As somebody the wrong side of 50 who grew up in England, I think the increase in obesity has a lot to do with the variety of available foods and the ease of preparation. I grew up in a relatively affluent household, but the diet was still fairly monotonous and labor intensive. In the dead of winter it was potatoes at most meals and carrots, cabbage, carrots, cabbage, brussel sprouts, carrots - you get the idea. The net result was that we ate to quench hunger but for the most part did not overindulge. Since most of the food had to be made, rather than just taken out of the package, having a snack was not so quick and mindless.

In those prehistoric days, lower caloric intake and more maunual labor probably hid the true genetics of lots of people. Now that we live in the era of ready instant delicious food, genetics rules.