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.

When S&P pre-announces index changes, arbitrageurs buy the incoming stocks and sell the outgoing ones before index funds must trade. This forces the funds to transact at worse prices, creating a consistent 15 basis point annual performance drag from what is effectively legal front-running.

The success of market-cap weighted indexes stems from a core trend-following principle: a few outlier stocks generate nearly all gains. The index's mechanism of increasing a stock's weight as its price rises and decreasing it as it falls mimics a basic, passive trend strategy.

A key flaw in pure indexing is the forced, predictable trading around index rebalancing. When a stock is added, all index funds must buy it, often at an inflated price due to front-running. David Booth estimates this systematic inefficiency costs index fund investors a "run up" of about 4% on new additions.

Index providers are no longer neutral. By changing inclusion rules to quickly add "hot" IPOs like SpaceX, they are making active bets on specific companies. This blurs the line between active and passive investing, requiring investors to have an opinion on the index's strategy itself rather than just blindly buying.

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.

Many stocks added to the S&P 500 are later removed. Index investors are forced to buy these "flip-flop" stocks *after* they have already appreciated significantly (avg. +75%), only to then participate fully in their subsequent decline (avg. -70%), locking in a substantial loss.

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.

The small portion of an index fund's portfolio that turns over annually (e.g., 5%) isn't passive. It actively buys stocks after they have significantly appreciated and sells them after they've declined, mimicking a poorly-timed, high-risk growth strategy.