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%.)
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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?