The scientific principle which states that if two opposing theories each explain the same phenomenon and are equally tenable, the simpler of the two is preferred. Applying this principle in psychology is nowhere near as easy as it might be in the physical sciences. For one thing, the evaluation of a theory as ‘tenable’ is a subjective judgement (and therefore subject to human bias) and also the assessment of a theory as simple or complex may be misleading in itself. To illustrate, behaviourism , with its emphasis on observable behaviour, is often valued as being a parsimonious explanation of all manner of things (such as gender role behaviour and mental disorders), whereas Freudian psychoanalysis, with its instinctual drives and repressed sexuality, is criticised for being a non-parsimonious of those same areas of functioning. To accept a theory because it is ‘simpler’ may miss the possibility that the more complex theory might have greater validity .