An agreement among two or more firms in the same industry to co-operate in fixing prices and/or carving up the market and restricting the amount of output they produce. It is particularly common when there is an oligopoly. The aim of such collusion is to increase profit by reducing competition. Identifying and breaking up cartels is an important part of the competition policy overseen by antitrust watchdogs in most countries, although proving the existence of a cartel is rarely easy, as firms are usually not so careless as to put agreements to collude on paper. The desire to form cartels is strong. As Adam Smith put it, “People of the same trade seldom meet together, even for merriment and diversion, but the conversation ends in a conspiracy against the public or in some contrivance to raise prices. ”