Attempts to read demographic dynamics using exclusively the tools of economics are bound to fail. The aim here is to construct an internally coherent theoretical model - not as an alternative but rather to complement the Rational Choice approach - for interpreting changing transitional behaviours. In such cases, people have to make decisions in a state of strong or even total uncertainty. We assume that when such a state of uncertainty is present, making a decision requires a relaxed state of mind, which makes it possible to let oneself go when faced with the incalculable risk of a transitional choice. The sole intention to make a transitional choice does not necessarily lead to it actually being implemented, since an underlying crisis mood can make intentions ineffectual. The onset of a crisis mood, in turn, could depend on a criticality that unbearably persists, transforming a circumstantial sense of inadequacy vis-à-vis a specific occasion into a generalized sense of inadequacy with regard to the whole world. Some implications of the model for social policies are briefly drawn in the conclusion.