Disposition effect

The observed pattern of selling investments that have risen while holding on to those that have fallen.

Example

A portfolio quietly drifts into a collection of its worst holdings, because every one that did well was sold to lock the gain in.

Whether you are currently up or down is a fact about your purchase price, not about the asset - yet it routinely decides which holdings get sold.

Loss aversion — The tendency for a loss of a given size to feel considerably worse than a gain of the same size feels good.
Anchoring — Letting the first number you encounter set the reference point for every judgement that follows, even when that number carries no real information about value.
Sunk cost fallacy — Continuing to commit money, time or attention to a position because of what you have already put in, rather than because of what it is now worth.

Loss aversion — Losing hurts roughly twice as much as the same gain feels good, and that asymmetry pushes investors into two opposite errors depending on which way the position has moved. (11 min)