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A contrast effect is the enhancement or diminishment, relative to normal, of a perception and related performance as a result of immediately previous or simultaneous exposure to a stimulus of lesser or greater value in the same dimension. (In this definition a "normal" perception is one free of immediate related context, greater or lesser, more appealing or less appealing.)

Contrast effects are ubiquitous throughout human and non-human animal perception, cognition, and resultant performance. A hefted weight is perceived as heavier than normal when "contrasted" with a lighter weight. It is perceived as lighter than normal when contrasted with a heavier weight. An animal works harder than normal for a given amount of reward when that amount is contrasted with a lesser amount and works less energetically for that given amount when it is contrasted with a greater amount. A person appears more appealing than normal when contrasted with a person of less appeal and less appealing than normal when contrasted with one of greater appeal.