IN THE early 20th century, a horse named Clever Hans was believed capable of counting and other impressive mental tasks. After years of great performance, psychologists put the ruse to rest by demonstrating that though Hans was certainly clever, he was not clever in the way that everyone expected. The horse was cunningly picking up on tiny, unintentional bodily and facial cues given out not only by his trainer, but also by the audience. Aware of the “Clever Hans” effect, Lisa Lit at the University of California, Davis, and her colleagues, wondered whether the beliefs of professional dog handlers might...