Key Takeaways
· Not all companies influence an index equally – Most major indices are market capitalisation weighted, meaning larger companies have a greater impact on overall index movements than smaller ones.
· A small number of companies can drive overall performance – The “Magnificent Seven” make up a significant portion of the S&P 500, meaning their performance can heavily influence the direction of the entire index.
· Index performance doesn’t reflect all companies equally – An index can rise even if many of its underlying companies are falling, due to the outsized influence of its largest constituents.
Important Information This article is for educational purposes only and does not constitute investment advice. Investments can go down as well as up and you may get back less than you invest.
When investors hear that a major stock market index such as the S&P 500 has risen or fallen, it might sound as though all companies within that index are moving in the same direction. In reality, stock indices are influenced more heavily by some companies than others. This is because most major indices are built using a system known as market capitalisation weighting.
Understanding how index weighting works can help investors better interpret market movements and understand why the performance of a small number of very large companies can sometimes have a significant impact on the overall index.
What Is Index Weighting?
An index is made up of a group of companies, but each company does not contribute equally to the index’s movement. Instead, most indices assign a weight to each company based on its market capitalisation, which is the total value of all its shares in the market.
Market capitalisation is calculated by multiplying the company’s share price by the total number of shares outstanding. The larger the company’s market value, the larger its weight within the index.
This means that when very large companies move in price, they can influence the overall index more than smaller companies because their weighting in the index calculation is higher.
For example, if a company with a large weight in the index rises significantly, it may push the entire index higher, even if many smaller companies within the index are declining.
The Magnificent Seven
A clear example of index weighting can be seen in the S&P500 with the influence of a group of technology companies often referred to as the “Magnificent Seven.” This informal name is commonly used to describe seven of the largest and most influential companies in the U.S. technology sector and this list details a rough weighting of each:
· NVIDIA: ~7.0–7.5%
· Apple: ~6.0–6.5%
· Microsoft: ~4.5–5.5%
· Amazon: ~3.5–4.0%
· Alphabet (Google combined): ~5.5–6.0% (Class A and C shares combined in index weighting)
· Meta Platforms: ~2.5–3.0%
· Tesla: ~2.0–2.5%
These companies have grown rapidly in recent years and now represent a substantial portion of the total value of the U.S. stock market.
At certain points in time, this concentration has become particularly significant.. At various points in 2024, the Magnificent Seven collectively represented around 30% of the total market capitalisation of the S&P 500, highlighting how a small number of companies can carry substantial weight within the index (Source: Bloomberg Intelligence; S&P Dow Jones Indices).
How Large Companies Can Influence the Index
To understand how weighting works in practice, imagine two companies within an index. One company might have a market value of several trillion dollars, while another might be worth only a few billion. Even though both are included in the same index, the larger company will have a much greater influence on the index’s movement.
If the larger company’s share price rises significantly, it can push the entire index higher, even if many smaller companies are not performing as strongly.
This is why investors sometimes observe situations where an index appears to be performing well even though a large number of individual companies within that index are experiencing more modest gains or even declines.
This effect can become more pronounced during periods when a small number of large companies are driving market returns. In 2023, for example, the Magnificent Seven accounted for more than 60% of the S&P 500’s total return for the year, demonstrating how the performance of a few very large companies can heavily influence the direction of the broader index (Source: Goldman Sachs Global Investment Research; figure varies depending on methodology and time period).
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Why Index Weighting Exists
Market capitalisation weighting is widely used because it reflects the relative size and importance of companies within the market. Larger companies tend to have a greater economic footprint, employ more people, generate more revenue, and influence broader economic activity.
By weighting companies according to their size, indices aim to provide a realistic representation of the overall market.
This approach also has practical advantages for investment funds that aim to replicate an index. If a company represents a large share of the market’s total value, it makes sense for it to have a larger presence within the index.
Index movements are a great introduction to the broader markets and may appear straightforward at first glance, but the forces behind them are often more concentrated than they seem. Recognising this can help investors develop a more informed view of the market.
Disclaimer
Your capital is at risk. All investments carry the risk of losing some or all your capital. The value of the investment can fall as well as rise, and the value of your investment may be less than the amount you invested. Tax rules can change and their effects on you will depend on your individual circumstance.
This information is for educational purposes only and does not constitute financial advice. Before investing, you should assess your financial situation, investment goals, and risk tolerance. If you are unsure, consider seeking advice from an independent financial adviser.
