I've actually desisted from talking about the social genesis of obesity, but perhaps now it's as good a time as any other to discuss it.
In many developed countries today, the obesity epidemic is associated with the poor and underclass – blacks and white trailer trash in the US, Maoris in NZ, Abos in Aus, Malays in Singapore, etc. Several factors are being put forward by sociologists and economists:
1. Lack of time for exercise. Most poor people are hourly rated and work long hours to make ends meet. Many hold down 2 or more jobs. Many run single-parent households. Ergo, no time for exercise.
2. Cheap fast food. The poor do not have time to cook healthy meals owing to long work and commuting schedules, so most consume fast food on the move. Moreover, it is actually cheaper to buy fast food than buy organic groceries and cook your own food in many Western countries. (A plate of chicken rice costs less than a balanced home-cooked meal in Singapore too.) But fast food is cheat food: it's processed, high in calories, low in nutrition and contains carcinogens as well as harmful substances like trans fats.
3. Lack of awareness. Many fat people are under-educated and semi-literate, as you'd expect those in the lowest social stratum to be. They don't have an inkling about what a balanced diet is, what kind of foods are harmful, how to count calories, what are the consequences of eating junk food. Esoteric terms like anti-oxidants and metabolic syndrome are Greek to them. I know a young Malay woman who buys french fries for her son every day, because she read somewhere that 'potatoes are good for your health'.
So, obesity is not just about poor willpower and discipline. In its broadest context as a social problem and medical epidemic, it is a problem of poverty. As long as the rich-poor gap keeps on widening, and the poor are perpetually caught in the poverty trap, expect the incidence of obesity to rise exponentially even as the rich get richer and society as a whole gets wealthier.