It's most likely a confluence of those, and other factors. My default inclination is to look to systemic effects first, though. So, in this case, whatever the s

It's most likely a confluence of those, and other factors. My default inclination is to look to systemic effects first, though. So, in this case, whatever the specific balance between the confounding factors may be, my bet is that it's weighted towards "food deserts" creating the unhealthy habits, rather than being their consequence. People don't generally have a lot of choice but to eat what's available to them, and the overwhelming majority of what's available in these places is packaged and processed beyond recognition. I grew up on the Great Plains, in the largest conurbation for hundreds of miles in any direction. I was literally surrounded by farm and ranchland. That area is classified as a food desert — a phenomenon I'm shockingly confronted with every time I fly home to visit the folks. Based on the conversations I've had with folks living there about this stuff, they (broadly) aren't even aware there are alternatives, or those alternatives are outright mocked. (Imagine growing up as a vegetarian when you have multiple relatives who ranch...)