Losing £100 hurts about twice as much as gaining £100 feels good

Daniel Kahneman and Amos Tversky's 1979 prospect theory found that people weigh losses roughly twice as heavily as equivalent gains — losing £100 takes about £200 of gain to offset psychologically. This 'loss aversion' broke from the standard assumption that a pound gained and a pound lost carry equal weight, and became the empirical backbone of behavioral economics. Kahneman won the 2002 Nobel Prize largely for this work; Tversky had died and was ineligible.

— Daniel Kahneman and Amos Tversky, Prospect Theory: An Analysis of Decision under Risk — Econometrica, 1979