Insecurity (partially) explains obesity, because body fat is an evolutionary adaptation to insecure food supplies. Our parents and grandparents were raised to eat what they were given and clear their plates. That's evolutionarily rational if you aren't sure where your next meal is coming from - food will spoil, but body fat will keep you alive through a famine.

Shaking off that habit is much easier if you never worry about whether you'll go hungry tomorrow; a lot of poor people don't have that luxury. If you're just scraping by, an unexpected bill or a parking ticket might force you to choose between a payday loan or an empty refrigerator. Combine economic insecurity with a food market full of cheap, high-calorie foods and you've got a recipe for nutritional disaster.

We're right to think that capitalism has created tremendous abundance, but we often under-estimate the pathological effects of insecurity. Without a sound safety net, the human brain just isn't capable of prudent long-term planning.