Why do dogs wag their tails?

Tail wagging is a communication signal, and reading it as simple happiness gets people bitten. Dogs wag when excited, when greeting, when uncertain, when anxious, and sometimes immediately before aggression. The rest of the body and the specifics of the wag carry the meaning. A loose, sweeping wag with relaxed body posture indicates friendliness. A high, stiff, fast wag with a tense body and direct stare is arousal that may not be friendly. A low or tucked tail with small wags indicates fear or appeasement. Speed, height, and stiffness together are the message.

Italian researchers found a lateral bias that has held up in follow-up work: dogs wag more to the right side when experiencing something positive, such as seeing their owner, and more to the left when confronting something they perceive as threatening, such as an unfamiliar dominant dog. Other dogs appear to read the difference, responding with more stress to a left-biased wag. The asymmetry is thought to reflect the different roles of the two brain hemispheres in processing emotion, a pattern found across many vertebrates.

Two practical points. Puppies do not wag in the first weeks of life and begin around the time they start interacting socially, which supports the reading that the behavior is primarily for communicating with others rather than an involuntary expression, and dogs wag far less when alone. And a wagging dog you do not know is not an invitation: let the dog approach, watch the whole body, and treat a stiff high wag with a hard stare as a reason to give space.