Disposition effect

The tendency to sell winners and hold losers, which is the behavioural consequence of loss aversion applied to a portfolio.

Realising a gain feels like confirmation of skill; realising a loss requires admitting an error, so the loss is deferred in the hope of recovery.

The behaviour is backwards on two counts. It cuts the positions where the thesis is working and keeps the ones where it is not, and in most tax regimes it also realises the wrong gains.

The countermeasure is a written sell rule that references the thesis rather than the price paid.