The Cox-Forbes theory is a theory on the evolution of chess put forward by Captain Hiram Cox and extended by Professor Duncan Forbes.
The theory states that a four-handed dice-chess game was played in India in approximately 3000 BC; due to the results of certain rules or the difficulty in getting enough players the game evolved into a two-handed game. Due to religious and legal objections to gambling the dice were dropped from the game, making it a game purely of skill.
The theory was mostly based on evidence in the Indian text Purana, but more recent study of the work has shown the evidence to be weaker than previously thought. Also the work is now assigned a more conservative date of 500 BC rather than the earlier 3000 BC. As a result the theory is now rejected by most chess historians.