The disposition effect is loss-aversion measured in the data. Studies of brokerage accounts show traders realize gains far more readily than losses, which inverts the cut-losses-let-winners-run rule that positive expectancy depends on.
Mechanical exits, such as a trailing-stop on winners and a hard stop-loss on losers, are the standard counter.
Example: across 1,000 trades a trader's average winner is +0.8R and average loser is -1.9R. The setups were fine; the exits were the problem.
Related: loss-aversion, trailing-stop, stop-loss, r-multiple