You treat a tax refund differently from an identical paycheck

Richard Thaler showed that people mentally sort money into separate 'accounts' based on its source or intended use, even though cash is completely interchangeable. A windfall like a tax refund gets spent more freely than the same amount earned as salary, and money earmarked for 'savings' stays untouched even while its owner carries high-interest debt elsewhere. Rational economics says a dollar is a dollar; mental accounting says otherwise.

— Richard Thaler, Mental Accounting Matters — Journal of Behavioral Decision Making, 1999