Tail Wag Bias

Giorgio Vallortigara, a neuroscientist at the University of Trieste in Italy, and two veterinarians, Angelo Quaranta and Marcello Siniscalchi, at the University of Bari published a paper       describing this phenomenon in the journal Current Biology. The researchers recruited 30 family pets of mixed breed and placed them in a cage equipped with cameras that precisely tracked the angles of their tail wags. Then they were shown four stimuli in the front of the cage: their owner; an unfamiliar human; a cat; and an unfamiliar, dominant dog.

When the dogs saw their owners, their tails all wagged vigorously with a bias to the right side of their bodies, while an unfamiliar human caused their tails to wag moderately to the right. Looking at the cat, the dogs' tails again wagged more to the right but more slowly and with restrained movements.  However the sight of an aggressive, unfamiliar dog caused their tails to wag with a bias to the left side of their bodies.

It is important to understand that we are talking about the dog's left or right viewed from the rear as if you are facing in the direction the dog is viewing. That means that if you are facing the dog and drew an imaginary line down the middle of his back that positive right-sided signal would appear as tail swings mostly curving to your left.