True or False?
There are many myths about dogs, and this one falls into that category.
It may be true that happy dogs wag their tails, but aggressive dogs often wag their tails too.
There are several physical body motions and cues that help dogs communicate their intent. A wagging tail can mean either agitation or excitement. A dog that wags his tail slowly and moves his entire rear end or crouches down in the classic "play bow" position is usually demonstrating a friendly wag.
Tails that are wagged when held higher, tails that "twitch" or a wagging tail held over the back may be associated with aggression.