The study, which examined the hippocampus region of the brain, found that Protestants who did not have a "born again" experience had significantly more gray matter than either those who reported a life-changing religious experience, Catholics, or unaffiliated older adults.
The study, funded by the National Institutes of Health and the Templeton Foundation, included at least two MRI measurements of the hippocampus region of 268 adults between 1994 and 2005.
It found an association between participants' professed religious affiliation and the physical structure of their brain. Specifically, those identified as Protestant who did not have a religious conversion or born-again experience — more common among their evangelical brethren — had a bigger hippocampus.
What does this all mean?
The hippocampus helps regulate emotion and memory, among other things, and shrinkage of it has been linked to Alzheimer's, dementia, and depression. Although the researchers believe there needs to be more research, they have speculated that perhaps the shrinkage in this case could be related to the stress of belonging to a minority group. (Atheists, if included in the religiously unaffiliated, would certainly be a minority as well.)
However, sociologists aren't so sure about this assumption. While born-again Christians are certainly a minority, they make up 40% of the population. The percentage increases as you move into the South. This is not exactly a minority in the truest sense.
If stress is indeed the reason behind the shrinkage, it would certainly dovetail with another recent study showing that those who deny evolution are more likely to experience anxiety about death (and we know that born-again Christians are more likely to deny evolution).
The research seems to present more questions than it answers: Could geography -- factoring in genetic, cultural, and environmental factors -- have something to do with the findings (the pool of participants were somewhat geographically constrained)? And most pressing, was this really a federally funded study?