Index investing gained popularity in the 1990’s and 2000’s, largely because it was an easy and cost effective way to get broad exposure to the stock market. Investors learned that being exposed to just a few stocks greatly increased their risks. Further, for anyone generating income from their portfolio, we previously posted on how volatility increases Sequence of Returns Risk and the detriment that has on Safe Withdrawal Rates.
How it Used to be, How it is Now
Above is a table showing the top 10 companies in the S&P 500, by market capitalization (share price * shares outstanding), from 2005-2025. (References: 1, 2)
- From 2005-2015, the 10 largest companies in the S&P 500 represented about 20% of the total weight of the index, versus over 40% in 2025. This demonstrates a significant increase in market concentration.
- From 2005-2015, the 5 largest companies in the S&P 500 each represented 1.6%-3.3% of the index; versus 4.0%-7.2% in 2025. (The largest company, Nvidia, holds a weight of over 7%.)
Some Background
What is Market Capitalization?
Market capitalization (frequently abbreviated “market cap”) is just the price of a stock multiplied by the total number of shares outstanding. Basically it is the price you’d pay for the company if you tried to buy all the shares. (Well, not really, because if you tried that, people would notice and that alone would move the price higher.) Fundamentally, it is a way to answer the question “how much does the stock market think this company is worth?”
Types of Stock Indices
Stock indices can be composed several ways; two common ways are market cap weighted and equal weight.
- Market cap weighted index: Each stock's impact on the index price is “weighted” based on its market cap. If you own an ETF that tracks a market cap weighted index, your money is effectively buying an amount of stock in each company according to its weight in the index.
- Equal weight index: All stocks have the same weight on the index price. If you own an ETF that tracks an equal weighted index, your money is effectively invested equally in every company in the index.
- Both statements above have the caveat that indices are only periodically updated to reflect constituent weights; frequent changes would wreak havoc for fund managers.
Why Does This Matter?
All that brings us to this fact:
If you hold an S&P 500 index fund (like SPY, VOO, IVV, etc.), you have essentially invested 40+% of your money into just 10 companies.
What About the Nasdaq 100?
Nasdaq 100 index market cap and index weights of the 10 largest companies
The Nasdaq 100 index is represented in the ETF world by ETFs like QQQ and QQQM.
The Nasdaq 100 index is a modified market cap weighted index; there are rules to limit the impact of a few large stocks on the index value. The left table is what the weights of each company would be on the index in a purely market cap weighted index. With “modified market cap weight“ it is actually the weights as shown in the right table.
The top 10 stocks in the Nasdaq 100 index currently have an unadjusted market cap weight of roughly 70% of the index total (left table above), while their current adjusted weights “only” represent about 53% of the index (right table above). (References: 1, 2)
- Even with the modified weights, 37% of the Nasdaq 100 Index is composed of just 5 stocks.
Details on Nasdaq 100 Index Methodology
In 2023, Nasdaq implemented a “special rebalance” to this effect; more on their methodology here. There have only been 2 prior special rebalances. Key from the linked methodology document:
- The aggregate weight of the companies whose weights exceed 4.5% may not exceed 48%.
What to Do?
If you were buying index funds 10 or more years ago, you likely didn’t make the purchase thinking:
I really want about half my money in 10 stocks, and the other half in diversified investment(s).
Simple Solution
The indices that stand out as having a problem with diversification are the S&P 500, Nasdaq 100, and Nasdaq Composite.
The simple solution to this is to recognize that market cap weighted index investing is not as diversified as it once was, and find ETFs that use equal (or other suitable) weighting.
Examples:
Another solution is an ETF that excludes the largest stocks from the index. An example of this is XMAG (Defiance Large Cap ex-Mag 7 ETF); “The First ETF Offering Exposure to the S&P 500 Excluding the “Magnificent 7” Tech Giants“.
I expect we will see more products like this as demand for investment diversification will drive product development in the ETF space.
(The above products are for reference to the type of product suggested; this is not an endorsement of any of the products.)
Fundamentally, you first have to recognize the problem. Hopefully this post helped with that. Now you need to review your investment holdings and decide what, if any, actions to take.
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View Video Transcript
So, there's a pretty good chance you own an index fund. It's kind of the default investment for smart, simple exposure to the market. But what if I told you that the diversification you think you have, well, it might not be the real story. Seriously, take a guess. If you've got money in an S&P 500 fund, what chunk of it do you think is tied up in just the 10 biggest companies? 10%, 20? Get a number in your head, cuz we're about to see how close you are. You've heard this a million times, right? For decades, the best advice was super simple. Just buy a lowcost index fund. It was the gold standard for getting into the market, spreading out your risk, and just letting your money grow. It was basically investing 101. And that entire strategy, that whole idea was built on this one powerful concept. Let's call it the diversification promise. The logic behind it was, you know, flawless. It's the classic don't put all your eggs in one basket. If you spread your money across hundreds of companies, a few of them having a bad year wouldn't totally wreck your portfolio. That was the whole point. Less risk, steady growth. But here's the thing. The market that gave us that advice isn't the same market we have today. Something really fundamental has changed. kind of right under our noses. So, let's dig into what we're calling the S&P 500's big shift. Now, this slide tells a pretty cool story. You can see how the top 10 companies have changed over time. You see old giants like GE and Exxon fade away and today's big tech names take over. But the most important part of this chart isn't the names, it's the numbers way down at the bottom. So, check this out. Back in 2005, the 10 biggest companies made up about 20% of the entire index. That meant the other 80% of your money was spread out among the other 490 companies. I mean, that sounds pretty diversified, right? That's what you signed up for. Okay, now fast forward to today. That number has more than doubled. It's over 41%. Just let that sink in for a second. Almost half of your diversified fund is riding on the success of just 10 companies. And this is the key takeaway really. If you own a standard S&P 500 index fund, you've accidentally made a huge bet on just a handful of stocks. The diversification you thought you had, it's been seriously watered down. So, how did this even happen? Is it some big conspiracy? Nah, not at all. It's actually a feature, not a bug, of how these funds are designed. To get it, we need to peek under the hood at how index waiting works. It all comes down to this thing called market capitalization, or just market cap. It sounds complicated, but it's super simple. You just take the company's stock price and multiply it by all the shares out there. It's basically the market's price tag for the whole company. See, most index funds are market cap weighted. This creates a kind of snowball effect. As a company gets bigger and its stock price goes up, it automatically takes up a bigger piece of the index. So, the winners just keep winning bigger shares of your money. The alternative is an equal weight fund where every single company from the biggest giant to the smallest player in the index gets the exact same slice of your investment. And hey, if you think the S&P 500 sounds concentrated, you got to see what's happening with the tech heavy NASDAQ 100. This is where it gets really wild. This slide shows you the problem in high definition. On the left, you see what would happen with pure market cap waiting. On the right is what the index actually looks like after NASDAQ steps in with special rules to try and spread things out a bit. But even with those rules, wow, you can see how topheavy it still is. And this chart really drives it home. If they didn't do anything, the top 10 companies would be over 70% of the entire index. 70%. Even after they try to fix it, those same 10 stocks still make up more than half the fund's value. Over 53%. It gets even more extreme. Just five companies, you can count them on one hand, make up 37% of the whole NASDAQ 100. So when you buy a NASDAQ 100 fund, you're not buying a 100 companies equally. You're making a massive bet on a tiny handful of tech titans. Okay, the problem is clear. Your index fund is probably way less diversified than you assumed. So the big question is, what can you actually do about it? Let's talk about how you can start reclaiming that diversification. Honestly, the first and most important step is just to recognize this new reality. That old set it and forget it approach now comes with this hidden risk of concentration that frankly most people just don't know about. The rules of the game have changed. So, what are some of the moves you could make? Well, first, just acknowledging the problem in your own portfolio is huge. Second, you can look into those equal weight ETFs we mentioned like RSP. They put the same amount of money in every company and bring that balance back. And third, there are even funds that kick out the biggest stocks entirely, specifically to avoid this problem. These are all tools designed to put the diversification back where it belongs. So yeah, the world of investing has shifted. The thing that was once the safest, most diversified foundation, it's become something else. Something way more concentrated and dependent on just a few massive companies. Armed with this knowledge, the real question isn't for me to answer. It's for you. What will you do now?