Overview of At The Money: Looking Beyond Market Cap Weighted Indexes
This episode of Bloomberg’s At The Money features Rob Arnott, founder of Research Affiliates, who critiques market-cap weighted indexes (e.g., S&P 500) and explains alternatives—especially fundamental (economic) weighting, aka RAFI. Arnott argues cap-weighting embeds a “buy-what-went-up” bias, creates hidden trading costs and concentration risk (the “Magnificent Seven”), and that fundamental/economic-weighted indices offer a durable way to mirror the economy, reduce valuation risk, and capture rebalancing alpha.
Guest and context
- Guest: Rob Arnott, founder of Research Affiliates, long-time critic of cap-weighted indexing and architect of fundamental (RAFI) indices.
- Host: Barry Ritholtz (Bloomberg).
- Topic relevance: market concentration, index construction, trading/frictional costs, and alternative index weighting methods for investors and advisors.
Key arguments against market-cap weighting
- Cap-weighted indexes mechanically overweight stocks that have recently risen in price—effectively “buying high” and later “selling low” when they fall.
- Index additions and deletions create predictable trading flows that are front-run and cause market impact—indexers pay higher prices when stocks are added and sell lower when removed.
- Large index funds and ETFs amplify these effects: S&P index holdings currently represent a substantial share (~25%) of the aggregate market cap of included stocks, increasing crowding and transaction friction.
- Market concentration (e.g., the MAG-7) increases portfolio risk; dominant stocks can reverse, causing underperformance for passive cap-weighted portfolios.
The “active side of indexing” and the flip-flop problem
- “Active side of indexing”: although index strategies are broadly passive, the small portion of turnover (e.g., 3–5% annually) behaves like aggressive active trading—buying the biggest winners and selling losers at big costs.
- Trading drag: Arnott estimates index turnover/announcement effects cost about 15 basis points per year in aggregate (from forced buying/selling at announcement/effective dates).
- Flip-flop statistics (from Research Affiliates work):
- Stocks are often added after large prior outperformance (example cited: ~75% outperformance before addition).
- Many added stocks get deleted later and are sold at much lower relative valuations (examples cited: deletions occurring after heavy losses, numbers on the order of tens of percentage points).
- Deleted stocks that later rejoin the index can show dramatic pre-addition rallies—creating a cyclical cost for indexers.
- Net effect: these dynamics can materially reduce index returns versus a theoretically frictionless reflection of the economy.
What is fundamental / economic weighting (RAFI)?
- Objective: weight index constituents by measures of economic footprint rather than market price.
- Typical inputs (Research Affiliates approach): average of four measures—sales, profits, book value (net worth, adjusted for intangibles), and shareholder payouts (dividends + buybacks).
- Practical result:
- Downweights frothy/high-multiple growth stocks relative to cap-weighting (they get weights closer to their actual economic contribution).
- Upweights undervalued/value stocks whose market caps understate their economic size.
- Produces a notable value tilt and systematic rebalancing (sell some winners, buy some laggards relative to market-cap weights).
Performance evidence and characteristics
- RAFI/fundamental indices have been live and investable since 2005 (PIMCO, Invesco, Schwab vehicles referenced).
- Relative performance:
- Arnott reports RAFI has outperformed cap-weighted value indexes by roughly 2%–2.5% per year compounded versus cap-weighted value indexes—resulting in a meaningful cumulative advantage over decades.
- Tracking error vs cap-weighted value is modest (cited ~2.5% variability), meaning the outperformance is relatively stable and persistent.
- Compared to equal-weight:
- Equal-weighting often produces similar long-term returns to fundamental weighting but with greater variability and a pronounced small-cap tilt.
- Equal-weighting the S&P can overweight recently enlarged (high multiple) names and miss very cheap stocks that never made the S&P—introducing its own biases.
Practical takeaways and recommendations
- Investors worried about concentration, valuation risk, and the “buy-what-went-up” bias should consider alternatives to pure market-cap weighting—fundamental/economic weighting is a proven option.
- Fundamental indices seek to mirror the economy rather than the market’s price signals, providing:
- A valuation-aware, value-tilted exposure.
- A built-in rebalancing mechanism that historically added ~2% p.a. vs cap-weighted value peers.
- Lower likelihood of extreme concentration into a handful of overvalued names.
- Equal-weight ETFs are a reasonable, simpler alternative but come with greater volatility and small-cap exposure—expect different risk-return tradeoffs.
- For advisors: evaluate index construction, hidden trading/friction costs, and how an index’s weighting interacts with client objectives (e.g., desire to avoid concentration vs. tolerance for value underperformance).
Notable quotes and soundbites
- “Indexing is described as passive, but if it has 5% turnover, the 95% is passive. The 5% looks like a hyper growth manager on crystal meth.”
- “If you weight stocks proportional to their price, you’re basically saying the more expensive they are, the bigger their weight—don’t you just love it?”
- “If you want an index that studiously mirrors the economy instead of studiously mirroring the market, don’t weight companies by market cap. Weight them by how big their business is.”
Bottom line
Cap-weighted indexes have structural and behavioral costs tied to market-price-driven weights, index additions/deletions, and concentration risk. Fundamental (economic) weighting—using sales, profits, book value, and shareholder payouts—offers a pragmatic alternative that historically produced a modest but persistent excess return versus cap-weighted value indices, with lower concentration risk and an automatic rebalancing benefit. Investors should weigh tradeoffs (value tilt, potential periods of underperformance vs growth-led rallies) and choose index construction aligned with long-term objectives and risk tolerance.