The term “reversion to the mean” is used somewhat interchangeably with the term “regression toward the mean”.
According to Wikipedia, a regression toward the mean is, “the phenomenon where if one sample of a random variable is extreme, the next sampling of the same random variable is likely to be closer to its mean.”
When it comes to finances, a reversion to the mean says it is likely that asset prices and investment returns will revert back towards their historical long-term average.
However, you do need to account for inflation here. If there is 20% annual price inflation and stocks go up by 20% in a year, they are effectively flat in real terms. A reversion to the mean would suggest that stocks might actually go higher since the real return has been zero.
In the definition above of the regression to the mean, it refers to a sample. In other words, let’s say you flip an evenly weighted coin ten times and you get heads nine times and tails one time. If you flip the same coin again ten times, there is nothing to suggest that you will get tails nine times to even things out. A regression to the mean will mean that ten more flips should tend toward the result of five heads and five tails.
When it comes to stocks, let’s say the historical average is a return of 8%. If you have one year where stocks go up by 50% (assuming little or no inflation), then it doesn’t mean the following year will tend toward the historical 8% return. It is not a new random sample. The stocks are already inflated by 50%.
In this scenario, if you are going to revert to the mean of 8%, the previous 50% rise in stocks will mean that they will go down approximately 22% in the following year to get back to the 8% average – in this case, over a 2-year span.
$100 x 1.08 x 1.08 = $116.64 (2-year return with 8% per year)
$100 x 1.50 = $150
$150 x .78 = $117 (2-year return with a 50% gain and a 22% loss)
The Nasdaq Bubble
In August 2025, U.S. stock market indexes are hitting all-time highs. Stocks are not the only asset at or near all-time highs, but it is the financial asset that is most commonly used by Americans to invest.
(While a primary residence is often considered to be a financial asset, it is really more of a consumption item at least up until the point of selling or renting it out.)
Let’s look at the Nasdaq. It has been so incredibly volatile over the last 3 decades, it is hard to pick a good starting point. The index closed the year at 1,577 in 2008 after the financial crisis had already hit. By the end of 2015, it was back up to 5,007, which is actually near where it peaked at the height of the dot-com bubble in 2000.
For easy numbers, let’s start in 2015 when the Nasdaq was around 5,000. The Nasdaq is now sitting above 21,000. In 10 years, the Nasdaq has gone up approximately 320%. That is a compounding return of a little over 15% per year over the course of 10 years.
If we go to the Bureau of Labor Statistics (BLS), it shows that $5,000 from June 2015 has the same buying power as $6,758.37 in June 2025. That is a little more than 3% compounded annually, but it was skewed higher because of 2022 and 2023.
Even if you think the BLS price inflation numbers are off, they aren’t off by that much. Compare $6,758.37 to over $21,000 (the Nasdaq). Even if you think stocks should average 5% higher than inflation, it isn’t even close.
Price inflation over the last 10 years is just over 3% according to the BLS. The Nasdaq has been returning over 15% per year on average over the last 10 years. This doesn’t connect unless you conclude that we are in a massive bubble.
The Higher It Goes, the Harder It Falls
Starting in 2015 is also rather generous. If we go back to 2008, the Nasdaq actually reached a low below 1,300. The Nasdaq is over 16 times higher than the low it hit in 2008.
The Nasdaq is extremely volatile. If there is a crash and the Fed doesn’t step in with massive monetary inflation, it will fall well below what would be considered a “normal” valuation. Even if the Fed does step in with massive monetary inflation, it might be too late to save the crash. And, of course, it will only make things worse economically.
From 2000 to 2002, the Nasdaq dropped almost 80%. In 2008 alone, the Nasdaq fell over 50% from its high to its low.
If the Nasdaq went down to 5,132, the peak of the dot-com bubble in the year 2000, it would mean a fall of over 75% from where we are right now.
The higher this thing keeps going, the harder it’s going to fall. We can go to the adage that things that are unsustainable have a tendency to stop.
Are you prepared if U.S. stocks crash by 75 or 80 percent? How will it impact you mentally? Will it significantly change your plans for the future?
Forget about the overall economy and inflation just for a minute. If stocks fell by 80%, how well will you handle this? If it will greatly impact your financial well-being, then you should consider a different strategy.
This isn’t an all-or-nothing game. It doesn’t mean you have to sell all of your stocks. It just means that you should be well-positioned if and when we have a stock market crash.