Veblen's theory of conspicuous consumption undermines rational consumer choice theory in several ways. It argues that consumers' preferences are dependent on price, with more expensive goods providing more utility due to their social status. This means demand would slope upward rather than downward. Galbraith also criticizes the idea that consumers have sovereign choice, arguing that producers use advertising to shape demand. Both theories show that consumer preferences are not stable or independent of external factors like price and marketing, weakening the rational choice model.