"Dogs evolved with humans, and a number of studies have suggested they are particularly sensitive to human cues," says psychologist William Roberts of Canada's Dalhousie University. Sentimental pet owners might even say their dogs know what they are thinking.The story brings to mind an example raised by a reader in response to an earlier post: Clever Hans, a German horse from a century ago who was famously believed to be able to perform arithmetic. After extensive testing, it was determined that his trainer needn't participate for the horse to perform its trick, ruling out fraud, but that "the horse got the right answer only when the questioner knew what the answer was, and the horse could see the questioner," according to the Wikipedia entry. In other words, subtle cues about human expectations were giving away the answer, but the horse couldn't independently discover it.
My suspicion is that the same thing may also occur with Deputy Pikett's "scent lineups," which have recently generated provably false accusations that resulted in ongoing civil suits. Dogs take no oath of honesty and have no understanding of their role in the legal system. Many sniffer dogs have high error rates. To the dog this is a game - a trick performed to earn a reward. But since Rover can't sit in the witness box and tell us what he smelled, the courts have allowed dog handlers to interpret for them, ignoring the strong possibility that the dog is only telling their trainer what they want to hear.