Masters in Business At The Money: Looking Beyond Market Cap Weighted Indexes
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Apr 22, 2026 
Rob Arnott, founder of Research Affiliates and creator of RAFI fundamental indexing, explains why cap-weighted indexes concentrate risks and hide active bets. He discusses index trading effects, edition-driven costs, and how fundamental weightings (profits, sales, net worth) and rebalancing can offer a different return profile. Listeners hear comparisons to equal-weighting and RAFI’s long-term performance patterns.
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Indexing Is Passive With An Active Tail
- Market cap indexing is effectively passive most of the time but behaves like an active hyper-growth strategy in a small high-turnover slice.
- Rob Arnott illustrates this by calling indexing 'blissfully indifferent' with a 5% active sliver that chases recent winners and amplifies momentum.
Index Additions Create Predictable Crowding Costs
- Large index providers force indexers to trade at announced effective prices, creating predictable crowding around additions and deletions.
- Arnott quantifies the drag: trading costs from these index flows can cost roughly 15 basis points per year, equivalent to high per-trade implicit costs.
The Flip Flop Performance Trap
- Stocks added to indexes typically arrive after large run-ups and, if removed later, suffer much larger subsequent losses, producing a net performance drag.
- Arnott cites numbers: additions occur after ~75% outperformance while deletions average ~7,000 basis points underperformance versus market.