SINISTER v dexterous. Commie v Tory. The difference between left and right carries more meaning to human beings than mere matters of handedness and symmetry. And so it is with man’s best friend as well. For in dogs, too, left and right signal different things. Specifically, it is in the way they wag their tails. And for dogs, like people, it is the left-hand side that is sinister.

The story started a few years ago when Giorgio Vallortigara of the University of Trento, in Italy, and his colleagues, established that dogs wag their tails to the right when they see something pleasant, such as a beloved human master, and to the left when they see something unpleasant, such as an unfamiliar dominant dog. What Dr Vallortigara did not establish then was whether such signals are meaningful to other dogs. Now, he and the team from the previous study have done just that.

As they report in Current Biology, they wired up several dozen dogs of both sexes and various breeds with electrodes, to record the animals’ heart rates, and then showed them videos of dogs, or silhouettes of dogs, head-on, with tail wagging to left or right. A left-wagging tail, they found, induced a higher maximum heart rate (in other words, an anxiety response) than a right-wagging tail, and this maximum heart rate lasted longer. A right-wagging tail, indeed, produced the same results as one that was stationary.

Dr Vallortigara and his colleagues also observed their animals during the experiment, noting behaviours such as ear-flattening, head-lowering and whining that are associated with stress. They found that stressed behaviours were more common in the presence of left-wagging than right-wagging.

All this suggests lateral specialisation in dogs’ brains. The nervous signals for left-wagging and right-wagging originate in different hemispheres. (Because of the peculiar way vertebrate brains are wired up, it is the left hemisphere that controls right-wagging, and vice versa—as is the case for handedness in people.) That they are triggered by different emotions shows that the two halves of a dog brain work, in this respect at least, differently.

Human brains are similarly lateralised. Handedness is one example. Another is language, a function predominantly of the left hemisphere. Whether it is just a coincidence that dogs and people agree about which side is sinister, or whether there is something deeper going on, remains to be determined.