recurring plots (spoiler)

I have just watched this on DVD, and remarkedon the close similarity of the twist to that of Psycho, and then in an amazing coincidence, watched an old episode of Dalziel and Pascoe on BBC Prime entitled 'Dialogues of the Dead' which had exactly the same plot twist! This time it was Dervla Kirwan who was murdering people due to being possessed by her dead twin brother. Can't these people think of anything original?

And another thing. Why is this? Every time a sad, lonely, emotionally vulnerable detective - good at his job, rubbish at life - scarred and wounded by hard experience, meets an attractive woman in the course of his investigations, and a mutual attracion more or less blossoms (some times they do it- Monroe, some times they don't - Morse) it turns out that she is the murderer - EVERY BLOODY TIME - without fail. Is this a purely British thing? Does it also happen to Americans? Australians? Anyone? It must so easy being a detective on a murder case - just arrest everyone you fancy - job done, best clear up rate in the county.