When I first started building my investment strategy, I worried about what might happen if one major corporation went bankrupt. What if the company I invested my hard-earned savings into suddenly suffered an accounting scandal or a devastating crisis?
That was when I discovered that regulated funds are not allowed to take that risk on your behalf. In the United States, diversification is written into the Investment Company Act of 1940 and reinforced by the tax code. In the United Kingdom, the FCA's COLL sourcebook applies the UK UCITS limits—the framework retained and renamed after Brexit, and still the gold standard for retail fund protection in Britain and across Europe. Both bodies of rules exist to ensure no single company, bank, or government can sink your entire portfolio.
Understanding these diversification limits gives you a peek under the hood of how professional index funds and mutual funds manage risk behind the scenes. Let's explore the core rules that keep your capital safe and properly diversified.
The Core Foundation: True risk management isn't about avoiding the market; it's about avoiding single-point concentration. By law, regulated funds on both sides of the Atlantic must spread risk across dozens of issuers so that a failure in one won't destroy your financial future.
What Can Funds Actually Buy? The Eligible Asset Universe
Before a fund manager ever worries about diversification formulas, they must choose from a strictly defined universe of eligible assets:
- Publicly Traded Securities: Equities and fixed-income bonds listed on regulated exchanges—the NYSE and Nasdaq in the US, the London Stock Exchange and other recognised investment exchanges in the UK.
- Money Market Instruments & Bank Deposits: Treasury bills, Gilts, commercial paper, and deposits maturing in under 12 months.
- Regulated Derivatives: Futures and options used for hedging or efficient portfolio management. US funds must also comply with SEC Rule 18f-4, which caps derivatives risk using a value-at-risk test rather than a simple notional limit.
- Other Funds: Investing in peer funds, provided those target funds don't hold more than 10% in other funds themselves—which is what prevents endless fund-of-funds chaining.
To maintain portfolio quality, the UK rules limit unlisted or non-approved securities to a maximum 10% bucket. The US equivalent is SEC Rule 22e-4, which caps illiquid investments at 15% of net assets and forces a fund to report and remediate any breach.
The UK Golden Rule of Diversification: 5/10/40
The central quantitative pillar of UK retail fund risk control is the 5/10/40 Rule, set out in FCA COLL 5.2. It limits how much exposure a fund can take to any single issuer:
- Base 5% Cap Per Issuer
- Up to 10% Exception
- Sum of >5% Positions ≤ 40% Total
- The 5% Base Limit: A fund cannot invest more than 5% of its total net assets in securities issued by a single entity.
- The 10% Expansion: A manager can raise that 5% cap up to 10% for specific holdings.
- The 40% Aggregate Cap: Here is the catch: the combined total of all individual positions that exceed 5% of the portfolio cannot exceed 40% of the fund's total value.
How the 40% Cap Works in Practice
Imagine a UK fund holds 10% in AstraZeneca, 10% in Shell, 10% in HSBC, and 10% in Unilever. Because those four companies each exceed the 5% threshold, they add up to exactly 40% of the portfolio. The remaining 60% of the fund's capital must be allocated in chunks of no more than 5% per issuer across at least 12 other distinct companies.
The US Rules: Two Tests, Not One
American funds arrive at the same destination by a different route, and it catches out even experienced investors: a US mutual fund has to pass two separate diversification tests.
1. The 1940 Act test (what the fund may call itself). A fund registered as a "diversified company" must, for 75% of its total assets, hold no more than 5% in any one issuer and no more than 10% of any issuer's outstanding voting securities. The remaining 25% is unconstrained. A fund that declines these limits is legally a non-diversified fund and must say so plainly in its prospectus—which is exactly why reading that one line matters before you buy a concentrated sector or thematic fund.
2. The Subchapter M test (whether the fund pays tax). To qualify as a Regulated Investment Company under the Internal Revenue Code and pass income through to shareholders untaxed at the fund level, a fund must also meet the IRS asset-diversification test each quarter: at least 50% of assets in positions no larger than 5% each, and no more than 25% of assets in any single issuer. Fail it and the fund is taxed as a corporation—a catastrophe no manager risks.
| Rule Set | Core Single-Issuer Limit | Consequence of Breach |
|---|---|---|
| US 1940 Act (diversified) | 5% across 75% of assets | Must re-register and disclose as non-diversified |
| US IRS Subchapter M | 25% cap; 5% across 50% of assets | Loss of pass-through tax status |
| UK COLL / UK UCITS | 5%, expandable to 10%, 40% aggregate | FCA breach report and correction |
Government Bonds and Counterparty Limits
Not all issuers carry the same default risk, so the rules flex with creditworthiness. UK funds may hold up to 35% in securities issued or guaranteed by a single government or public body—Gilts, US Treasuries, or other qualifying sovereign debt—and up to 100% if spread across at least six issues with none above 30%. It is this exemption that makes a pure Gilt or Treasury fund legally possible.
Exposure to any one bank or corporate group is separately capped at a 20% aggregate, combining shares, bonds, deposits, and derivative counterparty risk with that single institution. In the US, Treasury and agency securities are similarly exempt from the 5% test, which is why a Treasury money market fund can hold almost nothing else.
Passive Index Funds: Exceptions for Market Tracking
What happens when an index fund tracks a benchmark like the S&P 500 or the FTSE 100, where a single giant might represent 7%, 12%, or more of the index?
To allow index funds to mirror real market returns accurately, both regimes grant flexibility:
- UK replication funds can hold up to 20% in a single issuer, rising to 35% for one dominant constituent in exceptional market structures.
- US index funds frequently register as non-diversified precisely so they can track a concentrated benchmark without breaching the 1940 Act—while still needing to satisfy the IRS 25% ceiling, which is the real binding constraint when one stock runs hot.
This matters more than it used to. The S&P 500's largest holdings now sit close to the levels at which these caps start to bite, and it is worth knowing that your "diversified" index fund is diversified to the extent the index is—no more.
Ready to Take the Next Step?
Understanding how fund managers diversify risk behind the scenes is key to building a resilient, worry-free investment strategy. Take our habit assessment to evaluate your current portfolio allocation, check your asset concentration, and map out a diversified path to long-term wealth.