At The Money: Looking Beyond Market Cap Weighted Indexes (April 22, 2026)
Cap weighted indexes have come to dominate ETFs. Is it time for investors to consider a strategy based on fundamental weightings, such as profits or revenue growth?
Full transcript below.
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About this week’s guest:
Rob Arnott is known as the “godfather of smart beta” and founder of Research Affiliates, which oversees strategies for over $100 billion in assets.
For more info, see:
Professional Bio
Masters in Business
LinkedIn
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TRANSCRIPT: Rob Arnott
Intro:
Boy, you’re gonna carry that weight
Carry that weight a long time
Boy, you’re gonna carry that weight
Carry that weight a long time
Barry Ritholtz: Big, broad market-cap-weighted indexes, like the S&P 500, have dominated investor inflows and performance really since the financial crisis. But lately, critics of cap weighting point out that increased market concentration of just a handful of stocks — AKA the Magnificent Seven — is increasing risks for investors. What should a portfolio manager do about this? Well, to help us unpack all of this and what it means for your portfolio, let’s bring in Rob Arnott, founder of Research Affiliates and a longstanding critic of market cap weighted indexes. RAFI runs a variety of fundamental indexes that are based on things outside of cap weighting. Let’s jump right into it. So, Rob, you’ve spent decades challenging cap weighted indexes as simply just owning more of what just went up. Frame the case for alternative weighting — regardless of what it is, equal weight, fundamental, whatever — versus traditional cap weighted indices.
Rob Arnott: Let’s play a thought experiment. Suppose I came to you and said, I have a brilliant strategy. You’re gonna love it. This strategy involves watching companies and waiting until their market value gets above a certain threshold and buying them. On average, I’m buying them when they’re up 75% relative to the market in the last year and trading at twice the market multiple. Some of these go on to achieve great success, some don’t. And our sell discipline is very simple: when the market cap falls below a certain threshold, we’re gonna sell them, and we’ll sell them at, on average, half the market multiple, at a loss of about 7,000 basis points relative to the market. What do you think?
Barry Ritholtz: Hard pass. Hard pass.
Rob Arnott: What I’ve just described is the active side of indexing. I’ve got a monograph coming out shortly — CFA Institute Research Foundation — called The Active Side of Indexing. Indexing is described as passive, but if it has 5% turnover, the 95% is passive — it moves up and down with the market movements and it’s blissfully ignorant and indifferent to what’s going on in the economy or the companies or whatever it is. Really passive. The 5% looks like a hypergrowth manager on crystal meth. The weighting is also an issue. Why? If I came to you and said, I’ve got a brilliant idea, I’m gonna weight stocks proportional to their price — so the more expensive they are, the bigger its weight in your portfolio — don’t you just love it?
Barry Ritholtz: So let’s dive into that a little bit. Anybody who’s an indexer watches in horror every time something gets added to the index, and then there’s this grace period where the stock runs up and it’s even more expensive when it gets added. It’s even worse when there’s a deletion — they announce a deletion and the stocks plummet. Anticipating, front-running the sell — is this just a hidden cost?
Rob Arnott: It’s legal front-running.
Barry Ritholtz: I mean, if you’re gonna tell me — hey, we have $2 trillion in this index, we’re gonna sell this position in a month — why would you hold onto that?
Rob Arnott: Exactly. The S&P’s a beautiful example. The S&P is now big enough that the stocks held in S&P index funds represent roughly 25% of the total market cap of every stock that’s in the index — not each individual ETF or index fund, but aggregated. And that means that, to the extent that indexers are obsessed with having no tracking error with matching the index, they’re gonna buy that stock at the same price that it’s added to the index, which means a market-on-close price. I will pay whatever the price is at the close on the day that it’s added to the index. Now, if you’re a hedge fund, you’re gonna wanna accommodate that and help out by buying it early and then flipping it to the indexers. And that’s been going on for a quarter century or more. I documented the pattern back in 1986 in an article called “S&P Additions and Deletions: A Market Anomaly.” And I heard anecdotally that that article was used in part to lobby S&P to pre-announce so that the index funds wouldn’t get nailed by the index changes. Now they gotta buy this and sell that, and they’re buying it higher and selling this lower, and so they have an automatic drag. The magnitude of that drag is actually very simple. If you could transact at the price at which S&P announced the decision, not the price at which it becomes effective, you would add 15 basis points per annum. So the indexes lose 15 basis points just from trading costs, with 5% annual turnover or less — three to 5% annual turnover. That’s equivalent to three to 500 basis points per stock per trade. That’s a heavy trading cost, but it’s because it’s a crowded space. It’s a herd of elephants trying to go through a single revolving door.
Barry Ritholtz: Let’s talk about the flip-flop problem. Every time there’s an addition, something like 28% within a decade get dropped. And similarly, after there’s a deletion, almost half of those deletions rejoin the S&P, right, within a decade. What does this flip-flop do to performance?
Rob Arnott: Well, it does what you would expect. When I did my little thought experiment describing a brilliant strategy, I was actually citing statistics from our flip-flops paper. On average, stocks that are added are added after 75 percentage points of outperformance. If they falter and are kicked back out, they’re removed at a 7,000 basis point loss. Now, if you gain 75 and lose 70, you aren’t back where you started — you’re down 50. And it’s worse than that, because you didn’t participate in the 75, you did participate in the down 70. The deletion flip-flops — stocks that are deleted and re-added — are even more dramatic. They underperform by 3,500 basis points, give or take, in the year before they’re dropped, and then they outperform by 180 percentage points. They roughly triple relative to the market before they’re added back in. So flip-flops are very, very costly, and none of this is disrespect to the index providers. This stuff has not been studied much until we took a deep dive into it. If you don’t know you have a problem, how are you gonna fix it? And the problem is big, but it’s on a very small part of the portfolio. It’s on the active side of indexing — the little sliver of active trading.
Barry Ritholtz: Really, really interesting. So let’s talk about fixing it. You have been discussing for as long as I’ve known you — which is decades — economy-weighting indices rather than cap weighting or price weighting. Define what a fundamental economic weighting of an index is. What goes into that?
Rob Arnott: Let’s suppose you want an index that studiously mirrors the economy instead of studiously mirroring the market. Well, you wouldn’t weight companies by market cap, you wouldn’t choose them based on market cap. Let’s choose them based on how big their business is. Well, how do you define that? How big are its sales? How big are its profits? How big is its net worth? Today we would go a step further and say, net worth adjusted for intangibles. How much does it distribute to shareholders in dividends and buybacks? Four different measures. You could argue endlessly about which is right, or you could simply say, I’m gonna take the average of the four weights. So Nvidia is a decent slug of total profits in the economy, but it’s not seven or 8% — not its market weight. It’s in the 2% range in terms of sales. It’s in the 2% range in terms of dividends or net worth. It rounds to a very, very small number. So you could argue, is it half a percent or 1% or 2%? Average those, and you’re gonna say it’s about one, one-and-a-half percent of the economy. Okay, that’s big enough to make the cut. We’re gonna include it, and we’ll include it at a one, one-and-a-half percent weight. Now, if you do that, what you’re doing is taking the frothy growth stocks — beloved and expected to grow fabulously — and down-weighting them to their current economic footprint. You’re taking the value stocks — the unloved, out-of-favor, cheap stocks — and you’re saying, let’s reweight those up to their economic footprint. So you wind up with a stark value tilt. And that means the sensible way to measure RAFI, the fundamental index, is to measure it against the value indexes. And that’s where it gets really interesting. Schwab and Invesco have ETFs and mutual funds, PIMCO has some ETFs tied to RAFI — the fundamental index — and collectively those three organizations have over a hundred billion dollars in RAFI assets. So this is not new, it’s not small. We introduced the idea about 20 years ago. If you compare it with the cap weighted value indexes, you get an astonishing result. On average, RAFI beats the cap weighted value indexes by two to 2.5% per year compounded, and does so with variability.
Barry Ritholtz: How does that tracking error compare to the cap weighted growth indexes?
Rob Arnott: The growth indexes have outperformed hugely, but they’ve outperformed by dint of becoming more and more expensive relative to fundamentals. The underlying fundamentals of the value indexes — in terms of sales, profits, book value, dividends — have grown roughly pari passu with growth portfolios this century to date, which shocks most people, because the relative performance has been about two to 3% per annum for a quarter century. And the notion that, wow, this has beat this now by — call it something on the order of two to one, 10,000 basis points of outperformance — but the underlying fundamentals have grown in parallel.
Barry Ritholtz: Let me re-ask that question in a different way, which is: if we know there’s a disadvantage to cap weighted indexes, well isn’t the obvious and simple alternative just equal weight? Why not just go equal weight?
Rob Arnott: Equal weighting is a perfectly legitimate way to create a portfolio. It’s gonna have a stark small cap tilt, because a tiny company will get the same weight as Nvidia or ExxonMobil. It will have a stark value bias, because companies that are trading at low multiples will get the same weight as stocks trading at high multiples. It will have a rebalancing alpha. If a stock soars, you’re gonna trim it. If it tumbles, you’re gonna top it up. The only Achilles heel that I think matters for equal weighting is: equal weighting what stocks? Equal weighting the S&P, for instance — you’re going to be equal weighting a portfolio that includes companies that have soared into being big enough to be added. You’re gonna be leaving out companies that have performed badly enough to be really cheap, and the result is that you’re going to have a portfolio that’s biased towards higher multiple stocks. So, interestingly, equal weighting over long periods of time performs about the same as fundamental index, which we launched 20 years ago — but with much more variability.
Barry Ritholtz: Got it. That makes a lot of sense. So if we’re looking at a fundamental-driven index in a period where mega caps are dominating or growth is dominating, how do you ride that out? Up until last year, it felt like if you weren’t overweight the Mag Seven, you were underperforming — until we learned, last year, five of the seven Mag Seven underperformed in 2025.
Rob Arnott: Yeah, yeah. Shocking. The thing that I find interesting here is, we introduced fundamental index in 2005 — live strategies at PIMCO go back to mid-2005, at Invesco go back to late 2005. So it’s live, it’s been investable for 20 years. The thing that’s interesting is, just two years later, in 2007, value crested, and it underperformed ferociously until summer of 2020. Since then it’s been bottom-bouncing — outperforming handily, then crashing, outperforming, then crashing, bottom-bouncing. And so at the end of 2025, value had underperformed — Russell Value had underperformed the Russell 1000 peak-to-trough by 3,800 basis points. Wow. You were 38% poorer than a simple Russell or S&P index investor. That’s a horrific headwind for anything with a value tilt. RAFI Fundamental Index has a rebalancing alpha: a stock soars and its fundamentals don’t validate that, then you’re gonna say, thanks for the nice high price, I’m gonna trim it. If it tanks and the fundamentals don’t falter, you’re gonna say, thanks for the bargain, I’m gonna top it up.
Barry Ritholtz: So let’s talk a little more about that rebalancing strategy. What sort of alpha does that create? How does that drive returns?
Rob Arnott: The best way to measure the performance of RAFI is against the cap weighted value indexes. Relative to the value indexes — this is live — the RAFI indexes have beat the cap weighted value indexes by a little over 2% per year compounded. Now, with compounding, that’s a big number. That means that you’re over 50% richer than you were with a cap weighted value index after 20 years. So that’s important. Now the other thing that’s interesting is, relative to the value indexes, the tracking error is pretty tight. It’s about two-and-a-half percent variability in that 2% value add, which means that RAFI has beat cap weighted value in most years when value’s been winning and in most years when value’s been losing. It doesn’t matter. RAFI has been winning about three out of every four years. And this is live. This is not a back test.
Barry Ritholtz: So, to wrap up: investors who are concerned about market concentration, concerned about valuation, but a little skittish on the underperformance that value has created in a cap weighted format, should consider a fundamental index. It trades differently than both growth and value, and has a better risk profile and a better valuation profile. I’m Barry Ritholtz. You are listening to Bloomberg’s At the Money.
The post At the Money: Looking Beyond Market Cap Weighted Indexes appeared first on The Big Picture.
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