Biases in thinking
Cognitive dissonance
The discomfort of holding two beliefs that clash, or acting against what you believe, which people relieve by changing a belief or explaining it away.
When behaviour and belief disagree, people tend to adjust the belief. Someone who paid a lot for a product looks for reasons it was worth it.
It explains why small commitments lead to bigger ones, and why people defend choices they've made. A gentle, small step against an old belief can loosen it where an argument can't.
In practice
Customers who choose a more expensive plan often rate it more highly afterwards, partly to justify the choice.
Common mistake
Using it to trap people, by getting a small commitment and then using their discomfort to push them into a larger one they don't want.
Where it comes from
Leon Festinger set out the theory in A Theory of Cognitive Dissonance (1957). His 1959 experiment with James Carlsmith found people paid $1 to praise a dull task came to like it more than those paid $20.