Kahneman and Tversky described it in their 1979 prospect theory. Estimates of the ratio vary between studies, but a loss is commonly found to weigh about twice as much as an equivalent gain. The effect is an average and varies between people.
In trading it can show as holding a losing position past its stop in the hope of getting back to even, while closing winners early to lock in a gain before it disappears. Shefrin and Statman called this the disposition effect in 1985, and Odean's 1998 paper found evidence of it in the records of individual investors.
The arithmetic consequence is that average losses grow larger than average wins even when the win rate looks healthy. A trader who wins six in ten but takes profits at 0.5R and lets losses run to 1.5R has a negative expectancy while feeling right most of the time.
Defences are procedural. Place the stop when you enter and treat moving it further away as a rule breach that has to be logged. Decide the exit plan for winners in advance. Size positions so that a full loss is small enough that closing it does not hurt.
Measuring the average win and average loss in R, rather than the win rate, shows whether loss aversion is leaking into exits.