IAI Actuarial Core Principles · Economic Modelling
Rational Expectations Theory and the Efficient Markets Hypothesis
Rational expectations theory says people use all available information well, so their forecast errors are unpredictable and average zero. The efficient markets hypothesis applies this to prices: prices reflect available information. Learn the three forms (weak, semi-strong, strong), how each is tested, and what each means for investors and actuaries.
What this chapter covers
This chapter sits in CM2 Economic Modelling. It asks one question in two ways. How do people form expectations, and how much information do market prices already contain? Rational expectations theory answers the first. The efficient markets hypothesis (EMH) answers the second.
The chapter has four parts. First, you learn what rational expectations mean and how they differ from adaptive expectations. Next, you learn the definition of EMH and its three forms, which differ in the information set that prices reflect. Then you study how efficiency is tested and what the evidence says. Finally, you apply the ideas to investing and to actuarial work such as asset allocation, valuation and performance measurement.
It connects to the rest of the paper. Asset valuation, measures of investment risk and option theory all assume something about how prices respond to information. Pricing by no-arbitrage and random-walk models of prices rely on ideas close to efficiency. If you understand this chapter well, those later chapters make more sense, and you can criticise their assumptions in written answers.
This chapter is mostly conceptual, so marks go to clear definitions, correct distinctions and sound reasoning. It suits both short multiple-choice questions, such as which form of efficiency a given test checks, and longer written questions that ask you to explain, test or criticise the hypothesis. Many students lose marks by being vague, so precise wording earns an edge. The ideas also support later CM2 topics, so time spent here is repaid elsewhere in the paper. Exact weightings can change, so check the current IAI syllabus for the topic split.
Rational expectations theory and the efficient markets hypothesis: topics in the order to study them
- 1Rational Expectations TheoryIt is the base idea: expectations use available information efficiently, and you need it before you can read EMH as a statement about prices.
- 2Efficient Markets Hypothesis: Definition and FormsOnce you know rational expectations, you can define EMH and learn what information set each of the three forms covers.
- 3Testing Market Efficiency and Empirical EvidenceYou need the forms clear first, because each test targets a specific form and the evidence is read against it.
- 4Implications for Investors and Actuarial PracticeIt comes last because it pulls the definitions and evidence together into conclusions about strategy, valuation and modelling.
How to prepare Rational expectations theory and the efficient markets hypothesis
Treat this as a reasoning chapter. Aim to explain each idea in your own words, then practise using it on short scenarios.
- Read the rational expectations topic and write a two-line definition. Then write one line on how it differs from adaptive expectations, where people only look at past errors.
- Learn the three forms of EMH as a ladder. Weak: prices reflect past price and trading data. Semi-strong: prices also reflect all public information. Strong: prices also reflect private information.
- For each form, write down what it implies for technical analysis, fundamental analysis and insider information. Do this from memory until it is automatic.
- Study the tests by matching each to a form. Ask what information set the test uses and what result would count as evidence against efficiency.
- Collect the main reasons markets may be inefficient, such as costs, limits to arbitrage and behavioural effects. Be ready to give a balanced view, not a one-sided one.
- Write a short answer on the implications for an investor and for an actuary, for example a pension fund choosing active or passive management.
- Finish with past-style questions. Practise multiple-choice items against the clock, then one written answer in full sentences, checking that every definition is exact.
Common mistakes in Rational expectations theory and the efficient markets hypothesis
Saying efficient markets means prices are always correct or that nobody can ever beat the market.
Fix: State that prices reflect the stated information set and that no abnormal returns can be earned systematically from that information after allowing for risk and costs.
Mixing up the three forms or the information each covers.
Fix: Remember the ladder: past prices, then all public information, then all information. Link each form to the strategy it rules out.
Confusing rational expectations with perfect foresight.
Fix: Say that people can make errors, but the errors are random and not predictable from available information.
Treating a test result as proof for or against EMH without mentioning the joint hypothesis problem.
Fix: In any evaluation, note that efficiency is tested together with a model of expected returns, so conclusions are conditional.
Giving a one-sided answer on implications, such as 'passive is always best'.
Fix: Present the argument, the evidence and the limits, then give a reasoned conclusion that fits the scenario in the question.
Writing long descriptive answers with no structure.
Fix: Use short, labelled points: definition, form, test, implication. Make each point earn a mark.
Last-day revision: Rational expectations theory and the efficient markets hypothesis
- Rational expectations: forecasts use all available information efficiently, so forecast errors are unpredictable and have zero mean on average.
- Adaptive expectations rely on past values only, so errors can be systematic.
- EMH: market prices fully reflect the relevant information set.
- Weak form: prices reflect all past price and trading information, so technical analysis should not earn abnormal returns.
- Semi-strong form: prices reflect all public information, so fundamental analysis should not earn abnormal returns.
- Strong form: prices reflect all information, including private, so even insiders should not earn abnormal returns.
- Each form includes the one before it: strong implies semi-strong, which implies weak.
- Efficiency means prices react quickly to new information, so price changes follow new information and are hard to predict.
- Tests of weak form look at patterns in past returns; event studies look at price reaction to public announcements.
- Testing efficiency is a joint test with an asset pricing model, so a rejection may reflect a faulty model.
- Evidence is mixed: markets look largely efficient in the weak form, with some anomalies found in other forms.
- For investors, efficiency favours low-cost passive strategies; for actuaries, it supports using market prices in valuation.
Rational expectations theory and the efficient markets hypothesis practice questions
- An actuary values a life insurer's equity portfolio on a market-consistent basis. Which assumption about market efficiency best supports usi…
- An actuary values a with-profits fund's equity holdings using market prices, relying on the efficient markets hypothesis. Which consequence …
- Which of the following is a logical consequence of the semi-strong form of the EMH?
- Which finding would provide the strongest evidence against strong-form market efficiency?
- An event study examines the reaction of share prices to surprise dividend increases announced by Indian companies. In a semi-strong efficien…
- A share's price follows a process where price changes are independent of past changes and the expected return equals the required return. Re…
- A fund in India reports annual returns relative to a benchmark over 25 years. Its average alpha is 1.2% a year with a standard error of 0.6%…
- Which of the following findings would be regarded as evidence against strong-form market efficiency?
Rational expectations theory and the efficient markets hypothesis in other exams
The same ground in other exams, if you are preparing for more than one or want another angle on it.
Rational expectations theory and the efficient markets hypothesis: frequently asked questions
What is the difference between rational expectations and the efficient markets hypothesis?
Rational expectations is a statement about how people form forecasts: they use available information efficiently. The efficient markets hypothesis is a statement about prices: they reflect a given information set. EMH is often seen as rational expectations applied to financial markets.
How many forms of EMH do I need to know?
Three: weak, semi-strong and strong. Know the information set for each and the type of investment analysis it rules out. Also remember that each form includes the earlier ones.
Is this chapter more theory or calculation?
It is mainly theory. Expect definitions, comparisons and reasoned explanations rather than long calculations. Practise writing clear, structured answers and handling multiple-choice items that test the distinctions.
Why does this chapter matter for actuarial work?
Actuaries value assets, set investment strategies and model prices. Whether market prices can be trusted as fair information affects all of these. The chapter also helps you challenge assumptions in other CM2 models.