Diversification myths: why owning more funds does not always mean less risk
Diversification is about correlation, not count. Learn why more funds do not always mean less risk, and what a better spread really looks like.
There is a comforting idea that buying more funds automatically makes a portfolio safer, and at first glance it sounds reasonable. Spread your money around, and surely the bad days in one place will be balanced by the good days somewhere else. The trouble is that diversification is not really about how many investments you own. It is about how those investments behave in relation to each other, and that distinction is where a lot of beginner investors quietly go wrong.
The Short Version
- Diversification is about how holdings behave together, not how many line items you own.
- Five funds can still be one bet if they all lean on the same market driver.
- Costs, home bias and neglected rebalancing often undo the spread investors think they have.
- The practical test is simple: what job does each holding do that the rest of the portfolio does not?
What Diversification Actually Means
Two funds can sit side by side in your portfolio, with different names, different managers and different sector focuses, and still rise and fall in almost perfect unison. When that happens, holding both is closer to owning a single position with extra paperwork. The number goes up, the risk does not really come down, and the cost of running the portfolio creeps higher because you are paying charges on two funds that are doing the same job.
Correlation is the word that does the heavy lifting here, and it is a less intimidating idea than it sounds. It is simply a measure of how closely two investments move together. If one tends to rise when the other rises, they have a high correlation. If they often do the opposite of each other, they have a low or even negative correlation. Genuine diversification means mixing investments whose correlation is low, so that when one part of the portfolio is having a difficult spell, another part has a reasonable chance of behaving differently and softening the blow.
This is why a portfolio that holds five UK equity funds is not really diversified, even if it is split across five reputable providers. They are all exposed to the same broad UK stock market, which means they are likely to fall together in a UK sell-off and rise together when sentiment improves. Adding a sixth UK equity fund does not fix the problem. It just adds another fund that reacts to the same drivers, while quietly adding another layer of charges.
The FCA InvestSmart guidance is useful context because it keeps the focus on risk, cost and suitability rather than on collecting products for their own sake.
Why Asset Mix Matters More Than Fund Count
True spread tends to look across asset classes as well as across regions. A portfolio that combines UK equities with global equities, government bonds, investment-grade corporate bonds, and perhaps a smaller allocation to assets like gold or commercial property has a better chance of holding investments that respond differently to the same economic news. When inflation surprises on the upside, government bonds may struggle while certain equity sectors benefit. When growth slows, high-quality bonds often hold up better than shares. The mix is what creates resilience, not the count of line items.
There is also a practical limit to how much diversification a person can sensibly manage. Once a portfolio drifts past a certain size, monitoring every position becomes harder, costs accumulate, and rebalancing becomes a chore that gets postponed. A focused portfolio of perhaps eight to twelve thoughtfully chosen holdings across different asset classes will usually serve a long-term investor better than a sprawling list of forty funds that nobody has the time to review properly.
The Cost of Overlap
It is worth pausing on the cost angle, because it is the one that quietly erodes returns over decades. Each fund in a portfolio carries an ongoing charge, and many also have transaction costs when the manager buys and sells within the fund. A portfolio with overlapping exposure is therefore paying for diversification that does not exist. Over twenty or thirty years, even a small difference in charges compounds into a meaningful gap between what the markets delivered and what the investor actually keeps.
Another myth is that diversification is a one-off task. Markets shift, funds drift, and the balance of a portfolio changes without anyone touching it. A fund that started out providing genuine exposure to a particular corner of the market can quietly change its mandate, its manager or its style. Two funds that once had a low correlation can drift towards a high one if both end up chasing the same handful of large growth stocks. A yearly look through holdings, sectors and the basic story of each fund is a sensible habit, even if no trades are needed.
Home Bias, Sequence Risk and Drift
Home bias is a common example of where the count of funds can mislead. An investor who feels well diversified because they hold a UK equity fund, a European equity fund, a North American equity fund and a Japan equity fund is still mostly invested in developed-market equities, and within those markets, large companies dominate. Adding an emerging-markets fund, a global government bond fund and a short-duration corporate bond fund changes the picture meaningfully, because those assets tend to respond to different forces. A useful question to ask of any new fund is simple: what is it actually adding that the existing portfolio does not already have?
Sequence risk, where the order of returns matters as much as the average return, is also shaped by correlation. An investor who retires and starts drawing an income from a portfolio that is concentrated in one asset class is more exposed to a bad run in that class at the worst possible moment. A genuinely diversified portfolio, with assets that are not all pointing the same way, gives the investor a better chance that at least some part of the portfolio is doing reasonably well while withdrawals are being taken.
None of this means investors should chase complexity. A well-chosen global index fund, paired with a broad bond fund, can be a perfectly sensible core holding for many people, and there is no shame in keeping things simple. The point is that the word diversification describes a property of the whole portfolio, not a tally of line items. A two-fund portfolio with genuinely uncorrelated assets can be more diversified than a fifteen-fund portfolio full of overlapping exposure.
Tax wrappers change the picture at the edges but not the underlying principle. Within a stocks and shares ISA or a self-invested personal pension, the same logic about correlation and overlap applies. Wrappers are about how returns are taxed, while diversification is about the shape of the returns themselves. Both matter, and they sit alongside each other rather than replacing each other.
Rebalancing ties into this in a useful way. When one part of a portfolio does well, its share of the total grows, and the portfolio gradually becomes less diversified without anyone noticing. A calm, infrequent rebalance back towards the intended mix restores the original balance and forces a tidy-up of any drift. It is one of the few moments in investing when selling a winner and adding to a lagging part of the portfolio is a disciplined, evidence-based move rather than a guess.
A Worked Example
Imagine two investors each hold six funds. The first owns six UK equity funds with slightly different labels. The second holds a global equity fund, a broad bond fund, an emerging markets fund and a small position in gold. The first portfolio may look busier, but the second usually has a better chance of responding differently when one market or asset class stumbles.
That is the point of diversification in practice. You are not trying to maximise the number of lines on the statement. You are trying to avoid a portfolio where every holding tells the same story when markets turn difficult.
What This Means For You
Review your portfolio by role, not by brand. If two holdings would probably react the same way to the same shock, ask whether you need both. Our guides to value investing, growth investing and pound cost averaging are more useful when each fund in the mix has a clear job.
A simple annual check of overlap, charges and drift will usually do more for a private investor than adding yet another fund in the hope that complexity alone creates safety.
In Plain English
Diversification does not mean owning lots of things. It means owning assets that do not all move for the same reason at the same time.
If the portfolio would still rise and fall as one block in a bad week, the spread is weaker than it looks. Better diversification usually comes from clearer roles and calmer maintenance, not from more clutter.
This article is for general financial education only. It is not financial advice or personal investment advice. Investments can fall as well as rise, and you may get back less than you invest.