Why one good story is never enough

Most of us start investing by backing something we understand. A company we admire, a sector that's been in the news, a fund a friend recommended. It feels like conviction, and conviction feels good. The trouble is that a single holding carries a single set of risks, and those risks have a habit of arriving without warning.

Put some numbers on it. Suppose you have £10,000 invested entirely in one sector, and that sector falls 40% — not a fantasy, plenty of sectors have done exactly that within a single year. You've lost £4,000. Now suppose that same sector makes up 10% of your portfolio, with the rest spread across other sectors, regions and asset types. A 40% fall in that slice costs you £400, or 4% of your total. The loss is real, but it's survivable, and it doesn't derail your plans.

That's the whole idea in one line: diversification doesn't stop things going wrong, it stops any one thing going wrong from mattering too much.

Spreading across asset types

The first layer of diversification is the broadest. Different asset classes behave differently, often at different times.

• Shares offer the strongest long-term growth potential, but they fall hardest in a downturn.
• Bonds and gilts tend to be steadier, and government bonds in particular have historically held up when share prices tumble — though not always, so don't treat them as a guarantee.
• Cash protects your capital in the short term but loses buying power to inflation over long periods.
• Property can be accessed through funds, adding another source of return that doesn't move in lockstep with shares.
• Gold and other commodities are often held as a small hedge, typically 5% or less.

A common starting point for a UK investor with a medium-term horizon is somewhere around 60–80% in shares and 20–40% in bonds and cash, adjusted for how much volatility you can genuinely stomach. The right split is the one you'll stick with when markets fall, not the one that looks best on a spreadsheet.

Spreading across regions and sectors

Within shares, you can diversify geographically and by industry. The UK stock market is dominated by a handful of sectors — financials, energy and consumer staples — so a portfolio of only UK shares is far less varied than it looks, even if it holds 40 different companies.

Adding exposure to the US, Europe, Japan and emerging markets gives you access to industries that are barely represented here, from technology to healthcare to industrial machinery. Sector diversification matters just as much. If everything you own depends on oil prices or interest rates moving in one direction, you haven't really diversified — you've just bought the same bet in different packaging.

The home bias trap

UK investors have a well-documented tendency to overweight UK assets. It's understandable: we recognise the names, we read the news, the dividends arrive in pounds. But the UK accounts for only around 4% of global stock market value. Holding 50% of your equity portfolio here means a heavy concentration in a small slice of the world.

That's not an argument for abandoning UK holdings. Many people want some domestic exposure, and currency movements can work in your favour. But a deliberate, modest home weighting is very different from a portfolio that accidentally became a bet on one economy. If you're unsure where you sit, dig out your fund factsheets and add up the regional splits — the answer is often a surprise.

Diversification inside a fund

You don't need twenty funds to be diversified. A single global tracker fund can hold thousands of companies across dozens of countries, which is far more spread than most people could build themselves. In fact, over-diversifying across funds is a common mistake: hold five global trackers and you simply own the same shares five times, paying five sets of charges for the privilege.

Keep an eye on overlap. If your workplace pension, your stocks and shares ISA and your self-invested personal pension all invest in similar global equity funds, your true exposure is more concentrated than any single statement suggests. Check the top ten holdings in each and see how many names repeat.

Putting it into practice

A workable structure for many people looks like this: a global equity fund as the core, a bond or gilt fund for stability, a small allocation to something that behaves differently, and cash for emergencies. Use your ISA allowance — £20,000 for most adults in the current tax year — and your pension for long-term money, though rules can change, so check the current position.

Watch your costs, because they compound in reverse. A simple mix of index funds might cost you 0.1% to 0.25% a year in ongoing charges, plus a platform fee. Paying 1.5% instead sounds trivial until you realise it can swallow a quarter or more of your eventual returns over thirty years.

Finally, rebalance. Once or twice a year, check whether your target splits have drifted. If shares have raced ahead and now make up 80% of a portfolio you intended to be 70%, trim back and top up the steadier holdings. That simple habit forces you to sell what's expensive and buy what's cheap, and it keeps your risk where you decided it should be.