. A bond ETF may have a low fee and still lose value when interest rates rise. A broad equity fund may hold thousands of companies and still move largely with global equity sentiment.
Cost tells you what you pay to own an exposure. It does not tell you whether the exposure is suitable, balanced, or well timed.
This point gets missed because low-cost investing is often discussed as if it solves the portfolio problem. It solves a fee problem. That is useful, but narrower.
A portfolio built only around low fees can still suffer from:
Too much exposure to one country
Too much exposure to one sector
Too much dependence on equity markets
Too little inflation protection
Too little liquidity
Too much currency risk
Too little thought about future cash needs
A bad expensive product should not be replaced with a cheap version of the same bad idea without further thought.
Fees matter. So do valuation, role, behaviour, and risk concentration.
Most portfolio tools miss the most important question
Many portfolio tools are good at measurement. They can show past returns, volatility, fees, regional exposure, sector splits, and drawdowns. Some can estimate future scenarios.
Those inputs help. They can reveal overlap and risk that a normal statement hides.
Yet many tools are less useful at asking why an asset is held.
They show the portfolio as it is. They do not always challenge whether it still makes sense.
A tool may show that a fund has underperformed. That can be useful. But underperformance alone does not make a holding wrong. A defensive asset may lag in a rising market and still be doing its job. A value strategy may trail a growth market for years and still add useful balance.
The reverse is also true. A strong performer may be increasing portfolio risk. It may have grown from 5% to 18% of the portfolio. It may now dominate outcomes. It may still look attractive because recent returns look good, while the forward risk has increased.
The missing question is simple:
Why do I still own it?
That question does not appear on most portfolio dashboards, but it changes the review.
It turns the exercise from reporting into judgement.
A portfolio review should not only ask what happened. It should ask whether each holding still has a clear job.
A better portfolio review starts with ownership discipline
A useful review does not need to be complicated. It needs to be honest.
For each asset, fund, or strategy, ask five questions.
Why do I still own it?
This is the first filter. If the answer is “because it has done well”, that may not be enough. If the answer is “because I bought it years ago”, that is not an investment thesis.
A holding should have a current reason, not only a purchase history.
Good answers may include income, long-term growth, inflation sensitivity, capital preservation, diversification, or liquidity. Weak answers often sound like habit, hope, or fear of regret.
What role does it play?
Every holding should have a job.
Some assets aim to grow capital. Some aim to dampen volatility. Some provide income. Some protect against specific risks. Some exist because they can be sold quickly when cash is needed.
If two holdings have the same job, that may be fine. But if five holdings have the same job by accident, the portfolio may be more concentrated than it looks.
Clear roles also reduce emotional decisions. When an asset performs badly, the question becomes whether it failed at its job, not whether it was uncomfortable to own.
Which assumption am I relying on?
Every investment contains an assumption.
For equities, the assumption may involve earnings growth, valuation, margins, or economic resilience. For bonds, it may involve interest rates, credit quality, inflation, or central bank policy. For property funds, it may involve rental income, financing costs, occupancy, and liquidity.
The assumption should be visible.
If the portfolio relies on “rates will fall”, “large growth companies will keep expanding margins”, or “credit spreads will stay contained”, that needs to be recognised. Hidden assumptions create hidden concentration.
What would make me reduce or remove it?
This is where many portfolios improve.
Before buying or holding an asset, define what would change your mind. That condition may be thesis-based, valuation-based, risk-based, or role-based.
Examples include:
The holding no longer performs its intended function.
Its position size grows beyond a sensible range.
The valuation requires unrealistic assumptions.
A better, cleaner exposure becomes available.
The risk is already present elsewhere in the portfolio.
The liquidity no longer matches future cash needs.
This avoids the trap of endlessly defending a position because selling feels like admitting a mistake.
Is another asset already carrying the same risk?
This question often reveals the heart of the problem.
A portfolio may own different wrappers but repeat the same exposure. A Belgian investor might hold global funds, European funds, sector funds, and individual shares, yet still end up dominated by the same interest rate sensitivity, US market exposure, or growth style.
Overlap is not always obvious. It can sit inside funds, styles, factor tilts, currencies, or economic themes.
The aim is not perfection. It is awareness. If two assets carry the same risk, the investor should know that and size them accordingly.
A better portfolio is built on reasons, not just variety
Diversification remains one of the most valuable ideas in investing. The problem starts when the word becomes a substitute for thought.
A portfolio can be broad but fragile. It can be cheap but risky. It can be neatly rebalanced but built on an outdated thesis. It can contain many funds and still depend on one market story.
The better standard is ownership discipline.
Know what you own. Know why you own it. Know what risk it adds. Know what would make you change your mind.
A portfolio is not just a collection of assets. It is a collection of assumptions. PerCapita Asset Intelligence helps you structure, challenge and reassess the reasoning behind those assumptions. explore Asset Intelligence Pro.
A diversified portfolio is only strong when its parts earn their place. Variety is helpful. Purpose is better.
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