I've heard a different explanation (from Jared Diamond): it's because of the introduction and proliferation of high calorie, cheap foods. You see this in many 3

I've heard a different explanation (from Jared Diamond): it's because of the introduction and proliferation of high calorie, cheap foods. You see this in many 3rd world countries (e.g. central/south America, many poor towns in the US, etc) that have easy access to type of food.