Define the adaptation-level principle?

A psychological principle important in understanding happiness is the adaptation-level phenomenon. This theory, as described by David G. Myers in his work, The Pursuit of Happiness, describes the human tendency to judge various stimuli and situations relative to those we have previously experience.

Adaptation-Level Phenomenon is the tendency people have to quickly adapt to a new situation, until that situation becomes the norm.
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This theory states that an individual's basis of judgment of a stimulus is based on their prior experiences as well as their recollections of how they perceived similar stimuli in the past.

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