Thibaut and Kelly claim that all social behaviour is a series of exchanges where individuals attempt to maximise their rewards and minimise their costs. People exchange their resources with the expectation that they will earn a 'profit'. Rewards received in a relationship can be: being cared for, sex, and companionship. Costs may be: effort, financial investment, and time wasted. The 'economic' theory stresses that commitment to a relationship is dependent on the profitability of the outcome.