It is often thought that a happy dog wags it's tail but according to the scientists, a dog's tail wag can reveal its mood. Scientists have shed more light on how the movements of a dog's tail are linked to its mood. Earlier research had revealed that happy dogs wag their tails more to the right (from the dog's point of view), while nervous dogs have a left-dominated swish. Also dogs pick up whether a dog is going to be friendly or not from these tail wagging signals. So the next time you see your dog or any other dog wagging their tails take note.