Context effect
The same price, letter, or face reads differently depending on what surrounds it.
What it means
A context effect is any change in how a stimulus or option is perceived, judged, or chosen that comes from the surrounding information rather than from the thing itself. The mechanism is that the mind interprets rather than records—perception and valuation are relative and inferential, so the surrounding field serves both as evidence about what an ambiguous input means and as the yardstick against which magnitude is scaled. The family is broad: it covers top-down effects in perception, framing and question order in judgment, and the decoy, compromise, and contrast effects in choice. Context effects are usually adaptive, because the surroundings genuinely are informative, but they become a lever for manipulation and a threat to measurement whenever the context has been arranged rather than encountered. It matters because it means no item is ever evaluated on its own terms—there is no context-free judgment to appeal to.
Examples
The identical scrawled middle letter is read as an H in THE and as an A in CAT, because the surrounding letters decide what the ambiguous shape must be.
The same jacket at $120 feels like a splurge on a discount rack and a bargain in a designer boutique, because the neighbouring price tags set the scale.
In a questionnaire, a general question about life satisfaction draws different answers depending on which specific questions ran just before it, since the earlier items set the frame respondents answer within.
First described in Umbrella term; Palmer (1975) canonical demonstration.