Obesity has risen dramatically in the past few decades. However, the relative contribution of energy intake and energy expenditure to rising obesity is not known. Moreover, the extent to which social and economic factors tip the energy balance is not well understood. This exploratory study estimates the relative contribution of increased caloric intake and reduced physical activity to obesity in developed countries using two methods of energy accounting. Results show that rising obesity is primarily the result of consuming more calories. We estimate multivariate regression models and use simulation analysis to explore technological and sociodemographic determinants of this dietary excess. Results indicate that the increase in caloric intake is associated with technological innovations as well as changing sociodemographic factors. This review offers useful insights to future research concerned with the etiology of obesity and suggests that obesity-related policies should focus on encouraging lower caloric intake.