The Bocca della Verità (Mouth of Truth) is a large marble carving. Its origin is unknown, as is the reason why and when exactly it started being considered what we would call a 'lie detector' today. Persons who put their hand into the open mouth of the 'Bocca della Verità' and swearing a false oath were said to lose their hand because the mouth would snap shut and bite it off.
The plaque can be visited in the entrance hall of the church 'Santa Maria in Cosmedin', on the Piazza Bocca Della Verità in Rome, Italy.
Nowadays, the name 'Bocca della Verità' is a synonym for the marble mask, but this wasn't the case during the Middle Ages, when this term was commonly used for public letter boxes where people could, anonymously, leave hints about profane or sinful behaviour of their fellow men1.
Theories about the purpose and origins of the Mouth of Truth abound:
Some say the marble mask served as a place to confirm oaths as far back as 800 BC.
Other sources say that it served as a Roman drain cover in the ancient world, formerly located in the near-by Hercules temple, and only came into use as an instrument of truth-telling in the Middle Ages.
Some believe it was a fountain, claiming that the two holes at the sides of the medallion were used to keep it in an upright position.
Other people say as it is lacking any signs of erosion around the five holes for eyes, nostrils and mouth, the theories of the stone mask having served as a drain cover or a fountain/spouter can't be upheld.
It is thought that the face represents a deity; the following gods have all been considered as possibilities:
Mercury, the Roman god of thieves and merchants. Not far from the place where the Mouth of Truth is located, there is a spring dedicated to Mercury; but people didn't come here to tell the truth. The water of this spring was believed to bestow forgiveness to the thieves and merchants for the lies which they had told, so a connection of Mercury to the Mouth of Truth is highly unlikely.
Faunus, the Roman equivalent of the Greek god Pan, is being considered by some because of the horns on the mask and the testicles in the beard.
Jupiter, the Roman equivalent of the Greek god Zeus; he was a deity for various issues. There was a temple of Jupiter Jurarius (the god of law) not far from where the medallion is now, so people think that the stone mask was maybe once located there, and maybe people once swore oaths in this temple.
It could represent a river god or an ocean god, but again, whether it is Triton, a son of the deity Neptune, or Oceanus (Greek - Okeanus), a titan with horns on his head, is mere speculation.
The 'Bocca della Verità' has been in its current location since 1632. It is a round medallion of about 1.75m in diameter, made of Pavonazzetto marble, 19cm thick and has a weight of about 1,200kg. It has an estimated age of at least 2,200 years. Although Rome can be considered as the world's largest open-air museum with all the archaeological sites that can be visited, the 'Bocca della Verità' is one of the most visited places. It's very popular and modern fortune-teller machines on fun-fairs are often replicas and even have sound effects, although the real Mouth of Truth never wasted time speaking, it just bit.
The original mask plays a significant role in the 1953 film Roman Holiday starring Audrey Hepburn and Gregory Peck. According to Internet Movie Database, Gregory Peck pretended that the mouth actually bit his hand off, retrieving only an empty sleeve, and the horror of Audrey Hepburn is real.