Capgras delusion is the belief that someone, usually a spouse or close relative, has been replaced by an identical (or near-identical) looking impostor.
It most commonly occurs in the context of schizophrenia, but most of the research on the condition has been done on people who have developed it after brain injury.
It seems to be particularly associated with damage to the right-hemisphere of the brain (as most brain injury-related delusions are), but research suggests it might be particularly linked to the loss of the automatic emotional response to familiar faces.
Current theories suggest that this damage could lead to someone being able to recognise and identify a familiar face, while also having the feeling that something doesn’t feel ‘quite right’, potentially causing a delusional belief that the person is an impostor if there are also impairments with reasoning.
The Echo Maker won the National Book Award in the USA and has made several ‘best of 2006′ lists, but hasn’t been released in the UK yet, so, annoyingly, I’ve not been able to get hold of a copy.
Philip K. Dick’s short story Impostor is widely cited as another fictional example of Capgras delusion (most probably because it got made into a film), although another one of his short stories, The Father Thing, is a much better match.