I am going to keep this as simple as I can as I work my way down from organizing concepts to operational details. The social contract in a role playing game is between a game master and a collection of players. The game master agrees to provide rewarding opportunities for personal growth and the players agree to buy into the lusory goals. A few clarifications are in order.
First, this is a social contract, not an economic or legal contract. While there may also be economic benefits or legal restraints, the social contact, if satisfied, results in greater social status. The may be recognition, respect, goodwill, reputation, loyalty, or some other social capital. It is easy to see the distinction between economic and social gains. A game's high or low regard may well be inversely related to its profitability. A social contract differs from a legal contract in that a social contract is enforced in the court of public opinion rather than in a legal venue.
The obligation of the game master to provide rewarding opportunities for personal growth is derived from earlier comments in this blog about why people play. Play is fun because it provides rewarding opportunities for personal growth. So, a role playing game should as well.
The responsibility of the player to buy into the lusory goals is derived from Bernard Suit's definition of a game as a voluntary attempt to overcome unnecessary obstacles. We play games and games are fun because following the lusory goals of the game makes it so. It would not do to have a quarterback pull out a gun and shoot a rushing tackle. It is achieving the goals of the game within the constraints of the game that makes it fun and rewarding.