What Are Small-Cap Stocks and Are They Worth the Risk?
Small-cap stocks have historically outperformed large-caps over the long run — but with higher volatility. Here's what UK investors need to know.
What Are Small-Cap Stocks?
Small-cap stocks are shares in companies with a relatively small market capitalisation — typically defined as companies with a market cap below £2 billion in the UK context, though the exact threshold varies by index provider. In the UK, small-cap stocks are represented in the FTSE SmallCap Index — companies below the FTSE 250 — and in the AIM (Alternative Investment Market), which hosts smaller, typically younger, and higher-growth companies than the main market. The FTSE 350 combines the FTSE 100 and FTSE 250, while the FTSE All-Share adds the SmallCap to give a comprehensive picture of the full UK market.
The Small-Cap Premium: What the Evidence Shows
Academic research — beginning with the work of Rolf Banz in 1981 and subsequently formalised in the Fama-French three-factor model — has identified a persistent historical tendency for small-cap stocks to outperform large-cap stocks over very long periods. This excess return is sometimes called the small-cap premium or size premium. The intuition behind it is that smaller companies are riskier — they have less financial stability, narrower competitive moats, and less access to capital markets in a crisis — and investors expect higher compensation for taking on this additional risk.
Small-Cap Performance in the UK
Looking at the UK specifically, smaller companies have historically delivered higher returns than the FTSE 100 over very long periods, though the relationship has not been consistent and there have been extended periods when large-caps outperformed. During bull markets and periods of economic confidence, small-caps often lead the way. During recessions or financial crises, small-caps typically fall more sharply than large-caps and take longer to recover — which is part of why they offer higher long-run compensation.
AIM: Higher Risk, Higher Potential
The Alternative Investment Market is the UK's market for smaller growth companies. AIM stocks are generally higher risk than those on the main market — many are loss-making, pre-revenue, or in very early stages of development. However, AIM stocks carry some notable tax advantages: AIM-listed shares are exempt from Stamp Duty Reserve Tax on purchase, and qualifying AIM shares can attract Business Relief from Inheritance Tax after two years of holding. Some AIM shares are also eligible for inclusion in a Stocks and Shares ISA.
How to Invest in UK Small-Caps
The most diversified and cost-effective way to access UK small-cap stocks is through a small-cap index ETF. The iShares MSCI UK Small Cap ETF provides broad exposure to UK smaller companies within a single fund. The Vanguard FTSE UK All Share Index ETF includes small-cap exposure as part of a broader UK equity allocation. For those interested in AIM specifically, several investment trusts focus on AIM-listed growth companies, though these carry higher individual stock concentration risk.
Should You Add Small-Caps to Your Portfolio?
For most beginner and intermediate investors, a global index fund already includes thousands of companies of various sizes across multiple countries. Adding a specific small-cap allocation is a factor tilt that may enhance long-run returns but also increases volatility and complexity. If you choose to add small-cap exposure, keep it modest — perhaps 10 to 15 per cent of your equity allocation — and be prepared for it to underperform significantly in some years. The small-cap premium, if it exists, rewards patient investors with genuinely long time horizons.
