Adverse selection

What is adverse selection?

Adverse selection occurs when information differences affect who participates in a transaction, or which products are offered, in a way that leaves the less-informed party with poorer options. The problem is how the information gap changes the mix of buyers, sellers, or goods.

How it works

In Akerlof’s “Market for Lemons” (1970), sellers know more about used-car quality than buyers. If buyers offer a price based on average quality, owners of better cars may withdraw. The resulting quality decline can reduce trade further.

How a guarantee can change the deal

Akerlof discusses guarantees as one response to quality uncertainty. Imagine a used-car seller promising to pay for specified repairs during a warranty period. A buyer no longer bears all the cost if a covered fault appears; the seller has taken on some of that risk. The promise is useful only to the extent that the coverage is meaningful and the seller can be held to it. A credible guarantee can help support a transaction that uncertainty would otherwise prevent. It does not make the two parties equally informed.

Why it matters

Insurance provides a related example: people with privately known higher expected costs may be more willing to buy a given policy. Whether this produces a severe selection problem depends on pricing, information, eligibility, and other market features. These details determine how serious the selection problem becomes.

Moral hazard concerns incentives for behavior when a party does not bear its full consequences. Adverse selection concerns hidden characteristics that affect who joins the deal; moral hazard concerns incentives that affect what people do. Calling one “before” and the other “after” misses that distinction.