Abstract: We experimentally investigate the existence of the disposition effect and loss aversion as its potential cause. Our approach includes three key characteristics: (i) An environment closely mimicking actual stock markets; (ii) Individual-specific reference prices; (iii) A direct test of loss aversion as a cause of the disposition effect. We find strong support for the existence of the disposition effect as an independent hypothesis. This is an improvement over previous studies, which tested this hypothesis only jointly with others. Our results also strongly point to loss aversion, of the type postulated by prospect theory, being a source of the disposition effect.