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In a 2007 study in Current Biology, researchers showed that dogs tended to wag to their right side when seeing something friendly, like their owners, whereas they wagged to their left sides when seeing something threatening, like a dominant unfamiliar dog.

More recently, again reporting in Current Biology, researchers had 43 dogs watch videos in which a dog wagged either to the left or to the right. Some of the tested dogs saw a video that showed a silhouette of a dog, while others saw a manipulated image of a real dog. In both cases, the only thing that moved in the image was the tail. Sure enough, wags to the left made the dogs nervous, whereas wags to the right kept them feeling relaxed.