I remember being very surprised as a fresher when a lecturer informed us that Economics was premised on the idea of a rational consumer.
We frequently speak about the collective 'madness ' / irrationality of our house buying behaviour during the Celtic Tiger. However, what's irrational about buying an asset that's appreciating in value and that required none of your own money (100 percent mortgages) - especially, if the vast majority of the population have no formal training in Economics and are unaware of boom / bust cycles?
Selective defaulters are also acting rationally - if in significant negative equity and if little or none of their own money was involved in securing the property, especially in a largely consequence free environment.
It could be argued that those struggling to pay for an over priced 'asset' (liability) are acting irrationally - especially in areas outside of the main urban areas where rent is affordable.