In her book The Secret Language of Dogs, Victoria Stilwell explains how the direction a dog's tail wags are connected to different sides of the brain.
"Studies have shown that dogs wag their tail to the right when greeting people they know and like as unfamiliar people (if they are social dogs), although the tail wags lower," writes Victoria.
"They wag their tails more to the left when seeing an unfamiliar dog."
Additionally, different sides of a dog's brain are associated with particular behaviours.
"Again, the right tail wag is controlled by the left brain hemisphere, which is associated with more confident behaviours, while the left wag is controlled by the right brain hemisphere, associated with more cautious approach behaviours."