The disposition effect is a strange anomaly that effects humans and their handling of money and valuables. Coined by Shefrin and Statman in the 1980s, the disposition effect shows that people are more likely to sell stocks that have increased in value while hanging onto stocks that are static or losing value.
The researchers study showed that people dislike losing MORE than they like winning. So they are 'winning' by selling stocks that have increased in value but will hang on to stock that is decreasing in value - they would be 'losing' if they sold at a loss. It is a sort of profit-taking mentality and may be part of what causes gamblers to keep gambling until they have lost everything while pursuing the "big win."