Sunk Costs are those costs (in money, time, effort, whatever) that have already been paid and cannot be recovered through any means. The sunk cost principle is a somewhat counter-intuitive principle that says that your decisions should ignore sunk costs and only consider prospective costs. It's counter-intuitive because most people have a psychological investment in sunk costs. If you're three years into a college program that you hate and you're considering whether to drop out or not, most people think "but I've already invested THREE YEARS...I can't stop now!" In reality, you have to forget those three years - you can't get them back. Another example would be when you've written a post on the Internet that's really a bad idea. Maybe you are posting about how you found porno pics of a relative, or an embarrassing post about an ex-girlfriend who reads TiB or something. And you get to the end and you're thinking about clicking the submit button, and you're thinking "wait, this is a bad idea." But then, in ignorance of the sunk cost principle, you think "but I just spent all this time crafting the perfect post about this. I can't just throw it away now." So you post it anyway. It's always a bad idea. FOCUS: What things have you gotten so invested in that you couldn't back out regardless of the future costs and benefits? How did you rationalize staying invested? How did you get out, or did you?