Scientists at the University of Trento, in Italy, have found that the way a dog wags its tail shows whether it is happy or nervous.

According to their research, which has been published in the journal Current Biology, dogs that wagged their tails more to the right (from the dog’s point of view) were more likely to be happy, while dogs that wagged their tails more to the left were nervous, which determines how other canines will react to them. Researchers monitored dogs watching films of other dogs and measured their heart rates.

Professor Vallortigara said that dogs that saw a dog moving its tail to the right, stayed calm, but when they saw the tail wagging to the left, their heart rates increased and they became anxious.