cognitive dissonance
Cognitive dissonance is the mental itch you feel when two things you believe — or your beliefs and your actions — don't line up. Imagine someone who knows smoking is dangerous lighting another cigarette: the gap between "this is bad for me" and "I'm doing it anyway" creates a real, gnawing discomfort. That tension is the dissonance, and the mind hates leaving it unscratched.
To make the itch go away, we tend to change whatever is easiest to change. Sometimes that means changing the behavior, but shifting a belief or adding a comforting excuse is often easier than budging an entrenched habit. So the smoker may not quit; instead they reason, "My grandfather smoked and lived to ninety," or "It helps me handle stress." We're brilliant at inventing reasons that let us keep doing what we were already doing. This after-the-fact excuse-making is one common route, and it's called rationalizing.
The common misconception is that we form our opinions and then act on them. Often it runs the other way: we act first, then bend our beliefs to fit what we've already done. That's why people who suffer to join a group end up prizing it most, and why a small favor can warm someone toward you — having helped you, they conclude they must like you.
After buying an expensive phone, you suddenly notice all the glowing reviews and tune out the bad ones — quieting the doubt that you overpaid.
Post-purchase rationalization: we defend a choice after making it.
The term was coined by American psychologist Leon Festinger in his 1957 book A Theory of Cognitive Dissonance. His most famous case study, however, came earlier: described in the 1956 book When Prophecy Fails (co-authored with Henry Riecken and Stanley Schachter), it followed a small UFO group that predicted a flood would destroy the world. When the flood didn't come, committed members who were together didn't abandon their belief but reinterpreted it — concluding the group's devotion had persuaded the cosmos to spare Earth — and began proselytizing more energetically. Notably, members who faced the failed prophecy alone were more likely to lose faith.