There's been lots of research indicating that the left and right sides of human brains serve different functions. Turns out that birds, fish, and dog brains also have similar brain asymmetry, especially in the emotional area.
Sandra Blakeslee reports in the April 24 issue of the New York Times that "[w]hen 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."
Blakeslee reports on research published in the March 20 issue of Current Biology, in which Giorgio Vallortigara, a neuroscientist at the University of Trieste in Italy, and two Italian veterinarians, Angelo Quaranta and Marcello Siniscalchi, at the University of Bari, studied the tail wagging response of 30 dogs to known & unknown humans, an unknown cat, and an unknown dog.
"When the dogs saw their owners, their tails all wagged vigorously with a bias to the right side of their bodies, Dr. Vallortigara said. Their tails wagged moderately, again more to the right, when faced with an unfamiliar human. Looking at the cat, a four-year-old male whose owners volunteered him for the experiment, the dogs’ tails again wagged more to the right but in a lower amplitude."
Vallortigara says this "... suggests that the muscles in the right side of the tail reflect positive emotions while the muscles in the left side express negative ones."
Blakeslee cites more examples of how animals use different sides of their brains for important functions -- such as chicks who find food with their left eye and watch for predators with their right eye.
For more information:
A. Quaranta, M. Siniscalchi and G. Vallortigara, "Asymmetric tail-wagging responses by dogs to different emotive stimuli," Current Biology, Volume 17, Issue 6, 20 March 2007, Pages R199-R201. (not even the abstract is free, but if you have Current Biology at your library, try this DOI link.)
If You Want to Know if Spot Loves You So, It’s in His Tail
April 24, 2007
New York Times