Perfect competition assumes buyers and sellers know exactly the same things. Real markets are not like that. The person selling you a car knows what that noise is. You do not. That gap in knowledge is enough, on its own, to make a market fail.
📚 What you need to know
Symmetric information means both sides know the same things. It is one of the assumptions behind a perfect free market.
Asymmetric information means one side knows more, and can use that to their advantage.
It distorts prices and quantities, causing over-provision or under-provision.
Adverse selection happens before a deal: hidden information about who you are dealing with.
Moral hazard happens after a deal: hidden behaviour once someone is protected from risk.
Government responses: legislation and regulation, and provision of information.
Private responses: signalling by the informed side, and screening by the uninformed side.
Before or after? That is the whole distinction
Students lose marks here by learning two definitions that sound alike. Do not. Learn the timing instead, and the definitions fall out of it.
Any exam example can be placed on this line. Ask when the hidden thing mattered: at the point of agreeing, or afterwards.
Adverse selection
The classic case is insurance. You know your own health, your driving record and how carefully you live. The insurer does not. So people who expect to claim are the keenest to buy cover, and the insurer’s customers are not a random sample of the population — they are the riskiest slice of it.
The insurer responds the only way it can: raise the premium. But a higher premium is worst value for the low-risk people, so they drop out first. That leaves an even riskier pool, so the premium rises again. Left alone, the market can shrink until barely anyone is insured.
The loop matters. Adverse selection is not a one-off mistake — it feeds on itself until the good end of the market has gone.
Moral hazard
Now the deal is done and somebody is protected from the consequences of their own choices. Behaviour changes, and not for the better.
A fully insured driver takes slightly more risk. A bank that expects a government rescue lends more aggressively than it otherwise would. Nobody is committing fraud. They are just responding to the fact that somebody else now carries the downside.
The one-line test. If the hidden thing is a fact about who someone is, it is adverse selection. If it is a change in what they do once protected, it is moral hazard.
Both problems come from the same root: one side cannot see something. Every response in this topic is an attempt to make the invisible visible, or to give the informed side a reason to be honest.
Responses
What governments do
Legislation and regulation. Rules forcing disclosure — health warnings on packets, ingredient labels, rules on what a lender must tell you, penalties for misleading claims.
Provision of information. The state supplies or funds the information itself — nutritional labelling, public safety ratings, official inspection schemes.
What firms and consumers do without being told
Signalling. The side that knows more proves it, at a cost. A warranty, a full service history, an independent inspection certificate, a qualification.
Screening. The side that knows less digs for the information. Reading reviews, running a vehicle history check, insurers asking medical questions or using an excess.
A signal only works if it is expensive to fake. Anyone can say their car is fine. Only someone with a genuinely good car can afford to offer a three-year warranty, because a bad car would cost them a fortune in repairs.
Response
Strengths
Weaknesses
Legislation and regulation
Forces disclosure and punishes lying, which protects everyone
Costly to enforce, and firms find ways around the wording
Provision of information
Helps consumers make genuinely better choices
Costs money, and badly presented information gets ignored
Signalling
Reduces the information gap at no cost to the taxpayer
The signal costs the firm money, and only works if it is honest
Screening
Puts the buyer back in control of the decision
Takes time and money, and reviews can be faked
Worked examples
WORKED EXAMPLE
The insurance spiral
An insurer covers 10,000 people. 20% are high risk with an expected claim of $4,000; 80% are low risk with an expected claim of $500. Calculate the average premium, then explain what happens if all the low-risk people leave. [5]
Step 1: Expected cost per person(0.20 × 4,000) + (0.80 × 500) = 800 + 400Average premium = $1,200Step 2: How this looks to a low-risk person$1,200 charged against an expected claim of $500
They are paying more than double what their own risk is worth, so they cancel.
Step 3: The pool afterwards
Only high-risk people remain, so the premium must rise to their expected claim.
New premium = $4,000the market has shrunk to the riskiest customers: adverse selection at work
WORKED EXAMPLE
Is a warranty worth offering?
A dealer sells 5,000 cars a year. Offering a three-year warranty costs $80 per car but lets the dealer charge $200 more, because buyers trust the cars. Evaluate the signal. [4]
Step 1: Gain per car200 − 80 = $120Step 2: Gain across the year120 × 5,000 = 600,000Extra profit = $600,000 a yearStep 3: Why the signal works
A dealer selling poor cars would face far more than $80 of repairs per car, so they could not copy it.
a signal is only credible when the dishonest side cannot afford to fake it
WORKED EXAMPLE
Cutting moral hazard with an excess
An insurer has 20,000 policies. With no excess, 30% of policyholders claim each year. Adding a $500 excess cuts that to 18%. The average claim is $2,000. Calculate the saving. [4]
Step 1: Claims before and afterbefore: 0.30 × 20,000 = 6,000 claimsafter: 0.18 × 20,000 = 3,600 claimsStep 2: Fall in claims6,000 − 3,600 = 2,400Step 3: Value of the saving2,400 × 2,000 = 4,800,000Saving = $4.8 million a yearthe excess makes the customer share the risk, so behaviour changes back
💡 Exam tip
Define asymmetric information as one party having more information than the other in a transaction. Do not just say “unequal information”.
Anchor adverse selection to before the deal and moral hazard to after. Say the words “before” and “after” explicitly.
Link it back to market failure: prices and quantities are distorted, so resources are misallocated.
Separate the responses clearly into government and private. That structure alone earns marks.
Insurance and used cars are the two examples that fit almost any question. Learn one of each well.
Evaluation: information provision only works if people actually read and understand it.
⚠ Common mix-up
Swapping the two terms. Adverse selection is about hidden facts; moral hazard is about hidden behaviour afterwards.
Confusing signalling and screening. Signalling is done by the side that knows more. Screening by the side that knows less.
Assuming asymmetric information always means over-provision. Hidden dangers cause over-provision; hidden benefits cause under-provision.
Treating it as fraud. It is a structural feature of the market, not a crime.
Thinking more information always fixes it. Information that is confusing or ignored changes nothing.
Forgetting that signalling is costly. Warranties and certificates cost real money, which is exactly why they are believable.
Up next: Topic 3 — Macroeconomics, beginning with how we measure the size of a whole economy. Everything you have learned about one market now scales up to all of them at once.
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