Not all wags mean the same thing. Careful analysis reveals an emotional difference between wags to the right and wags to the left. This asymmetry reflects an underlying asymmetry built into the mammalian brain:
When dogs feel fundamentally positive about something or someone, their tails wag more to the right side of their rumps. When they have negative feelings, their tail wagging is biased to the left.
A study describing the phenomenon, “Asymmetric tail-wagging responses by dogs to different emotive stimuli,” appeared in the March 20 issue of Current Biology. The authors are Giorgio Vallortigara, a neuroscientist at the University of Trieste in Italy, and two veterinarians, Angelo Quaranta and Marcello Siniscalchi, at the University of Bari, also in Italy.
“This is an intriguing observation,” said Richard J. Davidson, director of the Laboratory for Affective Neuroscience at the University of Wisconsin in Madison. It fits with a large body of research showing emotional asymmetry in the brain, he said.
Research has shown that in most animals, including birds, fish and frogs, the left brain specializes in behaviors involving what the scientists call approach and energy enrichment. In humans, that means the left brain is associated with positive feelings, like love, a sense of attachment, a feeling of safety and calm. It is also associated with physiological markers, like a slow heart rate.
At a fundamental level, the right brain specializes in behaviors involving withdrawal and energy expenditure. In humans, these behaviors, like fleeing, are associated with feelings like fear and depression. Physiological signals include a rapid heart rate and the shutdown of the digestive system.
Because the left brain controls the right side of the body and the right brain controls the left side of the body, such asymmetries are usually manifest in opposite sides of the body. Thus many birds seek food with their right eye (left brain/nourishment) and watch for predators with their left eye (right brain/danger).
That said, the effect is relatively minor, so it’s probably not worth quantifying the wags of your best friend. The real question is whether or not dogs detect this asymmetry. Are they sensitive to the tail expressions of other dogs?