To find out, Dr. Vallortigara and his colleagues recruited 30 family pets of mixed breed that were enrolled in an agility training program. The dogs were placed in a cage equipped with cameras that precisely tracked the angles of their tail wags. Then they were shown four stimuli through a slat in the front of the cage: their owner; an unfamiliar human; a cat; and an unfamiliar, dominant dog.

In each instance the test dog saw a person or animal for one minute, rested for 90 seconds and saw another view. Testing lasted 25 days with 10 sessions per day.

Image Right Brain, Left Brain The muscles on either side of the tail apparently reflect emotions like fear and love registering in the brain. Credit... Marcello Siniscalchi/University of Trieste

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.

When the dogs looked at an aggressive, unfamiliar dog — a large Belgian shepherd Malinois — their tails all wagged with a bias to the left side of their bodies.

Thus when dogs were attracted to something, including a benign, approachable cat, their tails wagged right, and when they were fearful, their tails went left, Dr. Vallortigara said. It suggests that the muscles in the right side of the tail reflect positive emotions while the muscles in the left side express negative ones.

While some researchers have argued that only humans show brain asymmetry — based on the evolution of language in the left brain — strong left and right biases are showing up in the brains of many so-called simpler creatures, said Lesley Rogers, a neuroscientist who studies brain asymmetry at the University of New England in Armidale, Australia.