Does a wagging tail always mean a dog is happy?

No. A wagging tail means the dog is aroused — emotionally engaged and ready to act — and that arousal can be friendly, anxious, frustrated or aggressive. Reading a wag as automatic friendliness is one of the more common causes of dog bites.

What matters is the character of the wag and the rest of the body.

Speed and looseness. A broad, loose, whole-body wag — often with the hips swinging — usually accompanies genuine friendliness. A fast, stiff, narrow wag with a tense body is a very different signal and often precedes a warning.

Height. A tail held high and wagging stiffly generally indicates confidence or arousal that may include threat. A low tail wagging quickly often signals appeasement or anxiety. A tucked tail with a small wag usually means fear.

Direction. Research has found an asymmetry: dogs tend to wag more to their right when encountering something they view positively and more to their left when the stimulus is negative. Other dogs appear to read this, responding with more stress to left-biased wagging.

The wag must be read alongside the rest of the animal — ears, weight distribution, whether the mouth is loose or tight, whether the whites of the eyes are showing, and whether the body is stiff or fluid.

The practical advice is not to approach an unfamiliar dog on the basis of a wagging tail, and to ask the owner first regardless. Children are particularly at risk because they are taught that wagging means friendly.

Dogs do not wag at inanimate objects or when alone, which suggests the behaviour is primarily communicative rather than an expression of internal mood.