Definition
The contrast principle is a perceptual rule from psychophysics: when two items are presented in sequence, the second seems more different from the first than it actually is. A $40 sweater after a $1,000 suit feels cheap; the same sweater after a $5 tee feels expensive. A 60-degree room feels warm after the cold cellar; cold after the hot kitchen.
Cialdini imports the principle into compliance because it is a free amplifier on almost every other lever. Real-estate agents show two ugly houses before the target house. Clothiers sell the suit before the accessories. Car dealers negotiate the car price before introducing options. In every case, the sequence of presentation has been engineered so that the second item benefits from the contrast.
Why it matters
How it works
The brain processes sensory and evaluative information relative to a recent reference point because absolute values are computationally expensive and often less relevant than differences. This is good design for most of life — what matters about the room's temperature isn't an absolute Kelvin reading but whether it's warmer or colder than where you were before.
The pathology emerges when the reference point is chosen by someone else. A $20,000 car after a tour through the $40,000 models seems like a bargain; the same $20,000 car seen first in isolation would seem expensive. The price hasn't moved; the reference has.
The defense is to reset to the absolute value. Ask: "If this were the first and only item I saw, would I want it at this price?" The question forces an evaluation against your real preferences rather than against the staged reference.