Concentration You Didn’t Choose
Recent equity markets have been unusually narrow, with a small group of stocks and a handful of return drivers — momentum prominent among them — accounting for an outsized share of index gains. That narrowness matters because momentum does not announce itself the way a sector bet or a single large stock position does. It can accumulate quietly across quality funds, growth funds, index funds and individual stock picks, because all of them can end up owning the same names for related reasons.
As Fidelity portfolio manager Matt Jones puts it, quality investing is not momentum, growth investing is not momentum, and index investing is not designed to be momentum either — but "when enough capital crowds into the same ideas for long enough, many roads eventually lead to the same destination and the distinction between quality, growth and momentum begins to blur". That blurring is the core problem this article is about: concentration that hides beneath apparent variety.
It is worth being precise about what momentum actually is. As a factor, it is simply the tendency for securities that have recently performed well to keep performing well for some period, and for recent losers to keep lagging — a pattern built on price behavior rather than on business fundamentals alone. It is a well-documented phenomenon, not an exotic one. The issue the article raises is not that momentum exists or that it is inherently flawed, but that investors may be exposed to it without having decided to be.
Success Breeds Its Own Fragility
Momentum has a self-reinforcing mechanic: strong performance attracts capital flows, flows push prices higher, and higher prices attract still more capital. The cycle continues "until it doesn’t". Jones makes an important distinction here — this is not an argument that momentum is a bad strategy. It has been, in his words, "one of the strongest drivers of equity market returns in recent years". The caution is narrower and more practical: momentum often looks safest right before it becomes most vulnerable, because the very popularity that drives the returns is also what creates the crowding.
Research from MSCI adds a more granular lens to this dynamic. Using a model that scores how "crowded" a stock has become — based on valuation, trading activity and how sharply sentiment has shifted relative to the stock’s own history — MSCI found that, across global equities going back to 1997, stocks with higher crowding scores have tended to generate negative excess returns over the following months, with a fairly consistent decile-by-decile pattern. This is a historical, back-tested relationship rather than a guarantee about any individual stock or future period, but it illustrates a mechanism worth understanding: crowding and momentum often travel together, and the combination can carry a higher risk of sharp de-rating when sentiment turns.
Visible Diversification vs. Hidden Concentration
The practical difficulty is that two portfolios can look completely different on a fact sheet while sharing the same underlying driver. The table below outlines where that gap tends to open up.
| What looks diversified | What may actually be concentrated | Why the gap exists |
|---|---|---|
| Many individual stock holdings | Heavy exposure to one factor (e.g., momentum) across most of them | The factor, not the company, is the common driver of returns |
| Multiple fund managers or strategies | Similar underlying bets because managers use overlapping data and screens | Shared information sets produce convergent decisions |
| A cap-weighted index fund | Returns dominated by its largest constituents | Cap weighting automatically gives bigger weight to the biggest winners |
| Several asset classes on paper | Correlated sensitivity to the same macro theme or sentiment shift | Classification by label does not capture the actual return driver |
The common thread is that diversification is not just about the number of line items in a portfolio; it is about whether those line items respond to different underlying forces. Owning twenty stocks that all rise and fall with the same crowded trade is, in a meaningful sense, one bet wearing twenty costumes.
How Crowding Turns Into a Cost Problem
Hidden concentration is not only a risk-management question; it can also become a maintenance-cost question. The chain works roughly like this: strong trends attract more capital, crowding intensifies, and portfolios built to track or capture that trend need to trade more often to stay aligned with shifting leadership.
flowchart LR A[Strong trend performs well] --> B[Capital flows in, crowding rises] B --> C[Leadership narrows further] C --> D[Turnover increases to track it] D --> E[Trading and rebalancing costs rise]
More frequent rebalancing tends to increase trading costs, and in taxable accounts it can also trigger tax consequences that a buy-and-hold approach would otherwise defer. It is true that exchange-traded fund structures can, in general, reduce certain taxable distributions compared with some traditional mutual funds — but that structural feature does not erase the cost or tax effects of frequent trading inside a strategy that must constantly chase shifting momentum leadership. The wrapper can soften some frictions; it does not eliminate the underlying portfolio-construction issue of needing to rotate holdings as crowding and leadership evolve.
A Short Checklist for Reading Your Own Portfolio
None of this means a given allocation is wrong — Jones is careful to frame these as "healthy portfolio construction questions" rather than warning signs. A few useful ones for an individual investor to ask:
- How much of my recent return has come from a small number of stocks or themes, rather than from broad participation?
- If I look across my funds and individual positions together, do they tend to hold similar large-cap, high-momentum names?
- Would my portfolio’s performance change dramatically if market leadership rotated away from its current favorites?
- Am I rebalancing or trading more often than I used to, and do I understand why?
- Did I choose this exposure deliberately, or did it accumulate as a by-product of chasing what has recently worked?
What This Is — and Isn’t — Telling You
It is worth being clear about the limits of this argument. Nothing here establishes that momentum will reverse at any particular time, that concentrated portfolios are doomed to underperform, or that any specific fund or stock should be sold. Markets rotate on their own schedule, and factor behavior that has held historically is not a forecast of what comes next. What the evidence does support is more modest and more useful: that diversification is a question of return drivers, not headcount, and that crowded trends tend to carry higher turnover and higher implementation friction as leadership shifts.
The real question worth asking is not whether momentum will keep working. It is whether you know how much of your portfolio depends on it continuing to.
