The sunk cost fallacy

If you spent $150 on a ticket for a concert to hear your favourite singer perform, but had little interest in hearing the other acts, but then heard that your favourite singer wasn't going to appear, would you still go?

Many people would. Although they know that the one they want to see won't be there, and the other performers aren't worth driving across town to go attend, most people would attend the concert anyway, so their $150 is not "wasted". People will still go to the concert, even if nothing at that concert would make them glad they went. The fact is they paid for it, so they'll attend.

The sunk cost fallacy is a form of "throwing good money after bad", you look at what has been spent so far and can't bear to waste that money, so you press on regardless of how much more needs to be spent. Governments use this logic all the time to keep injecting cash into some giant folly even when it is clear that the amount remaining to be spent is not worth the gain it will bring.

If you had spent $1,000 buying a bed, but the shop was going out of business, and run by unethical shop owners, and these owners had sold the same bed to two customers, would you pay another thousand dollars to secure that purchase, so the money already spent is not wasted?

A little more related to trading, if you found that the thousand dollar course you attended by an expensive seminar guru was just a lead-up to a more expensive course, would you buy the next course, thinking the last purchase was rubbish but you still need to get your money's worth so you'll enroll in the next seminar for the advanced students?

Many traders and investors get so attached to a method of trading, an investment idea, an investment product or guru that they will keep throwing money at it for years. If you bought a very expensive black box trading package, you'll be absolutely determined to make it work, even if it has been a disappointment so far. You can't just put it back in its box and never trade with it again, that would be a waste. You keep attending the training courses provided and studying the finer points of trading the signals, because if you don't, all the money you've already spent would be wasted.

Traders sometimes do the same thing with their own ideas. If you lose a lot of money on a single play, say for example you had an idea that gold was going to go up so you start accumulating gold stocks, you'll seek any excuse you can find to keep playing that trade, because walking away and admitting it was a bad idea would only crystallise your loss.