Why the strong form EMH is the hardest version to believe

The efficient market hypothesis has three levels, and the strong form is the strictest. It claims that even secret information is already in prices. This page explains what that means and why most experts doubt it.

Quick answer

Strong form EMH says that share prices reflect all information, both public and private. That includes facts known only to company insiders. If it were true, not even directors or senior managers could earn higher returns than the market by trading on what they know. Most research and the very existence of insider trading laws suggest it does not fully hold.

Key takeaways

• It covers public and private information.
• It says even insiders cannot beat the market regularly.
• It is the strictest of the three EMH forms.
• Insider trading cases suggest it does not fully hold.
• SEBI insider trading rules exist because private information has value.

What is strong form EMH?

This is the most extreme level of the efficient market hypothesis. It says that every kind of information, including facts known only to insiders, is already part of the share price.

Under this view, a company director who knows about a large order before it is announced would gain nothing by trading on it. The market would somehow already reflect that private fact.

How is it different from the other forms?

Each level contains the one before it. If this version held, the semi-strong form and the weak form would also hold. The reverse is not true, which is why the strongest version is the easiest to challenge.

Why do most experts doubt it?

Studies of trades by company insiders have often found that these trades, on average, did better than the market. This suggests private information can give an edge, which goes against the theory.

Insider trading cases are also clear evidence. When people are caught trading ahead of price-sensitive news, it shows that secret information can be turned into gains before the public knows.

How do insider trading rules fit in?

Because private information has value, regulators ban trading on it. In India, the SEBI (Prohibition of Insider Trading) Regulations, 2015 stop insiders from trading while they hold unpublished price sensitive information, known as UPSI.

Companies must also close the trading window for insiders around key events like results. These rules aim to keep the market fair for ordinary investors, which matters only because this version does not hold.

Is the strong form useful at all?

Yes, as a reference point. It helps explain what a 'perfectly' efficient market would look like. By comparing real markets to it, researchers can see where information gaps exist and why rules on disclosure matter.

In short, this version of EMH is a useful idea in theory, but real markets do not meet it. The value of private information is exactly why insider trading is a serious offence.

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