To an extent, atheists and enthusiastic believers are products of their environment, so to speak. Growing up in secular or highly religious families and communities can strongly influence an individual's world view.
Geographically in the U.S, the affluent West Coast and Northeast are well-known for their higher-than-average levels of godlessness, and are home to a disproportionate number of atheists. In contrast, poorer states, especially those in the "Bible Belt" of the southeastern and south-central U.S., have more zealotry.
"Regional cultures are strong," said Kosmin. "It's the Vermonts and Oregons of the world where you find more atheists and more secular folk, and it's Alabama and Mississippi, the Bible Belt where you find more of the religious people.