The scientific method: used by physicists and chemists to make great progress in determining the laws of the physical sciences. Psychologists adopted the scientific method for behavioural science. Diffusion of responsibility a psychological state in which each person feels decreased personal responsibility for intervening. Life is lived forwards, but understood backwards. Understand behaviour in our everyday life is hindsight reasoning. Problem with relying solely on hindsight reasoning is that related past events can be explained in many creative, reasonable, and sometimes contradictory ways. Understand the causes of a given behaviour. Then easier to predict the conditions under which the behaviour will occur in the future. Theory development is the strongest test of scientific understanding. A good theory: incorporates existing facts and observations within a single broad framework, it is testable. Accuracy can be evaluated by gathering new evidence: predictions made by the theory are supported by the findings of new research, conforms to the law of parsimony.