The question of cooperation has fascinated and perplexed philosophers, economists, psychologists, sociologists and biologists from Aristotle to Darwin. It continues to do so today. At the heart of the debate is the relationship between the individual and the group or society to which he or she belongs: why cooperate when we believe our individual interests would be better served by acting unilaterally? This paper takes a broad, multi-disciplinary approach to answering the question 'why cooperate?'.