Knowing when a dog is happy is easy, but spotting fear is a lot harder, as Michele Wan, a certified applied animal behaviorist, and her colleagues showed in research examining whether people's perceptions of dogs' emotions vary according to experience. In the study, published in PLOS ONE in 2012, volunteers—who were grouped as having little or no experience with dogs, having lived with a dog at some point, or working with dogs for more or less than 10 years—watched short video clips of dogs. They then categorized the dogs' emotional state and noted which body parts tipped them off. Because the videos had no sound, participants had to rely on behavior to label a dog as, say, fearful or happy. These videos were not just any videos. They had been prescreened by dog-behavior experts whose schooling or professional experience had trained them to make science-based assessments of animal behavior.