We have seen what happens when the hypothesis attacks technical analysis. We have seen what happens when it attacks fundamental analysis. Now it is time to meet the argument itself — all three versions of it, laid out side by side.
There is a pattern you may have noticed in this series.
In Part 2, we introduced something called the Weak Form of the Efficient Market Hypothesis — the idea that all information contained in past prices is already reflected in today’s price. That’s what makes chart patterns useless.
In Part 3, we introduced the Semi-Strong Form — the idea that all publicly available information is already reflected in prices. That’s what makes fundamental analysis so difficult to do better than a room full of professional analysts who have already read the same filings you’re reading.
Both posts used those forms to make a specific argument. But we never stopped to look at all three forms together — to lay them out, compare them, and ask: which of these versions does the evidence actually support?
That’s what this post does.
Three Claims, Each Stronger Than the Last
Eugene Fama published his landmark paper on efficient markets in 1970. In it, he did something that proved extremely useful: he divided the hypothesis into three separate claims, each one bolder than the one before it.
Think of them as three nested circles — or, if you prefer, three Russian dolls. The Weak Form is inside the Semi-Strong Form, which is inside the Strong Form. Each larger form contains everything in the smaller one, and then adds more.
Here they are, from weakest to strongest.
The Weak Form
The claim: Stock prices already reflect all information contained in their own past trading history — price movements, trading volume, patterns, trends.
What it rules out: Technical analysis. If everything a price chart could tell you is already in today’s price, then studying that chart cannot give you an edge. The chart is a picture of history that the market has already processed and acted on.
The analogy: Imagine a racetrack that immediately updates its odds every time anyone places a bet. By the time you walk up to the window, the odds already reflect everything everyone has been willing to bet. Your fresh look at the horses’ recent race history isn’t going to tell you something the odds don’t already reflect — because everyone else’s fresh look has already been built in.
The evidence: The Weak Form has the strongest empirical support of the three. Decades of research — going back to Maurice Kendall’s 1953 study of British stock prices, and confirmed in markets around the world — consistently finds that past price movements do not reliably predict future price movements once trading costs are accounted for. The patterns that occasionally appear in backtests tend to dissolve once they become widely known and traded. We covered this in detail in Part 2.
Verdict: Well-supported.
The Semi-Strong Form
The claim: Stock prices already reflect all publicly available information — not just price history, but everything the public can know. Earnings reports. Revenue trends. Management commentary. Industry research. News coverage. Analyst opinions. All of it.
What it rules out: Outperforming the market using standard fundamental analysis. If everything that’s publicly known is already in the price, then building a financial model from public data cannot tell you something the market has missed. The market has already built that model — a hundred times over, by professionals with larger budgets than yours.
The analogy: Imagine you’re trying to find a great restaurant in a city where every food critic in the country has already reviewed every restaurant, their reviews are posted online, and every other diner has already read them. The hidden gem is no longer hidden. The market for reservations has already adjusted to reflect everything that is publicly known.
The evidence: The Semi-Strong Form is well-supported by the mutual fund data we discussed in Part 3. Study after study — including the SPIVA scorecard published by S&P, which tracks active managers against their benchmarks — finds that the majority of professional active fund managers underperform their relevant market index over ten-year periods, after fees. If professional analysts with deep research budgets and proprietary data tools cannot consistently extract edge from public information, the Semi-Strong Form seems to be doing a lot of work.
There are caveats, which we’ll explore in the next post. Smaller companies with less analyst coverage may be less efficiently priced. Patient, long-horizon investors may be able to exploit the short-termism built into how institutional money is managed. And qualitative judgment — assessing management quality or competitive durability in ways a financial model can’t fully capture — may still carry some signal.
A Real-World Illustration: What Zero Analyst Coverage Actually Looks Like
To make this concrete, here is what semi-strong form inefficiency looks like when it actually occurs.
Consensus Cloud Solutions — ticker symbol CCSI — is a small company that provides digital fax and secure document delivery services, primarily to healthcare providers. It had zero Wall Street analysts covering it. Not sparse coverage. Zero. No earnings models, no price targets, no quarterly preview notes, no buy or sell ratings. The information gap was absolute.
The financial statements were public. CCSI files quarterly reports with the SEC like every other public company. A determined investor could read them. The information existed. But when no analysts are actively processing public information and trading on their conclusions, prices can drift far from intrinsic value — the efficient mechanism breaks down not because of secrecy, but because of inattention.
The result: over roughly three months in 2025, CCSI’s stock rose approximately 54%. That is a large move in a short period. And here is the striking detail: even after that run, the estimated margin of safety — the gap between the company’s estimated intrinsic value and its market price — was still approximately 66%. The price had moved substantially toward intrinsic value, but intrinsic value was so far above where the stock had been trading that the market still hadn’t fully closed the gap.
This is offered as evidence about market structure, not as a recommendation to buy. The point is this: the semi-strong form predicts that prices will reflect all publicly available information. In a company with no analyst coverage, that mechanism is not functioning. When it eventually does function — when the information is finally processed, or when some catalyst draws attention to what the filings already showed — the price correction can be large. That is semi-strong form inefficiency caught in the act, produced entirely by public information that was simply not being noticed.
But the core claim holds: the bar for outperforming the market with publicly available information is much higher than most individual investors assume.
Verdict: Well-supported, with documented exceptions that suggest the edges are narrow and require genuine skill to exploit.
The Strong Form
The claim: Stock prices already reflect all information — including information that is not yet public. Private knowledge. Internal corporate data. What management knows about next quarter’s earnings before the announcement. What a scientist knows about a drug trial result before it is published.
What it rules out: Even insiders — people with access to non-public, material information about a company — cannot consistently beat the market on a risk-adjusted basis.
Why it’s controversial: This is the form of the hypothesis that most people — including most economists — believe goes too far.
Here is the simplest evidence against the Strong Form: insider trading laws exist. If insiders had no informational edge over the market, there would be nothing to prohibit. The laws exist precisely because insiders do have an edge — they know things the market does not, and that knowledge moves prices when they act on it.
The empirical record on this is not ambiguous. Studies of insider trading activity — using the legally required disclosures that corporate insiders must file when they buy or sell their own company’s stock — show that insiders do outperform. Executives who buy their own company’s stock do better, on average, than random. Not because they break the law, but because their disclosed purchases are a signal that they believe the stock is undervalued, and they have the best possible view of what the business is actually worth.
Beyond corporate insiders, there are other categories of traders with genuinely non-public information: specialists on exchange floors (in an earlier era), traders at funds that have developed proprietary research tools or datasets, and in some documented cases, individuals who obtained information through illegal channels. These examples all point in the same direction: when someone has information the market doesn’t have, they can and do profit from it.
Verdict: The Strong Form is mostly rejected by the evidence. Markets are not so efficient that insider knowledge provides no advantage. The existence of securities law, and the performance record of those who have traded on private information, confirms this.
The Nesting Structure — Why It Matters
These three forms are not independent claims. They build on each other in a specific way.
If the Strong Form is true, then the Semi-Strong Form must also be true — because if prices reflect all information, they certainly reflect all public information. And if the Semi-Strong Form is true, then the Weak Form must also be true — because if prices reflect all public information, they certainly reflect information about past prices.
In other words:
The three forms are nested: Weak Form is contained within Semi-Strong Form, which is contained within Strong Form.
Each inner form is a weaker version of the outer one. The Weak Form makes the smallest claim. The Strong Form makes the largest.
This nesting matters because it means the evidence doesn’t point in a single direction. We can believe the Weak Form is true (prices reflect past prices) and the Semi-Strong Form is mostly true (prices mostly reflect public information) without believing the Strong Form (prices reflect insider knowledge). That seems to be approximately where the academic consensus has settled.
Where the Consensus Stands
After more than fifty years of research, here is roughly where things stand:
Weak Form — largely confirmed. Price history does not reliably predict future price movements in a way that can be exploited after trading costs.
Semi-Strong Form — largely confirmed, with caveats. Public information is rapidly absorbed into prices, and most active managers do not consistently outperform. But genuine edges may exist in specific circumstances: less-followed securities, longer time horizons, or qualitative judgment that goes beyond what financial models capture.
Strong Form — largely rejected. Insiders have genuine information advantages. Markets do not price in non-public information before it becomes public — which is why it remains profitable (and illegal) to trade on it.
This framework is not the end of the conversation. There are documented anomalies — persistent patterns in market returns that don’t fit neatly into the efficient market picture. We’ll cover those in the next post.
What This Means for You
If you take this framework seriously — and you should — a few things follow.
Abandon the chart. The Weak Form is the most well-supported of the three. If you have been using technical analysis to make investment decisions, the evidence is unambiguous: stop.
Raise the bar for stock picking. The Semi-Strong Form says you need more than public information — you need to use public information in a way that the market hasn’t already priced. That means you need a genuine analytical edge, not just more effort. Reading the same 10-K that a hundred professionals have already read will not, by itself, give you an advantage.
Understand what insider buying tells you — without acting illegally. The failure of the Strong Form is a reminder that non-public information does matter. Legal insider purchase disclosures — which are publicly filed and thus semi-strong — can be a useful signal. What insiders do with their own money, when they’re allowed to, is information the market hasn’t always fully priced.
And above all: know which game you’re playing. The evidence suggests that for large, heavily covered companies, the Semi-Strong Form is close to true. For smaller, less-followed companies with patient investors and genuine analytical effort, the edges may be wider. Those are different games. Playing the right one matters.
A Note to Luca and Lili
You’ve now seen the whole map.
Three forms. Each one making a different claim about how much information prices reflect. Two of the three well-supported by evidence. One largely rejected — but for a specific reason that matters: insiders do have edges, which is precisely why securities law exists to constrain them.
What I want you to take from this is not discouragement, but clarity. The efficient market hypothesis is not an argument that investing is pointless. It is an argument about where the edges are and how hard you have to work to find them.
Your grandfather built the system you’ve been reading about — the screening tools, the intrinsic value models, the Lynch scoring — because he believes those edges are real. Not everywhere. Not easily. But real, for investors who are willing to look at the right places and wait long enough for the market to come to its senses.
Understanding these three forms — what they claim, what they don’t claim, and which ones the evidence supports — will help you think clearly about every investment decision you ever make. That clarity is a competitive advantage in itself. Most investors have never thought this carefully about it.
— Papa
The One-Sentence Summary
The Efficient Market Hypothesis comes in three nested forms — Weak (prices reflect past price data), Semi-Strong (prices reflect all public information), and Strong (prices reflect all information including insider knowledge) — and while the first two are well-supported by decades of research, the Strong Form is largely rejected by the evidence, because insiders with access to non-public information do demonstrably outperform, which is why securities law exists to prohibit trading on it.
Next: Efficient Markets, Part 5 — The Case for Index Funds. If prices mostly reflect available information, and most active fund managers can’t consistently beat their benchmark after fees, there’s a logical conclusion sitting at the end of this argument. In Part 5, we follow it to where it leads — and explain why Warren Buffett, the greatest stock-picker alive, told his wife to buy an index fund.
— Jim