Most of an index's returns come from a tiny fraction of its component stocks (e.g., 7% of the Russell 3000). The goal of indexing isn't just diversification; it's a strategy to ensure you own the unpredictable "tail-event" winners, like the next Amazon, that are nearly impossible to identify in advance.

The S&P 500 is no longer a passive, diversified market index. Its market-cap weighting has created a concentrated, active-like bet on a few dominant tech companies. This concentration is the primary reason it consistently beats most diversified active managers, flipping the script on the passive vs. active debate.

Passive, cap-weighted fixed income funds behave like momentum traders, buying more of a bond as its price rises. This is a flawed strategy for fixed income because many bonds are callable, meaning their upside is capped and rising prices increase call risk. Active management can exploit this inefficiency.

Market-cap-weighted indexes create a perverse momentum loop. As a stock's price rises, its weight in the index increases, forcing new passive capital to buy more of it at inflated prices. This mechanism is the structural opposite of a value-oriented 'buy low, sell high' discipline.

Owning a broad, cap-weighted index fund eliminates the need to predict market winners. As dominant companies like Sears fade, they are replaced by innovators like Amazon. The index automatically adjusts, selling off losers and increasing holdings in rising stars, ensuring you always own the future.

The underperformance of active managers in the last decade wasn't just due to the rise of indexing. The historic run of a few mega-cap tech stocks created a market-cap-weighted index that was statistically almost impossible to beat without owning those specific names, leading to lower active share and alpha dispersion.

Terry Smith contends that passive investing is mislabeled. It's a momentum strategy that forces capital into the largest companies regardless of valuation. With over 50% of AUM in passive funds (up from <10% in 2000), this creates a powerful feedback loop that distorts markets more than the dot-com bubble ever did.

Market-cap weighting turned the S&P 500 into a momentum fund for megacaps, leading to a decade of outperformance versus its equal-weight counterpart—a historical anomaly. Recent signs of equal-weight taking the lead suggest a potential market regime shift back towards value and smaller companies.

So-called passive indexes have a small but impactful "active side" in their turnover. This component behaves like a flawed momentum strategy, forcing the index to systematically buy stocks after they've surged and sell them after they've plummeted, creating a performance drag.