When you buy a fund that tracks a big stock market index, it feels like you are spreading your money across hundreds of companies. In theory you are. In practice, a handful of giants may be doing most of the work. That is index concentration, and it brings a risk that many investors do not realise they are taking.
What index concentration means
Most major indices, such as the S&P 500 and the FTSE 100, are weighted by market capitalisation. The bigger a company’s stock market value, the more it counts. When a few companies grow far faster than the rest, their share of the index swells. At times in recent years, the largest ten companies in the S&P 500 have made up roughly a third of its value.
So while an index fund may hold 500 companies, its performance can hinge on a small number of them. Our guide to market capitalisation explains how those weights are set.
Why it is a risk
The main danger is that investors are less diversified than they think. If the biggest companies share a theme, such as artificial intelligence, then a setback for that theme can drag down the whole index even if most other companies are doing well.
Concentration also makes indices more sensitive to individual results. When one of the giants reports earnings, the entire market can move. That is why results from the largest technology companies are watched by people who do not even own those shares directly. Our guide to mega-cap stocks covers this in more detail.
Why it is not always bad
Concentration often happens because the biggest companies have genuinely been the most successful. Their profits have grown fastest, and the index has simply followed the money. Trying to avoid them entirely has, at times, meant missing out on a large part of the market’s gains.
History also shows that concentration comes and goes. Leadership changes over time, and today’s giants are not guaranteed to stay on top.
How traders and investors manage it
Some investors use equal-weighted funds, which give every company the same weight regardless of size. Others mix index funds with funds focused on smaller companies or different regions. Some simply accept the concentration but keep an eye on how much of their total money depends on one theme.
Traders watch market breadth, which measures how many companies are rising compared with how many are falling. When an index climbs but most of its members do not, it suggests the rally is narrow and more fragile.
The bottom line
Index concentration means a few large companies have an outsized influence on a supposedly diversified index. It is not automatically a problem, but it is worth knowing how much of your money really depends on a handful of names.
Knowing your real exposure is the first step to managing risk. The free Samuel & Co Trading Trader Assessment can help you check yours.
