Until about 12 months ago I had a nice S&P 500 index fund, you might have one too. For years I assumed it was one of the safest, most sensible things I owned — spread across 500 American companies, nice and diversified, job done. Then I looked under the bonnet.
I ultimately ended up selling my S&P 500 ETF shares and re-investing the funds straight back into an All World ETF, my reasoning was two fold, firstly the rest of the world started out performing the S&P 500 returns, particularly around the time Mr.Trump started promoting tariffs, but secondly I wanted to make sure that my investments were more diverse.
When I first bought the S&P I didn't realise that "500 companies" doesn't mean each company gets an equal slice. The S&P 500 uses something called market-cap weighting, which means the bigger a company gets, the more space it takes up in the index. And right now, seven enormous tech companies are dominating the whole thing in a way that's starting to make a lot of people nervous.
The Magnificent 7 Problem
As of June 2026, seven companies — Nvidia, Apple, Amazon, Microsoft, Alphabet, Meta, and Tesla — make up 33.8% of the entire S&P 500, according to data from Stock Analysis. That means for every £100 you put into a standard S&P 500 tracker, roughly £34 goes straight to those seven businesses. The top ten stocks combined now account for over 40% of the index, according to RBC Wealth Management.
Just think about that, ten companies out of five hundred control nearly half the index!
This is not a new concern, but it's reached a level we haven't seen before. At the height of the dot-com bubble in 2000, the top ten companies in the S&P 500 made up around 27% of the index. Today's concentration is significantly worse than that — a period most investors would describe as obviously overheated.
What Happened in June 2026
This isn't just a theoretical worry. In early June 2026, the Magnificent 7 collectively shed approximately $2 trillion in market value over just a few weeks. Because those companies account for more than a third of the index, the whole S&P 500 was dragged sharply lower — even though hundreds of other mid-cap and value stocks were actually trading positively during the same period.
If you'd looked at your tracker fund and seen it fall, you might have assumed the whole American economy was in trouble. In reality, most of it was doing fine. It was your seven enormous tech holdings pulling the whole thing down.
This is the trap: what looks like broad diversification is actually a concentrated bet on a handful of AI-driven companies, all correlated to the same theme, all vulnerable to the same kind of bad news.
Is This Worse Than the Dot-Com Bubble?
In one important way, yes. Back in 2000, the tech companies powering the bubble were largely speculative — high valuations, little profit. Today's Magnificent 7 are highly profitable, cash-generative businesses with real dominance in their markets. That's partly why the concentration has been allowed to build without many people raising an alarm.
But Goldman Sachs research cited by Tema ETFs suggests that historical concentration levels like these are a reliable predictor of poor future returns. Their analysis points to a forward 10-year return on the cap-weighted S&P 500 of approximately -5%. That's not a guarantee, and you should treat any 10-year forecast with appropriate scepticism — but it's the kind of number that should at least make you think.
Terry Smith, the founder of Fundsmith, was blunter. In his January 2026 shareholder letter he warned that the shift into passive index funds is "laying the foundations of a major investment disaster" — though he was careful to add he couldn't say exactly when or how it would end. He has, of course, an interest in people preferring active funds. But the underlying maths he's pointing at is definitely real.
The Equal-Weight Alternative
There is a simple structural fix available to UK investors, and it's not especially expensive.
S&P 500 equal-weight ETFs track the same 500 companies but give each one the same 0.2% weighting, rebalanced every quarter. Nvidia gets the same share as a mid-sized pharmaceutical company or a regional bank. No single company can dominate your returns. The result is that your exposure to the Magnificent 7 drops from roughly 34% down to about 1.4%.
This approach has had a decent 2026. According to justETF data to 31st May 2026, S&P 500 equal-weight ETFs returned between 8.7% and 8.9% in GBP. For context, the Magnificent 7 themselves underperformed the broader S&P 500 in the first half of 2026, posting an aggregate return of 5.4% against the index's 7.9% — the first time in years the group has lagged.
For UK investors, these funds are accessible as UCITS ETFs inside an ISA or SIPP. The options include:
Xtrackers S&P 500 Equal Weight UCITS ETF 1C (IE00BLNMYC90) — the largest, at over £8.5 billion in assets, costing 0.15% per year
iShares S&P 500 Equal Weight UCITS ETF (IE000MLMNYS0) — £3.4 billion, also 0.15% per year
Xtrackers S&P 500 Equal Weight Swap UCITS ETF 1D (IE000FL46JJ1) — the cheapest option at just 0.08% per year
The Honest Tradeoff
Equal-weight funds won't always outperform. If the big tech giants continue to surge, a cap-weighted tracker will pull ahead because it holds more of the winners. Between 2023 and 2025, that's exactly what happened — the Magnificent 7 drove the index to record highs and equal-weight strategies lagged significantly.
The question isn't which approach is definitively better. It's about understanding what you actually own. If you hold a standard S&P 500 tracker and think you're diversified across 500 American businesses, you're not quite right. You own 500 companies in name, but your actual return is largely determined by seven of them.
For some investors — particularly those later in their journey who are more focused on not losing big than on maximum upside — knowing that feels important. For others with a long time horizon and strong nerves, the standard cap-weighted approach may still make sense. Neither is obviously wrong. But the choice should be a conscious one, not one made by default.
If you want to see what compound growth looks like across different assumptions for future S&P 500 returns, our compound interest calculator might help you think through the scenarios, you can store your various calculations using the store button on the calculator.
Final Thoughts
The S&P 500 uses market-cap weighting — bigger companies get a bigger slice. Seven companies now control 33.8% of the entire index.
In early June 2026, those seven companies lost around $2 trillion in value in just a few weeks, dragging the whole index down while much of the market was fine.
Today's concentration is higher than it was at the peak of the dot-com bubble in 2000.
Goldman Sachs research suggests current concentration levels imply poor forward returns for the cap-weighted index over the next decade.
S&P 500 equal-weight ETFs are available to UK investors inside ISAs and SIPPs from 0.08% per year — they give every company the same 0.2% weighting and reduce Magnificent 7 exposure from ~34% to ~1.4%.
The tradeoff: equal-weight funds can lag during tech-led bull markets, but offer more genuine diversification when concentration risk bites.
This article is here to give you some information and is for educational purposes only. It is not meant to give you financial advice. It is always a good idea to chat with a financial adviser who knows you well and can help you make the best decisions for your situation.
Sources
https://www.claritx.ai/blog/sp-500-concentration-magnificent-7-risk-2026
https://www.fool.com/investing/2026/05/15/magnificent-seven-growth-stocks-all-time-high/
https://www.fool.com/investing/2026/01/05/should-investors-be-worried-that-the-magnificent-s/
https://www.cnbc.com/2025/12/12/stocks-market-risks-investors-portfolios-2026.html
