A market-cap weighted index like the S&P 500 is, by its very structure, a momentum strategy. As a stock's price rises, its market cap increases, automatically boosting its weighting in the index. This forces passive investors to allocate more capital to outperforming stocks, a core tenet of momentum investing.

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.

Many trend followers only trade stock market indices, which dilutes performance by averaging all constituents. Trading individual stocks provides far greater potential for capturing extreme outlier moves, both long and short. An index, by definition, mutes the very trends the strategy seeks to exploit.

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.

By design, market capitalization-weighted indices increase allocations to assets as their prices rise. This forces investors to continuously buy more of what has already performed well, leading to concentration in popular, often expensive, assets and sectors from the previous market cycle.

Research by Bessenbinder shows that a tiny fraction of "superstar" companies drive all market gains. Since identifying these winners in advance is nearly impossible, indexing ensures you own them by default, capturing the market's overall growth without the risk of picking the wrong stocks.

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.