This may be true but aggressive dogs often wag their tails too. There are several physical body motions and cues that help dogs to communicate their intent. A wagging tail can mean agitation or excitement. A dog that wags his tail slowly and moves his all rear end or crouches down in the classic "play bow" position is usually a friendly wag. Tails that are wagged when held higher, twitches or wagging while held over the back may be associated with aggression.
