Genetic variation has not changed appreciably in the past 50 years and therefore cannot explain the secular increases in average body mass index observed over the past few decades. But changes in the environment (decreased need for physical activity and greater availability of cheap food) mean we are all at increased risk of obesity compared with our parents and grandparents. So why do many people remain slim, while others gain weight?

Genetic variation influences our appetites, metabolism, and tolerance of physical activity. This creates a strong genetic component to variation in body mass index in today’s environment. An analogy can be made with smoking—if everyone inhaled the same amount of cigarette smoke every day, the strongest risk factor for lung cancer would be genetic susceptibility to the adverse effects of cigarette smoke (G Davey Smith, personal communication).