Michael Shermer in Scientific American:
The next time you come face to face with a dog wagging its tail, you can make a quick determination on whether to reach out and pet it or step back in deference: check the tail-wag bias. If the wagging tail leans to the dog’s right, you’re safe; if the tail leans to the dog’s left, don’t move.
This tail-wagging bias was documented in a 2007 article in the journal Current Biology by Italian neuroscientist Giorgio Vallortigara and his veterinarian colleagues at the University of Bari. In an experiment, 30 mixed-breed dogs were each placed in a cage equipped with cameras that measured the asymmetrical bias (left or right) of tail wagging while the pooches were exposed to four stimuli: their owner, an unfamiliar human, a cat and an unfamiliar dominant dog. Owners elicited a strong right bias in tail wagging, and unfamiliar humans and the cat triggered a slight right bias. But the unfamiliar dominant dog (a large Belgian Shepherd Malinois) elicited a strong left bias in tail wagging. Why?
According to the researchers, because the left brain controls the right side of the body, and vice versa, the nerve signals cross the midline of the body and cause the dog’s tail to wag more to the right when its left brain is experiencing a positive emotion.