Ask a group of investors how they choose stocks and you will hear about hot sectors, clever technology, or the bank your grandfather worked for. Factor investing suggests a more systematic route. Instead of betting on individual companies, you bet on measurable characteristics that, across decades of data, have tended to earn extra returns. Those characteristics — size, value, momentum, quality and low volatility — are known as factors, and an entire industry of index funds, smart beta products and ETFs has grown up around them. The ideas are well supported by academic research, but they are not a free lunch, and they demand far more patience than most fund marketing suggests.
What a factor actually is
At its simplest, a factor is a trait that splits the market into groups and — historically, at least — gives one group a persistent edge over another. Size is simply the small-versus-large question: do small, nimble companies outperform their giant peers over time? Value is the cheap-versus-expensive question: do unloved stocks, trading below what their fundamentals appear to justify, beat fashionable ones? Momentum makes a different claim: recent winners tend to keep winning for a while, because investors are slow to digest new information fully. Quality favours profitable, stable, well-run businesses on the theory that durable earnings power compounds better than flashy turnaround stories. And low volatility, the most counterintuitive of the five, suggests that dull, defensive stocks deliver better returns for the risk you actually take than the explosive ones.
None of these are stock-picking tips in the usual sense. They are statistical regularities found in long runs of market history — mostly American history, where the records stretch furthest back and the evidence is therefore strongest. Understanding that distinction matters, because it shapes how much weight you should put on them.
The five main factors
| Factor | Description | How It Works |
|---|---|---|
| Size | Small-cap stocks outperform large-cap | Small companies have higher growth potential |
| Value | Cheap stocks outperform expensive ones | Low price-to-book stocks have higher returns |
| Momentum | Recent winners continue to win | Stocks that went up keep going up |
| Quality | High-quality companies outperform | Profitable, stable, well-managed companies |
| Low volatility | Low-risk stocks outperform high-risk | Defensive stocks have higher risk-adjusted returns |
The table summarises the idea behind each factor, but do not let the tidy phrasing fool you. Every one of these bets carries a cost — both in interpretation and in real money. Small-cap stocks are illiquid and volatile, and their companies are riskier precisely because they lack the scale and resilience of a mega-cap. Value stocks can stay cheap for years, because a stock is sometimes “cheap for a reason.” Momentum has a habit of reversing violently when the crowd suddenly changes its mind. Quality companies are well run and dependable, which is exactly why you pay a premium price for them. And low-volatility stocks tend to lag badly in strong bull markets, when it is the risk-takers who are rewarded.
The evidence behind the factors
| Factor | Annual Excess Return (US, 1927-2023) |
|---|---|
| Size (small caps) | ~2-3% |
| Value | ~3-5% |
| Momentum | ~8-10% (but high turnover) |
| Quality | ~2-4% |
| Low volatility | ~1-2% |
The rough figures above come from long-run US research covering 1927 to 2023, and they explain why factor investing became so fashionable. Momentum looks extraordinary — historically as much as 8-10% a year extra — while size looks comparatively modest at roughly 2-3%. Pause before getting excited, though, because two cautions apply. First, these are historical averages over nearly a century of American data; they describe what happened, not what is guaranteed to happen next, and similar effects elsewhere rest on weaker and shorter records. Second, the biggest premium belongs to the factor that is the most expensive to capture, because momentum strategies churn through holdings and generate heavy dealing costs and tax bills. An academic edge and an edge in your real-world account after fees and taxes are not the same thing.
Combining factors
Rather than bet everything on a single factor, investors commonly combine two or more to smooth the ride and reduce the damage when any one is out of favour. Value plus momentum is the classic pairing: value captures the long-term cheap-stock premium while momentum catches short-term trends, and the two have tended to do well at different times. Quality combined with low volatility builds a defensive portfolio that should fall less in downturns. Small-cap value — size and value together — has historically delivered the strongest returns of any combination. The logic sounds a little like portfolio construction by committee, and it works largely because factors that are not perfectly correlated dilute each other’s worst periods.
The two families of factors
It helps to think of the five factors as belonging to two broad families. Size, value and momentum are the aggressive, return-seeking side — they aim to earn more, and they bring bigger swings along the way. Quality and low volatility are the defensive side — they aim to lose less in bad markets and to produce steadier, more dependable returns. Knowing which family a fund belongs to tells you what you are buying and, more importantly, when you will be tempted to sell it. A momentum fund will look glorious for years and then give back a chunk of it in a sudden reversal; a low-volatility fund will bore you to tears during a bull market and quietly save you during the crash.
How investors actually implement it
You do not need to hand-pick a portfolio of factor stocks. The practical route is through factor ETFs and smart beta funds, where a rule-based index does the selection for you. Widely used examples include IUSN (iShares World Small Cap) and AVUS on the size side, VVAL (Vanguard Global Value Factor) for value, MTUM (iShares MSCI USA Momentum Factor) for momentum, and QUAL (iShares MSCI USA Quality Factor) for quality. Note that these are regional funds with US tilts; the exact tickers available on your UK platform will differ, and some factors are better covered than others in the European fund lineup.
Single-factor funds tilt your portfolio one way at a time, while multi-factor funds bundle several factors in one holding, which is more convenient and usually better diversified against any single factor falling out of fashion. The trade-off is cost: factor funds charge more than a plain global tracker, and multi-factor products can stack fees for a benefit you may struggle to see in the first decade. If you do build your own mix, treat individual factor funds as tilts around a cheap global core rather than as replacements for it.
The patience problem
The hardest part of factor investing is that premiums are irregular and only become visible over long periods. Value, the most famous factor of all, badly underperformed for roughly a decade after 2007 — exactly the kind of stretch that causes real investors to sell out just before the recovery. And a factor fund will not spare your portfolio from market falls: most track the broad market closely with a tilt, so when the tilt is wrong, you underperform the plain index for years. That is tolerable only if you are prepared to hold through it, which is why factor investing suits people with decades ahead of them far better than those hoping for an edge this year.
Bottom line
Factor investing is an extension of the same logic that makes index funds work: rather than argue with the market, you systematically overweight the characteristics that history suggests have been rewarded. The evidence is real, the rough US long-run premiums are meaningful, and factor ETFs make the approach practical for ordinary investors. But the premiums are irregular, they shrink after fees and taxes, and any single factor can underperform for a decade at a time. For most beginners, the sensible foundation remains a low-cost global tracker, with factor tilts as a modest, patient optional extra — never a substitute for time in the market or diversification across factors.