In economics, insurance, and risk management, adverse selection is a market situation where asymmetric information results in a party taking advantage of undisclosed information to benefit more from a contract or trade.

In an ideal world, buyers should pay a price which reflects their willingness to pay and the value to them of the product or service, and sellers should sell at a price which reflects the quality of their goods and services. However, when one party holds information that the other party does not have, they have the opportunity to damage the other party by maximizing self-utility, concealing relevant information, and perhaps even lying.

This opportunity has secondary effects: the party without the information may take steps to avoid entering into an unfair contract, perhaps by withdrawing from the interaction; a party may ask for higher or lower prices, diminishing the volume of trade in the market; or parties may be deterred from participating in the market, leading to less competition and higher profit margins for participants.

A standard example is the market for used cars with hidden flaws, also known as lemons. George Akerlof in his 1970 paper, "The Market for 'Lemons'", highlights the effect adverse selection has on the used car market, creating an imbalance between the sellers and the buyers that may lead to a market collapse. The paper further describes the effects of adverse selection in insurance as an example of the effect of information asymmetry on markets, a sort of "generalized Gresham's law".

Adverse selection was first described for life insurance. It creates a demand for insurance which is positively correlated with the insured's risk of loss.

For example, overall, non-smokers have a much lower risk of death than smokers of the same age and sex. If the price of insurance does not vary according to smoking status, then it will be more valuable for smokers than for non-smokers. Thus smokers will have a greater incentive to buy insurance and will purchase more insurance than non-smokers. This increases the average mortality rate of the insured pool, causing the insurer to pay more claims. The insurer relies on the premiums of the healthy non-smokers to cover the costs incurred by the smokers.

As more smokers purchase insurance, costs to insure them increases.

In response, the company may increase premiums to correspond to the higher average risk. However, higher prices cause rational non-smokers to cancel their insurance as insurance becomes uneconomic for them, exacerbating the adverse selection problem. Eventually, higher prices will push out all non-smokers in search of better options, and the only people left who will be willing to purchase insurance are smokers. The same applies to health insurance.

When raising capital, some types of securities are more prone to adverse selection than others. An equity offering for a company that reliably generates earnings at a good price will be bought up before an unknown company's offering, leaving the market filled with less desirable offerings that were unwanted by other investors. Assuming that managers have inside information about the firm, outsiders are most prone to adverse selection in equity offers. This is because managers may offer stock when they know the offer price exceeds their private assessments of the company's value.

Outside investors, therefore, require a high rate of return on equity to compensate them for the risk of buying a "lemon".

Adverse selection costs are lower for debt offerings. When debt is offered, this acts as a signal to outside investors that the firm's management believes the current stock price is undervalued, as the firm would otherwise be keen on offering equity.

Thus the required returns on debt and equity are related to perceived adverse selection costs, implying that debt should be cheaper than equity as a source of external capital, forming a "pecking order".

In modern contract theory, "adverse selection" characterizes principal-agent models in which an agent has private information before a contract is written. For example, a worker may know his effort costs (or a buyer may know his willingness-to-pay) before an employer (or a seller) makes a contract offer. In contrast, "moral hazard" characterizes principal-agent models where there is symmetric information at the time of contracting. The agent may become privately informed after the contract is written.

According to Hart and Holmström (1987), moral hazard models are further subdivided into hidden action and hidden information models, depending on whether the agent becomes privately informed due to an unobservable action that he himself chooses or due to a random move by nature. Hence, the difference between an adverse selection model and a hidden information (sometimes called hidden knowledge) model is simply the timing. In the former case, the agent is informed at the outset. In the latter case, he becomes privately informed after the contract has been signed.

In most adverse selection models, it is assumed that the agent's private information is "soft" (i.e., the information cannot be certified). Yet, there are also some adverse selection models with "hard" information (i.e., the agent may have evidence to prove that claims he makes about his type are true).

Adverse selection models can be further categorized into models with private values and models with interdependent or common values. In models with private values, the agent's type has a direct influence on his own preferences. For example, he has knowledge over his effort costs or his willingness-to-pay. Alternatively, models with interdependent or common values occur when the agent's type has a direct influence on the principal's preferences. For instance, the agent may be a seller who privately knows the quality of a car.

When banks and borrowers come together to determine the personal loans, mortgages or business loans, adverse selection is deeply rooted in the discussions.

For example, when a new customer approaches a bank seeking a personal loan, they will always know their spending, saving and potential income better than the bank would. This creates adverse selection as the customer possess information about their life which is unknown to the bank, and they can take an economic advantage due to this information.

Similarly, when a business requests a loan from a bank, this also creates adverse selection. The business possesses information about market trends, insider business knowledge, and other future happenings relevant to the business that a bank would not know when lending money to a company.