It all started with a chance encounter. Three scientists met at a social event and struck up a casual conversation about religion and mental health.
Wouldn't it be neat, they thought, if we could somehow measure religion's effect on the brain?
"I kind of thought about it later," recalled Martha Payne, a professor of psychiatry at Duke and co-director of its Neuropsychiatric Imaging Research Laboratory, "and I was like, 'You know, we could actually do that with the project I'm working on now.' "
Their casual conversation in 2009 began a study whose results surprised the researchers themselves. They found that our religious affiliations correlate with the way our brains age.
The mental health project Payne was working on had accumulated more than a decade's worth of brain images from volunteers. Scientists use these images to measure the volume of parts of the brain. That study wasn't at all focused on religion, but scientists had asked the volunteers a few basic questions about religion.
To tackle the new project, Payne teamed up with the two scientists, Amy Owen and David Hayward, who at the time were researchers in Duke's Center for Spirituality, Theology and Health.
The trio also collaborated with two other Duke researchers: Harold Koenig, a professor of medicine and the director of the center, and David Steffens, a professor of psychiatry.
The team decided to zoom in on an organ curled around the center of our brains for their study: The hippocampus helps us form new memories. It shrinks as we age, and our memories can suffer as that happens.
About 70 percent of earlier research on religion and memory has shown that religious people have better-performing memories, said Koenig, so the hippocampus made an intriguing target for their study.
These benefits - along with most health benefits associated with religion - seem to be more connected with spiritual practices like prayer than with religious affiliation, he said. The team expected that they might see the hippocampus shrink more slowly in the brains of those who pray and attend church most often.
What they found
But that's not what they found at all. Prayer and church attendance had absolutely nothing to do with it. Only religious affiliation mattered, and in a surprising way.
Born-again Protestants had the fastest-shrinking hippocampi - effectively, this part of their brains aged more quickly than the other groups. Catholics and those who claimed no affiliation weren't far behind, though.
Of all the groups they studied, only the mainline Protestants - Lutherans, Presbyterians, Methodists, and others - had hippocampi that aged more slowly than those who didn't identify with any religious group. There weren't enough non-Christian participants to look beyond Christianity.
Although the divide across Christians was boggling enough, people from all groups who said they'd had a life-changing religious experience before the study had the fastest-shrinking hippocampi of any group they examined.
Scientists had uncovered something new. Now, they were left with the challenge of explaining it.
First, what did their observations on aging really mean for memory?
"There's not a clear answer," Payne said. Everybody's hippocampus shrinks with age, but nobody knows whether there is a threshold at which it hurts the memory.
Because the data were collected for a different project, the team can't know how well the participants' memories functioned. There's no way to go back in time and test.
Another thing they would have loved to test is what study participants really meant by choosing a religious affiliation. Were they identifying their beliefs, or their active membership in a congregation? "No affiliation" might not mean that someone doesn't hold spiritual beliefs.
Even stickier, what does it mean to say you've had a life-changing religious experience? This might just be another way of identifying born-again Protestants, Hayward said, and the groups overlap significantly.
But being "born again" isn't the only kind of life-changing religious experience someone could have. People of all faiths - or even of no faith - might describe an experience this way. There's just no way to know without more questions, Owen said.
In the end, the researchers decided that chronic stress might be the best way to explain the brain aging they saw, even if they can't say what it means for memory.
"We know that the hippocampus is especially susceptible to stress," Payne said.
People are stressed by an experience they can't easily explain, the team suggested. Born-again Protestants, Catholics, and those with no affiliation might also suffer from this chronic stress because they hold minority beliefs.
Or, at least, they may have when the study started. Today, more than 40 percent of North Carolinians identify themselves as evangelical Christians, according to the Pew Foundation.
Owen suggested that this might not even matter.
"Perhaps, because of the national dialogue about religion and the culture war, a person who identifies as born-again may feel that their beliefs aren't reflected in the wider culture," she said.
It would be great to delve into this more deeply, she added. Until someone does, however, the team's idea about stress is just that - an idea.
But an idea can be dangerous. The researchers cringe to recall the initial public reaction to their study, which was published in March.
People tend to see studies comparing brains as a judgment about intelligence, Hayward said. He remembers seeing articles with headlines like "Christians have smaller brains," something he resents not only because it's inflammatory - it's not even what they measured.
Koenig, a veteran of studying religion and health, said people often twist research results into something that validates their own beliefs. He stressed that they're not out to convert anyone. They're genuinely curious as to why religion is associated with health benefits.
Religion is not directly doing anything, Koenig said. Instead, it's driving other factors that are causing health benefits. If they can figure out what those factors are, he said, they could one day package those things in a way that even nonreligious people could benefit from.
But Owen said that even the mildest interpretation was still exciting. To see that a life-changing religious experience has a measurable influence on the brain was absolutely astounding, she said.
"Maybe it was a moment, and yet that carried implications over time," Owen said.
But even a moment can leave behind a big effect - just like the chance meeting that started it all.