We are all familiar with the unsettling statistics on obesity. The World Health Organization projects that within the next three years, one in three people worldwide will be overweight, and one in 10 will be obese. The global obesity pandemic brings with it a host of health concerns, including an increased incidence of type 2 diabetes. The root cause of weight gain is energy imbalance – taking in more calories than you expend. But the societal causes of obesity, and the reasons behind the sudden increase in unhealthy weight gain worldwide, remain a matter of intense debate: are we eating too much, exercising too little, or both? In this report, Herman Pontzer shines a light on the complexities of energy expenditure and reveals some surprising yet crucial issues.