When people say "you should diversify," they usually mean well. But the advice lands without much explanation – as if diversification were something you either do or don't do, rather than something that can be done well or poorly.
In practice, it matters a great deal. Owning five companies is different from owning five hundred. Owning five hundred companies in one country is different from owning five hundred across many. Understanding the difference is what this article is about.
What diversification is not
Let us start with what diversification is not, because the most common versions of it offer less protection than people assume.
Owning several individual stocks is not enough. Suppose you hold shares in five domestic companies. They are different businesses in different industries. But they operate under the same regulations, move with the same national economy, and respond to the same local market conditions. If something goes wrong at home – a recession, a policy change, a spike in unemployment – all five positions tend to move in the same direction at the same time. That is concentration, not diversification.
Owning several funds is not enough either – if they track the same index. This is a more common situation than it might seem: two or three funds with different names, different providers, and broadly the same underlying holdings. More cost, no additional spread.
A 500-company index is not automatically well-diversified. The S&P 500 contains 500 of the largest US companies. It is one of the world's most respected equity indices. But its seven largest holdings currently account for about 31% of its total market value — far more than the name "500 companies" might suggest. And all of them are in one country, with one currency, and one set of political and economic conditions.
This is not a reason to avoid a US index fund. It is a reason to understand what you actually own.
What diversification actually means
True diversification means owning different types of risk, not just different names. It works across three dimensions.
Geographic diversification. Different countries and regions do not always move together. When one economy slows, others may be growing. When one country faces a currency crisis or a political shock, it does not automatically drag every other market with it. Owning investments across many countries means that problems in one place affect only part of your portfolio.
No one can reliably predict which region will outperform over the next decade. That is precisely why spreading across regions makes sense – it positions you for growth wherever it happens to occur, rather than requiring a correct prediction in advance.
Sector diversification. Technology stocks behave differently from healthcare stocks. Energy companies move differently from consumer goods companies. During any given period, sectors that have performed strongly can quickly underperform, and vice versa. A portfolio spread across many sectors moves more smoothly than one concentrated in any single area.
Time diversification. This dimension is the easiest to implement and often the most underappreciated. Investing a fixed amount each month – rather than everything at once – means you buy at many different price points over time. Some months you will buy when prices are high; others, when they are lower. Over time, these average out.
The practical benefit is not only financial. A regular monthly investment also removes the pressure of finding the right moment to invest, because you have stopped looking for one. Consider a simple example: an investor putting €200 per month into a broad index fund over ten years buys through volatile months and calm ones alike. No single entry point makes or breaks the outcome. The accumulation is what matters.
Why a US-only fund is not the complete answer
A US index fund is a strong investment. This argument is not against it.
But it is worth being precise about what it is: exposure to one country's equity market. The United States is the world's largest equity market by market capitalisation, and it has produced strong long-term returns over many decades. That is not in dispute.
What is worth noting is the degree of concentration within the index itself. The ten largest companies currently account for about 36.8% of the S&P 500's total value — considerably more than most investors realise when they think of themselves as holding "500 different bets." These companies are clustered in a small number of sectors and are all subject to the same US monetary policy, the same dollar, and the same regulatory environment.
A global index fund distributes this differently. It holds companies across North America, Europe, Asia, and emerging markets – thousands of companies in a single investment. The United States still represents the largest individual share, because US companies dominate global market capitalisation. But the degree of concentration is lower, and the geographic spread is real.
Whether a US index, a global index, or a combination suits your situation is a separate question – and one worth exploring with context specific to your circumstances. The point here is simpler: these are not interchangeable choices.
The simplest approach for a beginner
For someone starting out, a globally diversified index fund is the most practical foundation. Funds linked to broad global indices such as MSCI World or FTSE All-World hold shares in hundreds to several thousand companies in a single investment. At the index level, MSCI World currently has 1,319 constituents, while FTSE All-World has 4,230 constituents. You do not need to decide which country to favour, which sector to overweight, or which companies to select. The index does that work, weighted by market size.
One honest note: even broad global index funds remain significantly tilted toward the United States, because US companies are the largest by market value. Buying a global index fund does not eliminate US exposure – it reduces its share relative to your total portfolio.
Combined with a habit of regular monthly investing, a global index fund provides a coherent starting point for most long-term investors. It is not the only valid approach, and individual circumstances vary. But it is a well-evidenced foundation that requires no specialist knowledge to maintain.
What diversification does not protect against
This is the part that is often left out, and it should not be.
Diversification reduces what is called unsystematic risk – the risk specific to a single company, sector, or country. It does this well. But it does not eliminate systemic risk – the risk that markets fall broadly and simultaneously.
In 2008 and again in early 2020, equity markets around the world declined sharply at the same time. A globally diversified portfolio would have fallen too. Diversification limited the damage relative to more concentrated positions, but it did not prevent loss altogether.
This is not a flaw. It is an accurate description of what diversification is: a tool for managing concentration risk, not a way to remove market risk entirely. The residual risk – the possibility that markets broadly decline – is also why, historically, holding a diversified equity portfolio over long periods has produced returns that cash and bonds alone have not matched. Risk and long-term return are connected.
What to keep in mind
- Diversification is not about how many assets you own. It is about owning genuinely different risks.
- Geographic diversification – spreading across countries and regions – is the most important dimension for most long-term investors.
- A US equity index fund is a strong investment but represents a single country's market, with significant concentration in a small number of large companies. It is a starting point, not a complete picture on its own.
- A globally diversified index fund is the simplest way for a beginner to achieve meaningful diversification in a single investment.
- Investing regularly each month adds time diversification and removes the pressure of timing the market.
- Diversification does not protect against broad market downturns. That is a normal part of investing – and it is also why long-term investing has historically paid off.
A well-diversified portfolio will not be the top performer in any given year. It holds assets that are doing well and assets that are not – which is precisely the point. You are not trying to predict which part of the world will win next. You are making sure that wherever growth happens, some of your money is there.
That is not a sophisticated strategy. It is a sensible one. And for most people starting out, it is a stronger foundation than any approach that asks you to predict the future correctly.