Is based upon the premise that motivation can be a reflection of the extent to which an employee perceives him / herself to be fairly treated. The theory explains why an employee can be content and motivated at work one day and, with no change in pay, terms and conditions, can become discontented and demotivated the next, if they learn that someone in a similar situation is enjoying greater ‘outputs’ in relation to ‘inputs’. It also explains why promoting one person over another can demotivate those not promoted.