Changeling is supposed to have missed out by two votes on the Golden Palm at last year’s Cannes Film Festival because some of the jury found the story far too incredible even for an American movie. It is easy to see why. A boy disappears from the home of his working-class single mother. After some months the police return a different child. Nevertheless they insist the mother keep the boy. When she demands they look for her real son the officer in charge of the case commits her to an insane asylum. If this were not sufficiently melodramatic, the film…