A Social dilemma is a paradox that arises from a social decision situation in which contributions are needed to attain some common and shared goal or end-result, and where the rational choice of the individual is to "free-ride". For instance, if it was possible for you to be a non-paying passenger on a bus ride, that's the "rational" thing for you to do, in so far as it would leave you better off. You would both get the ride and keep the fare money.