Why Information Can Affect Trade
A buyer may value an item more than its seller does, or a client may value a service more than it costs a provider to supply. If both parties know the relevant facts and can agree on a transfer, they can share the gains from trade. But a buyer may not know the quality of an item, and a client may not be able to observe the care a provider takes after signing an agreement. These are different information problems, and they can interfere with exchange in different ways.
The key distinction is when the information concerns. A hidden type is a characteristic that exists before the parties contract but is privately known. A hidden action is a choice made after an agreement that one party cannot observe or verify. These problems do not automatically prevent trade: the effect depends on what the parties can observe, what they can commit to, and whether the proposed terms still make both sides willing to participate.
This distinction is aligned with the SOA VEE Economics (Validation by Educational Experience) syllabus. It is useful to ask three questions: What is private? When does it exist or occur? Can the other party condition the agreement on it? The earlier material on market failure provides the broader context: an outcome can be inefficient when a barrier prevents mutually beneficial exchange.
Hidden Types: Private Characteristics Before Agreement
Suppose a seller knows the quality of an item but a potential buyer cannot distinguish qualities before making an offer. The buyer may have to base an offer on the distribution of possible types rather than on the actual item’s quality. As a result, a price acceptable for an average item may not be attractive to a seller with a high-quality item. If the buyer instead offers enough to attract that seller, the price may be too high relative to the buyer’s expected value across the possible items.
The basic benchmark is gains from trade. If a buyer’s value for an item is \(V\) and the seller’s reservation value is \(R\), the gains from trade when their information is known are \(V-R\). A positive difference means there is a range of prices acceptable to both: any price \(p\) satisfying \(R\leq p\leq V\). Hidden information can make it difficult to set a price that works for the relevant type, even when the underlying gains are positive.
Worked Example: An Unobserved Equipment Type
A used-equipment buyer considers purchasing one machine from a seller who knows its type. With equal probability, it is a basic machine or a precision machine. The buyer values the basic machine at $300 and the precision machine at $1,200. The seller’s reservation values are $100 and $900, respectively. The buyer cannot inspect the type before making a single take-it-or-leave-it price offer, and sellers accept any offer at least as large as their reservation value. Find the gains from trade by type and assess whether a uniform offer can produce a precision-machine sale without giving the buyer negative expected value across the two types.
Method. First compare buyer value and seller reservation value for each type. Then test whether one price can both attract the precision seller and give the buyer nonnegative expected value if either type accepts.
For a basic machine, the gains from trade are \(300-100=\$200\). For a precision machine, they are \(1{,}200-900=\$300\). With equal probabilities, expected gains from trade if both types could be identified and traded are:
A price below $900 cannot attract the precision-machine seller, whose reservation value is $900. At a price of $900, both types accept. The buyer’s expected value across the equally likely types is:
At a $900 offer, the buyer’s expected net value is therefore \(750-900=-\$150\). A higher offer makes that expected net value still lower. Thus, under the stated single-offer rule, there is no offer that attracts the precision seller while leaving the buyer with nonnegative expected value across both possible types. The precision machine would create $300 of gains from trade if its type were known, but that trade is blocked by the information constraint. This does not imply that no basic-machine trade is possible: a price between $100 and $300 can be acceptable to a basic-machine seller and leave the buyer with nonnegative value for that type.
Check. The $900 offer does not make the precision seller worse off, since it equals that seller’s reservation value. The obstruction is that the buyer cannot identify the precision machine and loses in expectation if both types accept. A common mistake is to compare $1,200 with $900 and conclude that the precision-machine trade must occur. That comparison establishes positive gains for a known precision type; it does not establish that a feasible offer under hidden type is profitable for the buyer.
Hidden Actions: Private Choices After Agreement
With a hidden action, the parties may agree before the relevant choice is made. The problem is then whether the party choosing the action has a reason to take the action the other party expects. If a payment does not depend on an unobservable choice, the chooser may prefer a less costly action, even if that action reduces the total value of the transaction. A client who anticipates that choice may decline to contract.
To evaluate the basic issue, compare the parties’ expected values under the actions they can actually enforce. If an action \(a\) produces a benefit \(B(a)\) and uses resources costing \(C(a)\), total surplus is \(B(a)-C(a)\). A positive surplus under a desired action is not enough by itself: a contract must also leave the parties willing to participate given the action that can be observed or verified.
Worked Example: A Service Provider’s Unobservable Effort
A hypothetical client considers a one-time technical inspection. The provider incurs an unavoidable resource cost of $450 if the job is undertaken. The provider can also choose careful effort, which costs an additional $300, or minimal effort, which costs nothing extra. Careful effort gives the client an expected benefit of $900; minimal effort gives an expected benefit of $400. Effort cannot be observed or verified, and the proposed contract pays a fixed fee whether effort is careful or minimal. The provider accepts a fee only if it covers the provider’s costs, and the client accepts only if expected benefit is at least the fee. Determine whether trade is feasible when effort is verifiable and when it is hidden.
Method. First identify the action the provider would choose under a fixed fee. Then compare the fee the provider requires with the maximum fee the client will pay under that action.
If careful effort is verifiable and contractually required, the provider’s total cost is \(450+300=\$750\). The client’s expected benefit is $900, so a fee of $750 covers the provider’s cost and leaves the client with expected net value \(900-750=\$150\). A fee of $750 therefore permits trade.
If effort is hidden and the fee is fixed, the provider receives the same payment under either action. Minimal effort costs $0 extra, while careful effort costs $300 extra. The provider therefore chooses minimal effort. Given that choice, the client’s expected benefit is $400, so the client will pay at most $400. But the provider needs at least $450 to cover the unavoidable resource cost. The required fee conditions would be:
No fee satisfies both conditions. Thus, the hidden action blocks a transaction that would have been mutually beneficial if careful effort could be verified and required. A common mistake is to use the $900 benefit when evaluating the hidden-action contract. That benefit applies only if careful effort is supplied; under the stated fixed-fee arrangement, the provider chooses minimal effort, so the relevant benefit is $400.
Diagnosing the Information Problem
A careful diagnosis prevents confusing hidden types and hidden actions. Start by fixing the time of the agreement. If the private fact is already true at that point, it is a type. If the private choice is made afterward, it is an action. Then identify the party who knows or chooses the relevant information and ask whether the other party can verify it in time to make the agreement depend on it.
A pre-existing quality or characteristic indicates hidden type. A later effort or care decision indicates hidden action.
Name the informed party and the party whose decision depends on the information. Avoid saying only that “the market” lacks information.
Information may be private without affecting trade if it can be credibly disclosed, measured, or made contractible under the situation’s assumptions.
Use the type the buyer expects to face or the action the chooser actually has an incentive to take. Then compare the parties’ acceptable terms.
For example, a buyer’s uncertainty about an item’s condition before purchase concerns type. A buyer’s inability to monitor how carefully a repair is completed after hiring a repairer concerns action. The object of uncertainty may be related to the same transaction, but the timing changes the economic problem.
Worked Example: Separating a Worker’s Type From an Action
A hypothetical firm is considering a six-month data-cleaning project. An applicant privately knows whether they already have advanced data skills; this characteristic exists before hiring. After hiring, the worker chooses whether to spend additional time checking difficult records, a choice the firm cannot observe. If the worker has advanced skills, the project’s value before additional checking is $8,000; with basic skills, it is $5,000. The firm values the project at $10,000 if records are carefully checked, but at $7,000 if they are not. Identify the type and action, then calculate the project’s gross value above its stated value in each relevant case, before wages or other costs.
Method. Classify each piece of private information by timing, then subtract the project’s stated value from the firm’s value for the relevant combination. The project values are not added together: each pair describes an alternative case.
The worker’s pre-existing skill level is the type. The choice to check difficult records after hiring is the action. If skills are advanced and records are carefully checked, the firm’s value above the stated project value is \(10{,}000-8{,}000=\$2{,}000\). If skills are advanced but records are not carefully checked, the difference is \(7{,}000-8{,}000=-\$1{,}000\). If skills are basic and records are carefully checked, the difference is \(10{,}000-5{,}000=\$5{,}000\); if skills are basic and they are not carefully checked, it is \(7{,}000-5{,}000=\$2{,}000\).
These differences are not total gains from trade because the worker’s compensation and other costs have not been specified. They show why the firm needs to distinguish the pre-existing skill information from the later checking decision. A common mistake is to call both facts “hidden quality.” Skill is a characteristic the worker has before the agreement; checking is a choice the worker makes afterward.
When Hidden Information Does—and Does Not—Block Trade
Information asymmetry is a possible source of market failure, not a proof that failure must occur. Trade can continue if the parties can observe the relevant fact, verify it, agree on terms that remain acceptable despite uncertainty, or otherwise make the transaction worthwhile. Conversely, a transaction can fail even when the underlying surplus is positive if no feasible terms address the information constraint.
| Question | Hidden type | Hidden action |
|---|---|---|
| When is the relevant information present? | Before agreement | Chosen after agreement |
| What is private? | A characteristic, such as quality | A decision, such as effort |
| What can go wrong? | A price based on uncertainty may not attract a particular type | The chooser may take an action that makes the agreed transaction less valuable |
| What should be evaluated? | Expected value and participation under the offer | Incentives, actual action, and participation under the contract |
The practical exam discipline is to state the information structure before calculating. Identify who knows what, whether it is a pre-existing type or a later action, and what the other party can condition on. Then calculate gains from trade under that information structure—not under an assumption of perfect information that the scenario does not permit.
Check Your Understanding
For each problem, identify the information issue and show the relevant comparison before checking the answer.
- A seller knows whether a batch of specialty bulbs has a 70% or 95% expected first-year survival rate before sale. The buyer cannot test the batch before offering a price. Is the private information about type or action? (Answer: Hidden type; survival quality is a pre-existing characteristic.)
- A cleaning firm agrees to service an office each week. The firm chooses the time spent cleaning after the contract is signed, and the client cannot observe the time. Is the private information about type or action? (Answer: Hidden action; time spent is a post-agreement choice.)
- A buyer values a known instrument at $720, and the seller’s reservation value is $510. Calculate gains from trade and give the range of mutually acceptable prices. (Answer: Gains are \(720-510=\$210\); any price from $510 through $720 is acceptable to both.)
- A client’s expected benefit is $560 if a provider takes a costly, careful action and $280 if the provider takes a cheaper action. The provider’s unavoidable cost is $320, and careful effort costs an extra $180. A fixed fee does not depend on effort, which cannot be observed. What action does the provider choose, and can a fee satisfy both parties’ participation conditions? (Answer: The provider chooses the cheaper action; the client’s maximum fee is $280 and the provider needs at least $320, so no fee works.)
- A buyer faces a hidden type that is high quality with probability 0.4 and low quality with probability 0.6. Buyer values are $1,000 and $400, respectively. The seller’s reservation value is $700 for high quality and $200 for low quality. Find expected gains from trade if the type were known, then the buyer’s expected value at a uniform offer that attracts both types. (Answer: Expected gains are \(0.4(1{,}000-700)+0.6(400-200)=120+120=\$240\). The buyer’s expected value is \(0.4(1{,}000)+0.6(400)=\$640\); an offer of $700 attracts both types but gives the buyer expected net value \(640-700=-\$60\).)