Look around you. The way we live explains why we are increasingly polarized

In 2016, I set out to understand why a border wall appealed to so many. I realized Americans are increasingly boxing themselves in - with vast impacts on the way we see the world

The border's like our back door," a concrete salesman named Chris told me in January 2017. You leave it open, and anyone can walk right in." It was the day of Trump's presidential inauguration, and we were chatting on the exhibition floor of a trade show in Las Vegas, called World of Concrete. Circular saws, cement mixers, gleaming new trucks - it was an unusual place to talk about the politics of immigration.

But the simple promise of a concrete wall between the US and Mexico had flung a business tycoon into the White House, and I wanted to understand what this was about.