Are Stock Markets Efficient or Not?

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Is the Stock Market Efficient or Not? Investing Question Answered asked, “Are markets efficient or not?”

After spending a few minutes thinking about how I would tackle this widely researched topic, I decided to do what any researcher would do, I went to google and typed in, “Are markets efficient or not?

300 million responses later I am no less daunted. This question is one of the most widely debated investing questions.

Find out if stock markets are efficient or not. Can you beat market returns?

What is the Efficient Market Hypothesis?

The efficient market hypothesis was created by Noble prize winner, Eugene Fama.

According to the efficient market hypothesis is:

“A market theory that evolved from a 1960’s Ph.D. dissertation by Eugene Fama, the efficient market hypothesis states that at any given time and in a liquid market, security prices fully reflect all available information. The EMH exists in various degrees: weak, semi-strong and strong………. This theory contends that since markets are efficient and current prices reflect all information, attempts to outperform the market are essentially a game of chance rather than one of skill.”

The Three Types of Efficient Market

  1. Weak market efficiency
  2. Semi-strong market efficiency
  3. Strong market efficiency

Weak Stock Market Efficiency

The efficient market hypothesis (EMH) is broken down into the “weak form” which states that stock prices reflect all publicly available information.

Semi-Strong Stock Market Efficiency

The “semi strong form” of the EMH includes the weak form and adds that stock prices also adjust rapidly to the release of all new public information. Practically, this means that stock prices adjust so quickly to information that investors can’t profit from technical analysis (analyzing past stock price trends and movements) or even from fundamental analysis.

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Strong Stock Market Efficiency

The third type of the EMH, the “strong form” includes the weak and semi strong and adds on insider information. If the markets were “strong form” efficient, then investors couldn’t profit from securing insider information. We know that markets are not “strong form” efficient because it is so widely accepted that investors can profit from insider information, that it is illegal to trade on insider information.

So we’ve ruled out the possibility that markets are “strong form” efficient, but are they weak or semi-strong form efficient?

In other words, can investors make a profit greater than is expected by the riskiness of the security?

The index fund industry, spearheaded by John Bogle and Vanguard Investments states that investors are well served to invest in a diversified portfolio of index funds and that they will beat active and professional investors most of the time.

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This sounds almost too simple to be possible, and there is loads of research that supports that finding. Yet, not everyone buys into the simple EMH theory, including Nobel prize winner, Robert Shiller.

‘Is the Stock Market Efficient?’ Controversy

If markets were proven to be completely efficient, then investors wouldn’t look to exploit market inefficiencies.

Shiller challenges the EMH with evidence that markets move away from their fair price and create over or undervalued scenarios. Think about the internet bubble of the late 1990’s. Even now, in late 2017, stocks seem to be more highly valued than their historical averages.

Read: What to Do Now That Markets Are Peaking? Should I Sell Stocks Now?

During stock market bubbles, such as in the late 1990’s stock average PE (price earnings) ratios sky rocket driving prices way above fair value as defined by companies’ underlying business models. Shiller was among the forecasters who warned us about overvalued assets during the dot com boom and the more recent housing price bubble of the middle of last decade.

Market anomalies may also cause question about the efficient market hypothesis. These anomalies seem to persist over the long term in financial markets. (Be aware that they may persist over the long term, but in short periods of time there is not consistency to support that the market anomalies always “outperform”)

Do Stock Market Anomalies Disprove the Efficient Market Hypothesis?

What is a stock market anomaly?

 “In financial markets, anomalies refer to situations when a security or group of securities performs contrary to the notion of efficient markets, where security prices are said to reflect all available information at any point in time.”

Stock market anomalies include:

1. Momentum– A stock going up in price usually continues to go up (or down) even past the point of “fair value”.

2. Value Stocks-Lower valued stocks usually appreciate more than over valued stocks.

3. Small Cap Stocks-Small cap stocks usually outperform larger capitalization stocks.

4. The January Effect-Stocks usually go up in January.

But, do these anomalies disprove the efficient market hypothesis or are there logical explanations that explain the incidence of out performance other than the fact the markets are not efficient?

Why Market Anomalies Don’t Disprove the Efficient Market Hypothesis

The Investor’s Business Daily founder, William O’Neil is a big proponent in the momentum approach and it is true that for a period, stocks do seem to continue moving in the same direction. Frequently, investors overreact to a positive or negative news incident causing the company’s stock price to deviates from fair value. The problem with making an outsized gain, above that predicted by markets, is persistency.

Can an investor, over time, beat the EMH by investing in momentum stocks? It is unlikely, because at some point, momentum stops and share price returns to fair value. A successful momentum investor must be correct twice; when to buy in and when to sell.

Small stocks usually grow faster than large stocks. Practically speaking, if a company sells $10 billion per year, in order to grow 10% they would need to sell $100 million more in the next year. That’s a lot of sales.

But, if a company sells $5 million in a year, they only need to increase sales by $500 thousand in order to grow 10%. Earning $500 thousand is much easier than selling $100 million more products or services. Thus, it’s only logical that a small company is going to grow faster than a larger one.

At the end of the the year, many investors sell stocks with losses in order to benefit from the capital loss on their taxes. Thus, in January, they buy back stocks to remain in the markets. Simple explanation for the January effect.

If you want to capitalize on the market anomalies, put some money into a small cap and value ETF.  

 Are Markets Efficient or Not?

I’ve taught the “efficient market hypothesis” in an Investments class at Santa Clara University and Lebanon Valley College. The conclusion is not a simple one.

If investors could make a profit greater than market returns most of the time, that would disprove the efficient market hypothesis.

Can investors outperform markets over the long term without taking outsized risk (or without being overly lucky) and continue to beat the markets year after year?


But, if there are investors beating the markets year in and year out, without taking large risks, they probably aren’t telling. After all, if these extraordinary investors divulge their methodology, then their advantage would disappear. Maybe a few investors such as Warren Buffett or George Soros beat the odds, but not many.

The Efficient Market Investing Takeaway

After decades of investing in the markets, picking stocks, and index funds, I’m convinced that over the long term an index fund approach, in line with one’s risk tolerance is the most effective way to invest. A passive index fund investing strategy is easy to implement and market returns are good.

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It’s unlikely that over the long term you can beat the market returns. 

Stocks have averaged about 9% over the last 80 years or so and bonds about 5%.

If you do want to try to outperform the markets, go with indexing for the majority of your portfolio and invest 10 to 15% of your financial assets with an active strategy.

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1. SEP IRA versus Solo 401(k); Which is Better?

2. Should I Pay off my Mortgage or Invest in the Stock Market?

What is your opinion? Are markets efficient or not? Do you use index funds or actively manage your investments?

Updated; November, 2017


  1. I don’t think all the factors are priced in immediately which means that they are not efficient. I think the market is always lagging, it’s lagging less than before because more information is readily available. Technical analysts will argue that all they need to do is look at a Chart to determine direction, I don’t think that’s the case.

  2. I am a big fan of index funds although I use some sector funds and individual stocks to balance out my portfolio. My sector funds and individual stocks are used for growth and less volatile than other choices.


    January 9, 2014

  3. I’m much honored you picked my question. What an excellent explanation! Thanks for taking time to address this very complex topic, Barb!


    January 9, 2014

  4. @Nick, Actually, a research study which looked at how quickly stock prices reacted to an earnings surprise showed that there was a big jump in price immediately after the earnings announcement, but the stock price continued to move in the same direction for up to three months after the announcement, giving investors some time to get in (or out) on the trend.
    @Krantcents- A bit of both, that’s covering your bets.
    @Moneycone-My pleasure.

    Barbara Friedberg

    January 9, 2014

  5. I studied this when getting an MBA in finance, and think it’s a more challenging question than meets the eye. Your write up is really good.

    Ultimately, for the average investor, your suggestion makes sense to me – in terms index funds. After all, many if not the majority of actively managed funds tend to underperform index funds after factoring in costs.

    Ray @ Squirrelers

    January 9, 2014

  6. Are Stock Markets Efficient? They’re more efficient then I’ll ever be!


    Rick Ferri

    January 9, 2014

  7. @Ray, My portfolio is almost all index funds and I’m selling any remaining individual stocks I hold from years ago!
    @Rick-Me too! 🙂

    Barbara Friedberg

    January 10, 2014

  8. The inefficiencies of the market may allow some people to outperform beyond just dumb luck, but certainly not me. (I thought I could using momentum in the late 90s but got hit pretty hard in 2000.) I am now perfectly happy with market returns minus cost.

    Bryce @ Save and Conquer

    January 11, 2014

  9. I would have to say markets could be much more efficient than they currently are. High frequency trading and insider trading have the effect of draining shareholder equity from the market and serve no other purpose in my book.

    Paul @ The Frugal Toad

    January 11, 2014

  10. 10 – 15 % with an active strategy is very good advice. I see novice investors getting burnt because they invest 100% of their capital in active strategies with high volatility, when things are going well they get incredibly rich and when things go wrong they lose money and panic sell. I have 50% of my money in Fixed Term Deposits, 10% in Dividend Aristocrats UK, 10% in Dividend Aristocrat US and the remainder scattered between Peer to Peer, Index Funds and Property. Great advice, thanks for sharing.

  11. @Bryce-You are not alone. Many (including Nobel prize winners and well known economists)well respected financial professionals support an indexed based approach to investing.
    @Paul-The easy access to information may be a blessing or a curse, depends upon the circumstances. I agree that it’s tough to beat any market inefficiencies. Hopefully, insider trading laws deter!
    @Nick, Sounds like you have a well thought out conservative portfolio. What factors led you to this allocation?

    Barbara Friedberg

    January 13, 2014

  12. Barbara, this topic leads me in loops all day as well, and I agree it is not easy to make a quick determination one way or the other.

    I’ve often read that ideas that sound great on paper don’t lead to beating the market in practice in the long-run.

    I did find one study, however, that showed some investors can do better than average somewhat consistently. It is a paper called “Can Individual Investors Beat the Market?” from the Harvard University School of Finance.

    Bill Nast

    January 27, 2014

  13. Hi Bill,

    If one can beat markets consistently with a reasonable effort without taking on excess returns, I’d certainly like to hear about it. Bill, thanks for weighing in.

    Barbara Friedberg

    January 27, 2014

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