The current obesity epidemic is largely driven by environmental factors, including nutritional transition towards refined and fatty foods with the growing production of energy-dense food at relatively low cost, increased access to motor vehicles, mechanisation of work and sedentary lifestyles. These influences in modern society are modified by individual characteristics. Ultimately, energy intake in excess of caloric expenditure causes obesity, but why this occurs in some but not all individuals is not known. Obesity is more prevalent in the lower socioeconomic classes but even so, there is a varying relation of socioeconomic status with obesity between countries at different stages of development and, even in the Western world, socioeconomic gradients with respect to obesity are both heterogeneous and in transition. Potential mechanisms for an effect of obesity on subsequent social status have been proposed, the most obvious being related to the stigmatisation experienced by the obese. Obesity seems to be causally related to mood disturbances, whereas there is no conclusive evidence that the reverse is true. When considering psychological aspects of obesity, depressive symptoms are more likely to be consequences, rather than causes of obesity.