An unprecedented retail trading collapse in South Korea triggered over 1.2 million margin calls, affecting roughly 3.4% of the adult population
Every week, investors make a supposedly simple choice. Two ETFs sit side by side on a platform. Same asset class label. Same stated remit. Fees within a basis point. The cheaper one wins, the box is ticked, and the allocation is considered done.
Then the portfolios begin to diverge. A year later, the gap is so wide that clients start asking questions, and advisors scramble for explanations. Nothing malfunctioned. Neither fund broke its mandate. They simply weren’t built to do the same thing - a truth the marketing label never disclosed. The index methodology is the portfolio. By treating passive funds as interchangeable commodities, the industry has handed the keys to a silent active manager. And as concentration risk reaches historic extremes, that blind trust is no longer a structural quirk. It is becoming a systemic threat.
The Illusion of Uniformity: The Korea Distinction
The easiest way to see how a single definitional choice reshapes a portfolio is to look at emerging markets. The standard MSCI Emerging Markets Index historically included South Korea, where it often represented nearly a quarter of the total weight. The MSCI Emerging Markets ex Korea Index excludes it entirely. And some emerging markets ETFs exclude it too as many index providers classify South Korea now as a developed market.
That one rule creates a structural gulf. Remove Korea and giants like Samsung Electronics and SK Hynix, together more than 15% of the standard index, disappear. The performance gap is staggering over one twelve month period, the standard index returned 43.51% in USD, while the ex Korea version returned 23.02%. A 20%+ percentage point divergence born from a single methodological decision. Fee comparisons only matter once the structural definitions match. Most buyers never look past the ticker.
The Concentration Trap
This is not an emerging markets anomaly. The fiction of passive uniformity is breaking down across the world’s most trafficked benchmarks, where extreme concentration has quietly turned diversified indices into narrow, high beta bets on a handful of companies.
The S&P 500, the global default for broad US equity exposure, is now dominated by the Magnificent Seven. Investors think they are buying American commerce; in reality, they are buying a concentrated tech portfolio with a long tail attached.
The distortion became so severe in the Nasdaq 100 that the index provider executed a rare special rebalance to forcibly cut back the largest constituents. The rules had to be altered mid stream because concentration had breached the index’s own diversification thresholds. MSCI China has faced similar pressure as Alibaba and Tencent grew so large that a single regulatory crackdown dragged down an entire national benchmark.
These are not theoretical risks. They are structural realities created by the design of the index.
The Silent Active Manager
Passive funds are only passive for the investor. Behind every ticker is an index provider making active, consequential decisions: which countries qualify, which companies enter the universe, how they are weighted, and when the entire structure is rebalanced.
Selection determines the boundaries of the portfolio. Weighting determines how those holdings behave - whether market cap weighted, free float adjusted, or capped to prevent dominance. Reconstitution schedules, often quarterly, trigger predictable trading flows that the rest of the market anticipates and exploits.
These choices are made by committees and rulebooks, wrapped in the comforting language of passivity. They are rarely scrutinized as active decisions at all.
From Market Shock to Systemic Calamity
For years, concentration risk was treated as an academic talking point. Today, with trillions of dollars flowing into market‑cap‑weighted strategies, the feedback loop between passive inflows and increasingly dominant mega‑caps is approaching a breaking point.
When a small cluster of stocks dictates the behaviour of every major benchmark, a correction in one sector doesn’t stay contained. Passive funds sell pro‑rata, and pro‑rata selling hits the largest weights hardest. The same structural forces that drove those names to outsized gains now amplify their declines. As fear builds at the end of an extended rally, behavioural over‑reactions compound the mechanical selling pressure. Each leg down triggers more passive selling, and the loop tightens.
This is how a routine market shock becomes a systemic event, not because advisors changed their process, but because the market’s most crowded winners have become so large, so quickly, that even a modest reversal can cascade through every benchmark that holds them.
The Rulebook Is What You Own
Coming back to choosing one fund over another, across long horizons in developed markets, two broad trackers that follow the same benchmark may behave similarly. In those cases, once methodology, replication, tax treatment, and lending policies align, the cheaper one should win.
But the fee must be the final comparison, never the first. The quiet structural choices, a capping rule here, a float adjustment there, a reclassification buffer somewhere else, compound inside products sold as simple.
A disciplined process earns its keep by monitoring benchmark changes, country classifications, and concentration limits. When ETF selection begins with the methodology rather than the ticker, the silent active manager stops being silent. The firms that will be blindsided in the next systemic shift won’t be the ones who chose the wrong index. They will be the ones who didn’t realise they were choosing an index at all.
Read the rulebook. The ticker was never the thing you owned.
Until next time.
Allan Lane