Every time you buy a sandwich, you know instantly whether it was worth the money. But what happens when you purchase a service and never truly know if you got what you paid for? Welcome to the murky economic world of "credence goods."

Unlike a search good (like a banana, where you see the quality before buying) or an experience good (like a haircut, where you judge it afterwards), a credence good leaves the consumer permanently flying blind. Because the product or service requires highly specialized knowledge, the buyer suffers from extreme information asymmetry.

The market failure here is structural: the seller acts as both the diagnostician and the provider. When the person telling you what is broken is the exact same person profiting from the repair, it creates a massive financial incentive for "overtreatment" (selling you fixes you don't need) or overcharging.

The classic UK example is the annual MOT test and car service. You hand over your keys, and an hour later, the mechanic solemnly informs you that your brake pads are dangerously thin and need immediate replacement. Did they actually need replacing? Unless you are a trained mechanic yourself, you will never know. You simply have to take their word for it.

We are also seeing this accelerate in the UK’s wellness and private health sectors. With NHS waiting lists driving a boom in private dentistry, patients are increasingly vulnerable. If a new private dentist suddenly finds three "micro-cavities" that require expensive fillings, the patient cannot interpret the X-ray to verify the claim. Similarly, if you pay a premium for trendy vitamin supplements pushed by a TikTok influencer to "boost immunity," and you avoid the flu all winter—did the pills work, or did you just get lucky?

The problem even extends to the environment. When you pay £3 to "offset the carbon" of your easyJet flight to Malaga, you are buying a macro-level credence good. You have no way to verify if a tree was actually planted or if it survived long enough to capture your emissions.

Credence goods prove that free markets only function efficiently when buyers and sellers share equal information. When we lack that information, we are forced to rely on reputation, regulation, and second opinions to protect our wallets.