Picture two screens. The first shows a position up 18% — \$1,800 of unrealized gain on a \$10,000 cost basis. The second shows a position down 28% — a \$2,800 unrealized loss.

Most investors, almost reflexively, sell the winner to 'lock in' the gain and hold the loser hoping it recovers. Both moves feel rational in the moment. Both are usually wrong.

The same wiring that helped our ancestors survive on the savanna systematically destroys investment returns — and it's the most consequential force in your portfolio that has nothing to do with which stocks you pick. Loss aversion is the single most-replicated finding in behavioral finance. Daniel Kahneman and Amos Tversky's 1979 paper 'Prospect Theory: An Analysis of Decision under Risk' established the empirical fact: when people are offered a 50/50 coin flip — heads you win \$100, tails you lose \$100 — almost no one accepts the bet despite its zero expected value.