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.

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.

Contrary to classic theory, markets may be growing less efficient. This is driven not only by passive indexing but also by a structural shift in active management towards short-term, quantitative strategies that prioritize immediate price movements over long-term fundamental value.

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.

Many assume the S&P 500 is a purely rules-based, passive index. In reality, a committee makes discretionary decisions on inclusions and exclusions. For example, MicroStrategy met the technical criteria for inclusion but was denied by the committee.

Market cap indexing acts like a basic trend-following system by buying more of what's rising. However, its Achilles' heel is the lack of a valuation anchor, causing investors to over-concentrate in expensive assets at market peaks. In high-valuation environments, almost any other weighting method, like equal-weight or value, is likely to outperform over the long term.