What social factors have given rise to the so-called ‘epidemic’ of depression? You will need to show your understanding of the social construction of ‘depression’ as a category of suffering (i.e. changes in formal definitions of depression), and the social, cultural, and economic conditions that may have contributed to increased rates of depression.
Use statistics early in your essay as ‘evidence’ that there is something of an ‘epidemic’ (or you may reject this argument, based on evidence).