When did the middle finger become offensive?

An American television network has apologised after pop star M.I.A. extended her middle finger during Sunday night's Super Bowl halftime show. What does the gesture mean, and when did it become offensive?

A public intellectual, expressing his contempt for a gas bag politician, reaches for a familiar gesture. He extends his middle finger and declares: "This is the great demagogue".

The episode occurred not on a chat show nor in the salons of New York or London, but in Fourth Century BC Athens, when the philosopher Diogenes told a group of visitors exactly what he thought about the orator Demosthenes, according to a later Greek historian.

The middle finger, extended with the other fingers held beneath the thumb, is thus documented to have expressed insult and belittlement for more than two millennia.