"Don't put all your eggs in one basket" is the oldest investment advice in existence. Diversification — spreading investments across many assets so that the failure of any one does not destroy the portfolio — is the foundational principle of modern portfolio construction.
Yet diversification fails more often than investors expect. It fails when assets they believed were uncorrelated move together during crises. It fails when investors have concentrated positions they do not recognise — in their employer's shares, their home property, their currency, their country. And it fails when portfolios are "diversified" across assets that all share underlying risks.
This article examines the forms of concentration risk that catch international investors off guard, the limits of diversification, and how to build portfolios that are genuinely robust rather than just superficially spread.
What Is Concentration Risk?
Concentration risk is the risk that an excessive allocation to a single asset, sector, geography, or risk factor means that one bad outcome damages the portfolio disproportionately.
The most obvious form is owning too much of a single stock. If you hold 50% of your portfolio in one company's shares and that company fails, you lose 50% of your wealth regardless of what the rest of the portfolio does.
But concentration risk exists at multiple levels:
- Security concentration: Heavy allocation to individual stocks or bonds
- Sector concentration: Heavy allocation to one industry (technology, financials, energy)
- Geographic concentration: Heavy allocation to one country
- Currency concentration: Holding most assets in one currency
- Factor concentration: Overexposure to a specific investment factor (momentum, growth)
- Manager concentration: Relying heavily on one investment manager
- Human capital concentration: Employment income, pension entitlements, and employer shares all dependent on one company or sector
- Counterparty concentration: Holding deposits, structured products, or insurance policies with too few counterparties
Hidden Concentrations in "Diversified" Portfolios
Many investors believe their portfolios are well diversified because they hold multiple funds or a large number of securities. This belief is often misleading.
The US Technology Problem
A market-cap weighted MSCI World ETF in 2026 holds approximately 65% US equities by value. Within US equities, a significant proportion of the index weight is concentrated in a handful of technology megacaps — Apple, Microsoft, Nvidia, Alphabet, Amazon, and Meta collectively account for approximately 20–25% of the MSCI World index weight.
An investor who holds one "global" equity ETF and believes they are diversified across the world economy has roughly 15–20% of their equity portfolio in six US technology companies.
This is a genuine concentration. If the tech sector undergoes a prolonged correction — as it did from 2000–2002 and briefly in 2022 — this concentrated position significantly damages the portfolio.
Employer Concentration
For many internationally mobile professionals — particularly those in finance, oil and gas, technology, or law — their salary, bonus, pension, and long-term incentive plan may all depend on the same employer. Some may also receive employer shares as part of their remuneration.
The aggregate "exposure" to employer performance can be enormous: current salary, future salary, pension entitlements, unvested share options, redundancy entitlements. If the employer faces difficulties, all of these are at risk simultaneously.
Investors with significant employer share allocations should typically reduce this exposure as vesting occurs — selling employer shares and reinvesting in diversified assets. The discomfort of selling (tax events, appearance of disloyalty) should not override the risk management need.
Property and Human Capital
For many internationally mobile individuals, their most significant assets are:
- A property in their home country (often mortgaged)
- Human capital (the present value of future earnings)
Both tend to be linked to their home country economy. A British investor's property is priced in sterling; their earnings are likely to be broadly correlated with the health of the UK (or their employment sector's) economy. In a severe UK economic downturn, property prices, employment prospects, and asset values may all fall simultaneously.
True diversification requires holding a meaningful portion of financial assets in geographies and currencies other than the home country — something international investors are often better placed to achieve than domestic ones.
When Correlations Rise in Crises
The second major form of diversification failure is the behaviour of correlations during crises.
Under normal market conditions, different asset classes have modest correlations with each other. Equities and bonds are often cited as negatively correlated — when equities fall, bonds tend to rise (as investors seek safe havens). This relationship underpins the traditional 60/40 portfolio.
But during acute market crises — when investors are forced to sell — correlations between all assets tend to rise sharply. This is because selling is driven by liquidity needs and risk appetite, not by the fundamental differences between asset classes.
During the Global Financial Crisis of 2008–2009:
- Global equities fell approximately 50%
- Commercial property fell 40–50% in many markets
- High-yield corporate bonds fell 30–40%
- Private equity and infrastructure (illiquid assets) declined significantly in value though this was often obscured by infrequent valuation
- Hedge funds of all strategies declined, as many used leverage that was called in simultaneously
Assets that appeared uncorrelated during calm markets moved together in crisis.
The assets that genuinely held their value or appreciated during 2008–2009: government bonds (particularly US Treasuries and UK gilts), gold, and some absolute return strategies.
This is the fundamental limitation of diversification: it works less well when you most need it.
Factor Concentration
Smart beta and factor ETF investors can inadvertently introduce factor concentrations that only become apparent in specific market environments.
A portfolio tilted towards value and momentum simultaneously might seem well-diversified at the factor level. But both factors experienced significant underperformance simultaneously in the 2010–2019 period as growth stocks dominated.
More subtly, a portfolio holding global equity, emerging market equity, global high-yield bonds, and private credit may appear diversified by asset class. But all of these are "risk-on" assets with high exposure to the credit cycle and to global growth. In a global credit crunch, all would likely fall simultaneously.
True factor diversification requires holding assets across risk exposures: risk-on assets (equities, credit) and genuine risk-off assets (government bonds, gold, trend-following strategies).
Counterparty Risk
Investors holding cash deposits, structured products, offshore bonds, and derivatives with concentrated counterparties face counterparty risk — the risk that the institution itself fails.
In most developed markets, deposit protection schemes cover deposits up to a limit (£120,000 per person per firm in the UK since 1 December 2025, €100,000 per institution in EU member states). Balances above these thresholds are unsecured creditor claims on the bank.
Investors holding large cash balances should spread them across multiple institutions, ensuring no single balance exceeds the deposit protection threshold in the relevant jurisdiction.
Similarly, structured products, offshore bonds, and annuities involve concentration with specific insurance or banking counterparties. The failure of Lehman Brothers in 2008 crystallised losses for many structured product investors. Most European insurance-linked products have policyholder protection schemes, but these have limits and conditions.
Measuring Concentration Risk
Several metrics help investors quantify concentration:
Herfindahl-Hirschman Index (HHI): Measures portfolio concentration across securities. Calculated as the sum of the squares of each position's weight. A value of 1 indicates a single-position portfolio; values below 0.10 indicate reasonable diversification at the security level.
Effective number of positions: The inverse of the HHI. A portfolio with an HHI of 0.05 has an "effective" number of 20 positions, even if it holds 100 individual securities (many of which are tiny positions).
Country concentration: What percentage of the portfolio is in each country? Compare to global market weights.
Sector concentration: What percentage is in each sector? The MSCI Global Industry Classification Standard (GICS) divides equities into 11 sectors — technology, financials, healthcare, and so on.
Factor exposure: What are the portfolio's factor loadings on market, size, value, momentum, quality, and volatility? Factor analysis tools (available through Bloomberg, MSCI, or specialist portfolio analytics software) can decompose returns into factor exposures.
Managing Concentration Risk
At the security level: Limit any single security to a maximum of 5% of the total portfolio (or 3% for higher-risk investors). Actively reduce employer share concentrations as they vest.
At the sector level: Monitor sectoral weights. No single sector should exceed 25–30% of the equity allocation.
At the geographic level: Hold genuinely global exposure, not home-country-biased. The world is larger than the UK or US.
At the currency level: Hold meaningful assets in at least two or three major currencies. For internationally mobile investors, this may happen naturally.
At the factor level: Balance risk-on and risk-off exposures. Include genuine safe-haven assets (high-grade government bonds, gold) that will hold value in a crisis.
At the counterparty level: Spread deposits across multiple institutions below deposit insurance thresholds. Use multiple investment custodians for large portfolios.
At the human capital level: Invest financial assets in sectors and geographies uncorrelated with your employer and home market.
When Concentration Is Acceptable
Concentration is not always inappropriate. There are situations where it is justified:
High-conviction specialist knowledge: Some investors — typically former industry practitioners — have genuine informational or analytical advantages in specific sectors. A retired pharmaceutical executive may justifiably hold a larger pharmaceutical allocation.
Tax planning constraints: Large unrealised gains in concentrated positions may make diversification prohibitively expensive in the short term. A phased reduction using CGT annual allowances and tax-loss harvesting may take several years.
Liquidity constraints: Employer share schemes with unvesting periods or sale restrictions create temporary concentration that cannot be immediately resolved.
Very short investment horizons: If you need to sell most assets within two years, ultra-diversification across illiquid alternatives adds cost and complexity without benefit.
Acknowledging accepted concentration and monitoring it explicitly is better than pretending it does not exist.
Compliance Caveats
Diversification does not guarantee against loss. All investments can fall in value as well as rise. The examples and analysis in this article are illustrative and do not constitute personal investment advice. Portfolio concentration is highly individual — what is excessive concentration for one investor may be acceptable for another. Always seek professional advice tailored to your circumstances.
How Global Investments Can Help
At Global Investments, we conduct rigorous concentration analysis for every client portfolio, examining concentration at the security, sector, geographic, factor, and counterparty levels. For clients with significant employer share exposure or concentrated property holdings, we develop phased diversification strategies designed to reduce risk efficiently over time. Contact us to arrange a portfolio concentration review.
This article is for general information only and does not constitute financial, legal or tax advice. Rules, prices and regulations change; verify current requirements with a qualified adviser before acting.