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We all know that Americans — leading the way for the rest of the developed world — are getting fatter. We hear about the “obesity epidemic” on the TV news, with footage of people depicted from the waist down shuffling around in XXL sweatpants and carrying supersized sodas. The majority of us are overweight, complaining about how our jeans are getting tighter and wondering why, despite all our efforts to diet and go to the gym, the number on the scale keeps edging higher.

For years, the explanation for weight gain was straightforward: it was all about energy balance, or calories-in versus calories-out. This Gluttony and Sloth theory held that obesity simply came from overeating and underexercising, and the only debate was about dieting — whether it was better to join the low-fat or the low-carb camp. Some scientists explored genetic differences associated with fat, but others said genes couldn’t possibly explain the rate at which Americans were gaining weight: “We just aren’t evolving that fast,” one obesity expert noted.

Environmental scientists have long suggested that there were likely external factors at work, but until recently, the traditional obesity-research community rejected such claims. Now it seems that the tide is turning: This month’s issue of Obesity Reviews features an extensive look at the accumulating body of research linking the environment with obesity.

The idea of our surroundings contributing to weight gain is nothing new, of course. But past discussions about the role of the “environment” focused mostly on the fast-food culture that we live in, where highly processed, highly caloric foods are constantly available, eating times are chaotic, kids run around drinking sugar-saturated sodas all day, no one has time to cook, fruits and vegetables are scarce in low-income urban areas, a venti frappuccino has 760 calories, and muffins are the size of melons. Add to that our changing physical environment — the fact that everyone sits in front of computers every day, instead of working out or working on the farm — and the “calories in” excess of the weight equation seems obvious, and obesity over-determined.