Shefrin and Statman (1985) introduced the disposition effect, whereby investors tend to sell stocks with unrealized gains (where the current price has risen above the purchase price) to realize profits while holding stocks with unrealized losses (where the current price has fallen below the purchase price) to avoid realizing losses. Lakonishok and Smidt (1986) demonstrated this phenomenon through the positive correlation between stock prices and trading volume, and Odean (1998) provided empirical evidence by analyzing the relative difference between the proportion of gains realized and the proportion of losses realized using individual investors' stock trading data. This study applies prior research findings on individual investors' stock trading behavior to the Korean stock market to analyze the selling behavior of Korean individual investors and aims to contribute to this field of research by testing the logical basis for the disposition effect, which prior studies have not been able to differentiate. The phenomenon of individual investors exhibiting asymmetric selling behavior depending on whether their stock holdings are in gain or loss positions is a consistent finding across prior studies analyzing actual trading data from various countries. However, the converged research findings on individual investors' asymmetric selling decisions have not identified the decision-making heuristics of individual investors that give rise to this phenomenon. Loss aversion based on prospect theory and investors' bias based on mean reversion—the belief that losing stocks will have higher returns than winning stocks—have been commonly proposed as causes of the asymmetry, but no study has yet analyzed the differentiated explanatory power of these rationales. This study aims to contribute to enhancing the understanding of individual investors' stock trading behavior by identifying the logical underpinnings that can explain individual investors' disposition effect. Additionally, this study seeks to extend prior research on differences in trading behavior according to the characteristics of individual investors, who are an important investment group in the Korean stock market, and on the trading behavior of online investors, whose numbers are rapidly increasing. Analysis of actual trading data from individual investors trading through a mid-sized securities firm revealed that the disposition effect is a general phenomenon in Korea as well. The average holding period for stocks sold at a gain was relatively shorter than that for stocks sold at a loss, and the proportion of gains realized was higher than the proportion of losses realized. The disposition effect was markedly stronger in bear markets characterized by a downward trend, and the disposition effect differentiated by overall market trends is difficult to explain by mean reversion but can be explained by loss aversion. Thus, this study provides differentiated explanatory power regarding loss aversion and mean reversion, which have been commonly proposed as the basis for this asymmetry in prior research. Analysis by individual investor characteristics showed that investors with relatively longer stock holding periods and investors managing large amounts of investment capital exhibited a higher disposition effect, whereas differences in trading medium (online vs. offline) and trading frequency showed no statistically significant relationship with the disposition effect.