Ask many UK investors what "the stock market" means, and the FTSE 100 is often the first thing that comes to mind — it is the index reported nightly on the news, referenced in newspapers, and frequently the default choice for a first equity fund. Yet the UK represents a relatively small fraction of global stock market value, and heavy allocation to it can leave a portfolio less diversified than an investor might assume. This tendency to overweight one's home market is known as "home bias", and it affects investors in nearly every country — but its particular shape and cost look different for UK investors specifically.
What home bias is and why it happens
Home bias describes the well-documented tendency of investors worldwide to hold a disproportionately large share of their portfolio in companies from their own country, relative to that country's actual share of the global stock market. It is not unique to the UK — American, Japanese and Australian investors show similar patterns relative to their own markets — but it is worth understanding in a UK-specific context because of how large a mismatch it can create.
Common reasons for home bias
- Familiarity. UK companies are more visible in day-to-day life and the domestic media, making them feel more "knowable" than an unfamiliar overseas business.
- Historical habit. Many older investment products, workplace pensions and legacy portfolios were built with a strong UK equity weighting as a default, a pattern some portfolios still carry forward.
- Currency comfort. Holding UK assets avoids currency fluctuation relative to sterling, which some investors perceive, rightly or wrongly, as reducing risk.
- Perceived tax or dividend familiarity. The UK market's dividend culture, and its long history of dividend-focused funds, has made UK equity income a popular default choice.
The scale of the mismatch
The UK stock market has historically represented only a small single-digit percentage of total global stock market capitalisation, dominated as global markets are by the United States and, to a lesser extent, other developed and emerging economies. An investor allocating, say, 40% or 50% of their equity portfolio to UK shares is therefore holding vastly more domestic exposure than a market-capitalisation-weighted global index would suggest — a significant divergence from a globally diversified starting point.
Sector concentration within the UK market
The FTSE 100 in particular is also heavily weighted towards a relatively small number of sectors — historically energy, financials, materials and consumer staples have made up a large share of the index, while it has comparatively little exposure to some fast-growing sectors, such as large-cap technology, that are more prominent in indices like the S&P 500 or MSCI World. This means a heavy FTSE 100 allocation is not just a bet on the UK, but implicitly a bet on the relative performance of a particular set of sectors.
Why this matters for diversification
The central benefit of diversification is combining assets whose returns do not move in lockstep. A portfolio dominated by a single country's market — however well-established that market is — forgoes much of the diversification available from spreading exposure across many countries, currencies, industries and economic cycles that do not always move together. A period of UK-specific weakness, whether driven by domestic economic conditions, currency movements, or sector-specific headwinds, can disproportionately affect a UK-heavy portfolio in a way that a globally diversified one would be more insulated from.
| Consideration | Heavy UK weighting | Globally diversified weighting |
|---|---|---|
| Sector balance | Concentrated in financials, energy, materials, staples | Broader spread including technology, healthcare, industrials globally |
| Currency exposure | Predominantly sterling | Mixed currencies, adding a further diversification factor (and its own risk) |
| Familiarity | High — domestic companies and news coverage | Lower — requires more research into overseas markets |
| Diversification benefit | Limited to one economy's cycle | Spread across many economic cycles |
Is some home bias reasonable?
Not all home bias is necessarily a mistake. There are some genuine, if partial, arguments for holding some UK exposure:
- UK investors typically have future liabilities in sterling — day-to-day living costs, and eventually retirement income — so some sterling-denominated assets can reduce currency mismatch risk, though bonds and cash are often a more direct tool for this than equities.
- The UK market has historically offered comparatively attractive dividend yields, which may suit investors specifically seeking income.
- Some investors are more willing to hold through volatility in a market they understand well, which has behavioural value even if it is not strictly optimal on paper.
The distinction many portfolio commentators draw is between deliberate, modest UK exposure held for a stated reason, and unexamined home bias that has simply never been questioned.
Home bias in workplace pensions specifically
Home bias often enters a portfolio without any deliberate decision at all, through a workplace pension's default investment fund. Many default workplace pension funds, particularly older ones, were built with a meaningfully higher UK equity weighting than a globally market-capitalisation-weighted fund would suggest, reflecting historical convention in UK pension design rather than a current assessment of optimal diversification. Employees who have never actively reviewed their workplace pension's fund choice may be carrying a significant, unexamined UK weighting purely as a legacy of the scheme's original default setting, sometimes without realising it, since a workplace pension statement does not always make the underlying regional breakdown obvious at a glance.
Checking a workplace pension's regional exposure
It is worth requesting or looking up the factsheet for a workplace pension's default fund, checking specifically for its regional equity breakdown, and comparing this against a global index's typical UK weighting. Where a meaningful gap exists and the pension provider offers alternative fund choices — which most workplace schemes do, even if the default option is rarely reviewed — switching some or all of the pension into a more globally diversified fund option is often possible without needing to change employer or scheme.
The FTSE 100 versus the FTSE All-Share
It is also worth distinguishing between the FTSE 100, which covers only the 100 largest companies listed in London, and the broader FTSE All-Share, which includes a much wider range of UK-listed companies including small and mid-cap businesses. A portfolio described as "tracking the UK market" through a FTSE 100 fund specifically is arguably even less diversified within the UK itself than one using a FTSE All-Share fund, since it excludes a large number of smaller UK companies entirely, concentrating exposure further into the largest, most internationally-facing businesses — many of which, incidentally, already generate a large share of their revenue from outside the UK, which somewhat blurs the line between "UK exposure" and genuinely global exposure in any case.
Addressing home bias in a portfolio
- Establish what proportion of the current equity allocation is UK-focused, looking through fund factsheets to underlying holdings if necessary.
- Compare this to the UK's actual weighting in a global index, to see the scale of any gap.
- Decide whether the current UK weighting reflects a deliberate choice — such as an income objective — or simply historical accumulation.
- Where reducing UK weighting is desired, consider redirecting new contributions towards a global or international fund rather than necessarily selling existing UK holdings outright, which can be a gradual, tax-efficient way to shift the balance over time.
A worked example
Suppose a hypothetical investor holds a £70,000 equity portfolio, of which £35,000 (50%) is in various UK-focused equity funds, accumulated gradually over 15 years of workplace pension contributions and ISA top-ups. If a broad global index would suggest a UK weighting closer to the low single digits of the equity portion, the investor's 50% weighting represents a very large overweight to a single economy. Rather than selling all the UK funds at once — which could trigger Capital Gains Tax if held outside an ISA or SIPP, subject to the £3,000 annual exempt amount — the investor could direct all new contributions to a global fund for a period, gradually reducing the proportional UK weighting over several years without an abrupt change.
A note on revenue exposure versus listing location
An important nuance often missed in discussions of home bias is the distinction between where a company is listed and where it actually earns its revenue. Many of the largest FTSE 100 constituents are genuinely multinational businesses generating a substantial share of their profits overseas, in foreign currencies, from customers around the world. This means a UK-listed portfolio is not necessarily as purely exposed to the domestic UK economy as its label might suggest — though it remains true that such a portfolio is still concentrated in a specific, relatively narrow set of large companies and sectors, regardless of where those companies happen to generate their underlying revenue.
A final practical checkpoint
A simple, useful habit is to check the UK weighting of every account held — ISA, SIPP, and any workplace pension — at least once a year, alongside any broader portfolio review, since home bias tends to build up gradually and unnoticed rather than arriving as a single deliberate decision.
Key takeaways
- Home bias — overweighting domestic shares relative to their global market weight — is common among investors in most countries, including the UK.
- The UK represents a comparatively small share of total global stock market value, so a heavily UK-weighted portfolio is far from globally representative.
- The FTSE 100 is also concentrated in a narrower set of sectors than a global index, adding a further, less visible concentration.
- Some UK exposure can be a deliberate, reasonable choice — for income, currency matching, or investor comfort — rather than a mistake in itself.
- Redirecting new contributions towards global funds is often a more tax-efficient way to reduce an existing home bias than selling outright.
- Always check current HMRC and FCA figures and allowances, as these change from tax year to tax year.