This article originally appeared on Tonic.

My belief in God didn't spontaneously combust—it faded. I lost my virginity at 16. I stopped going to church. I snuck out past curfew. As punishment, my mom made me memorize Bible verses, and I recited them like recipes.

I wasn't the only kid who stopped believing. A record number of young Americans (35 percent) report no religious affiliation, even though 91 percent of us grew up in religiously affiliated households.

Our disbelief was gradual. Only 1 percent of Americans raised with religion who no longer believe became unaffiliated through a onetime "crisis of faith." Instead, 36 percent became disenchanted, and another 7 percent said their views evolved.

It's like believing in Santa Claus. Psychologists Thalia Goldstein and Jaqueline Woolley have found that children's disbelief in Santa Claus is progressive, not instantaneous. First kids think that the Santa in the mall or library is real, then they think he's not real but still magically communicates with the actual Santa, and so on, until they finally realize that Santa is composed of costumed actors. "Kids don't just turn [belief] off," Goldstein says.

Likewise, losing faith happens in pieces.

I used to love this illustrated children's Bible my mom gave me. Long-faced Jonah inside a yawning blue whale felt warm and right. My brain made these feelings. When we enjoy religious or associated experiences, like snuggling up with Mom reading the Bible, our brain's reward circuits activate. Over time, religious ideas become rewarding in and of themselves. This is a powerful, unconscious motivation to keep believing.

"Religion works exactly like a drug—like cocaine, or methamphetamine—or like music, or like romantic love," says Jeffrey Anderson, a radiology professor at the University of Utah who studies religion in the brain. "All of those experiences on some level tap into rewards. The physiology is really the same."

When I began to see my colorful Bible as boring and childish, those same reward circuits likely became less active. Religious experiences produced less pleasure. This happens involuntarily in people with Parkinson's disease, which compromises the brain's reward centers. In turn, Anderson tells me, people who develop Parkinson's are much more likely to lose their faith.

In sixth grade, I learned that humans evolved over six million years, not seven days. Ironically, the brain's evolution is what enables us to believe in religion at all. Most components of religious belief are stored in the most evolved region of the brain, the frontal lobe. This may explain why religion is uniquely human.

For many years I believed in both creationism, with a God whose hand I could shake, and evolution, a cold, scientific world that cared nothing about me. Because when we lose faith, our brain's preexisting belief networks don't dissolve. They're updated, like a wardrobe. "Even if someone abandons or converts [religions], it's not like they're throwing out all the clothes they own and now buying a whole new set," says Jordan Grafman, director of brain injury research at the Shirley Ryan AbilityLab and a professor Northwestern University. "You pick and choose what you leave and what you keep."

New beliefs join the same neurological framework as old ones. It's even possible that an existing belief network paves the way for additional beliefs. Woolley has found that kids who believe in fantastical beings are more likely to believe in new ones invented by researchers. "I think it's because they already have this network that [the new belief] kind of fits into," she explains. Sometimes the new beliefs resemble the old ones; sometimes they don't.

As I tried to reconcile my belief in God with my growing knowledge of the natural world, I drew arbitrary distinctions. God couldn't see me poop but he could hear me pray, I decided. Eventually I couldn't figure out how, physically, he could do either.

This scientific descent from religion is common. Pew's 2016 survey on why now-unaffiliated Americans lost faith yielded explanations such as, "Rational thought makes religion go out the window," "Lack of any sort of scientific or specific evidence of a creator," and "I'm a scientist now, and I don't believe in miracles."

But it's not just science that sways our beliefs; it's the culture of science. Others' testimony critically influences our belief systems. We arduously convince young children to believe in Santa, and they do. Testimony dictates religious beliefs, too. For example, psychologist Rebekah Richert has found that if you frame a fantastical story as a religious story, children raised in religious households will believe it. If you don't frame it religiously, they'll call your bluff.

When we get to college, however, cultural testimony changes. An analytical, scientific view reigns, and there's little room for God. We staggered home from parties pontificating on the pointless evil of Western religion. We made friends by cynically confessing our doubt. College is "very likely to challenge the more conservative belief systems we have in our brains," Grafman says. It deflates our adolescent faith.

When we finally break up with religion, we rebound. Eventually, non-religious people who once had religious epiphanies get those same feelings from being in nature, or from seeing profound scientific ideas expressed, Anderson says. "The context changes but the experience doesn't." Most non-religious people are "passionately committed to some ideology or other," explains Patrick McNamara, a neurology professor at Boston University School of Medicine. These passions function neurologically as "faux religions."