The Theory of Individual Behavior'

'The Theory of Individual Behavior', is devoted to the use of indifference analysis to build a framework for demand theory. Indifference analysis goes to great lengths to describe consumer behavior and how consumers make choices between options available to them. The options they face include budget constraints, different preferences for risk, and a multitude of products and services from which to choose. If one steps back and looks at consumer behavior, it seems to be based upon a premise that is not actually stated in our book. That premise is that consumers behave rationally in their consumption expenditures.
Do you believe consumers do behave rationally? Is so, how do you describe impulse buying, buying as a result of advertising or strong sales pitches, buying for purposes of conspicuous consumption, or buying to simply have something bigger and better than someone else? If consumers do not behave rationally, how can this theory account for the above behavior? Is the theory still valid? If you think consumers do behave rationally, how can rational thinking describe the above behaviors? How do your thoughts impact, if at all, your opinion of the theory?