Post-purchase rationalization, also known as Buyer’s Stockholm Syndrome, is a cognitive bias whereby someone who has purchased an expensive product or service overlooks any faults or defects in order to justify their purchase. It is a special case of choice-supportive bias.

Expensive purchases often involve a lot of careful research and deliberation, and many consumers will often refuse to admit that their decision was made in poor judgement. Many purchasing decisions are made emotionally, based on factors such as brand-loyalty and advertising, and so are often rationalized retrospectively in an attempt to justify the choice.

For example, a consumer cannot decide between two popular video game consoles, but in the end decides to purchase the one that many of their peers also own. After purchasing it, they may find few games for their console worth purchasing, and more for the console they did not purchase. However, they do not wish to feel they made the wrong decision, and so will attempt to convince themselves, and their peers, that their original choice was the correct one, and the consumer’s opinion is better than everyone’s opinion, i.e. using sour grapes arguments.

This rationalization is based on the Principle of Commitment and the psychological desire to stay consistent to that commitment. Some authorities would also consider this rationalization a manifestation of cognitive dissonance.