New studies show that your dog's tail wag tells a lot: a wag to the right of its body shows positive emotions; to the left negative.

Imagine if your dog could talk. What does he really think of your new boyfriend? How badly does he want to go nose to nose with the overaggressive dog at the park whose owner assures you just loves everybody?

Most dog owner will look at their dog’s posture for some clue, and consider a wagging tail one of the best indicators. But which way is it wagging?

Dog’s tails fall in the mid-line of their bodies, neither to the right or the left. Now researchers in Italy have discovered that when dogs feel fundamentally positive about something or someone, their tails wag to the right. When they have negative feelings, their tail wagging tends left.

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, recruited 30 family dogs of mixed breeds that were enrolled in an agility training program. The dogs were placed in cages with cameras angled to precisely track the direction of their tail wags. Then they showed the each dog four stimuli through a slat in the cages: the dog’s owner, an unfamiliar human; a cat; and an unfamiliar, dominant dog.

When the dogs saw their owners, their tails all wagged vigorously toward the right side. Their tails wagged moderately and more to the right, when looking at unfamiliar human. When faced with a cat, the dogs’ tails again wagged more to the right but in smaller sweep.
When viewing an aggressive, unfamiliar dog – a large Belgian shepherd Malinois – the dogs tails all wagged to the left side of their bodies.

“This is an intriguing observation,” said Richard J. Davidson, a neuroscience researcher at the University of Wisconsin in Madison.  Not surprisingly, the findings play up some of the fundamental differences that have been shown between the left and right brain in humans — research that has been replicated in other animals.

In most animals, including fish and birds, the left brain deals with behaviors scientist term approach and energy enrichment. In humans, that means the left brain is associated with positive feelings, like love, or a feeling of safety and calm.  The opposite occurs in the right brain with behaviors involving withdrawal and energy expenditure. In humans, these behaviors are associated with fear or depression.

The study which appears in the March 20 issue of Current Biology, suggests that the muscles in the right side of the tail reflect positive emotions while the muscles in the left side express negative ones.