Why do we shake hands?

Handshakes are very old: an Assyrian relief from the 9th century BC shows King Shalmaneser III clasping hands with a Babylonian ruler to seal an alliance, and Greek grave stones from the 5th century BC show people clasping hands. The best-known theory for why the gesture began is about trust. Most people held a sword or dagger in their right hand, so offering that hand empty showed you came in peace. Some historians suggest the shaking motion was meant to make sure no knife was tucked up the other person's sleeve. Nobody can prove exactly how it started, but by the 1800s the handshake had become an everyday greeting in Europe and America, helped along by Quakers, who preferred it to bowing or doffing a hat because it treated everyone as equal.