The evolutionary biology of self-deception, laughter, dreaming and depression: some clues from anosognosia.

Patients with right hemisphere strokes sometimes vehemently deny their paralysis. I describe three new experiments that were designed to determine the extent and depth of this denial. Curiously, when asked to perform an action with their paralyzed arm, they often employ a whole arsenal of grossly exaggerated 'Freudian defense mechanisms' to account for their failure (e.g. 'I have arthritis' or 'I don't feel like moving it right now'). To explain this, I propose that, in normal individuals, the left hemisphere ordinarily deals with small, local 'anomalies' or discrepancies by trying to impose consistency in order to preserve the status quo. But when the anomaly exceeds threshold, a 'devil's advocate' in the right hemisphere intervenes and generates a paradigm shift, i.e. it results in the construction of a new model using the same data. A failure of this process in right hemisphere stroke would partially explain anosognosia. Also, our model provides a new theory for the evolutionary origin of self-deception that is different from one proposed by Trivers. And, finally, I use anosognosia as a launching-off point to speculate on a number of other aspects of human nature such as Freudian defense mechanisms, laughter, dreams and the mnemonic functions of the hippocampus.