Adding a third option that nobody wants can change what people buy.
Offer a small popcorn for 3 and a large for 7, and most people pick the small. Add a medium for 6.50 and suddenly the large looks like a bargain, so many more people choose it. The medium exists only to be rejected.
Why is this true?
This is the decoy effect. People judge value by comparison rather than in absolute terms, and a deliberately worse option makes its neighbour look better. Subscription tiers and menu design use it constantly.
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More about psychology
Psychology facts describe the shortcuts your mind takes without asking permission. We misjudge randomness, remember interrupted tasks better than finished ones, and change our answers when a group disagrees with us. The entries here stick to findings that have held up under replication, avoid the pop-science versions that overstate them, and explain what the effect really predicts about everyday behaviour.