Sunk Cost Fallacy
Past investments shouldn't determine future decisions.
Explanation
This describes our tendency to continue investing in something because we've already put time, money, or effort into it—even when it's clearly not working. Our brains hate feeling like we've 'wasted' previous investments, so we keep throwing more resources at failing projects. But money already spent is gone no matter what you do next.
Real-World Example
Watched 2 hours of bad movie? Those 2 hours are gone whether you watch the last 30 minutes or not. Spent $100k on failing project? That money is gone—don't spend another $100k just to 'not waste' the first. Dated wrong person for 3 years? Those years are gone—don't waste a 4th.
How to Apply
Ask: 'If I were starting fresh today, would I make this choice?' Ignore all past investments. Pretend you inherited this situation. Would you continue? Set 'kill criteria' in advance: 'If we don't have 100 users by month 3, we stop.' Pre-commitment prevents sunk cost thinking.