While virtually any cognitive bias or other psychological pitfall can hurt trading, two of the most common are confirmation bias and the sunk cost fallacy. Confirmation bias is when you give more weight to data that confirms what you already believe while giving less to info that proves you wrong. It’s critical to try and work extra hard to evaluate and take onboard data that disproved your ideas, as your first instinct will usually be to discount it. The sunk cost fallacy is the idea that previous losses or costs should contribute to your decision, making when nothing you can do will get them back, and so you should only make trading decisions based on their effect on the future. For example, if you’ve invested a lot in a business that turns out to have a bad business model, you should act as if you were looking at it as a fresh investment without regard for money you’ve lost previously.