Why do cats wag their tails?

I will quote a world expert in answering this question. There are lots of answers on the internet to this question. Lots are guesses from personal observation. Confusion is caused because what does "wagging" mean? A cat's tail is moved in different ways. The Free Dictionary says that to wag means: To move briskly and repeatedly from side to side, to and fro, or up and down...

On page 28 of Dr Desmond Morris's famous book, "Cat Watching" he answers the question, "Why does a cat wag its tail?"

He says that most people believe it is a sign that the cat is angry. He says that this is "only a partial truth".

The answer, he explains, is that the cat is in "a state of conflict". The cats wants to do two things at the same time and one desire stops the other.

A typical example, he says, is when a cat wants to go outside. You let him out. It is pouring rain. He wants to go out but does not want to get wet. There is a conflict in his mind. His tail wags. Any situation that causes this emotional conflict will lead to tail wagging. Another typical instance might be when a cat wishes to catch a bird on a lawn. There is no cover on an open lawn. He needs cover to approach without being seen. This causes mental conflict and his tail will wag, swish from side to side.

Dr. Desmond Morris explains how this tail movement evolved. It started under a physical rather than a mental situation. Cats use their tails for balance. The small tree dwelling wildcats such as the margay and the larger clouded leopard have thick long tails. They are used for balance. The cheetah too uses its tail for balance when chasing prey.

When a cat is off balance he or she will move the tail out to help regain balance. This tail movement over time evolved into a form of useful body language reflecting mental balance rather than physical balance. The movement speeded up and it became a sign of emotional conflict rather than the need to physically rebalance.

Cats still use the tail for balancing.