Temperament
Thinking, Fast and Slow
Daniel Kahneman's map of the two systems making every financial decision you have ever made.
Thinking, Fast and Slow lays out the two systems running your brain, System 1, fast and automatic, and System 2, slow and effortful, and most investing mistakes happen because System 1 is making a decision that should belong to System 2.
Kahneman is a psychologist, not an investor, and the book barely mentions markets directly. That is exactly why it belongs here. The biases he documents were discovered in a lab, not a trading room, which means they apply to you whether or not you think of yourself as an emotional investor.
Anchoring on the price you paid
The idea from this book I use the most is anchoring. The price you paid becomes a silent reference point your brain keeps checking against, even though the stock has never heard of that number and does not care what you paid for it. Loss aversion, the deeper reason that anchor stings so much, gets its full treatment on its own page here. Nothing in life is as important as you think it is, while you are thinking about it, and that line is not really about investing, which is what makes it useful for investing. The stock consuming your attention right now feels like the most important thing in your portfolio. It usually is not.
Build the rule before you need it
We can be blind to the obvious, and we are also blind to our blindness. The practical takeaway is not to out-think System 1 in the moment, that fight is not winnable in real time. It is to write the rule down while calm, before the moment you will most want to break it.
“Nothing in life is as important as you think it is, while you are thinking about it.”
“A reliable way to make people believe in falsehoods is frequent repetition, because familiarity is not easily distinguished from truth.”
“We can be blind to the obvious, and we are also blind to our blindness.”
“The confidence that individuals have in their beliefs depends mostly on the quality of the story they can tell about what they see, even if they see little.”