The Nun Study
The sisters are being investigated. No, I'm not referring to that investigation.  This investigation, dubbed the Nun Study , began in 1990 as the project of David Snowdon at the University of Minnesota. (Kuddos to NPR for pointing out that the sisters in the study aren't nuns, but sisters .) Snowdon, who "wanted to look at aging over time, and decided to focus on sisters because they all had fairly similar histories and backgrounds," began studying the brains of a group of School Sisters of Notre Dame by conducting yearly memory tests. When he learned that most of the sisters had to write their autobiographies upon joining the congregation (many of which were written 50 years before the study began), he began to evaluate them based on how grammatically complex they were as well as something called idea density, "the average number of discrete ideas contained in every 10 written words."
What Snowdon discovered was that those autobiographies written with a higher idea density tended to correlate to sisters who weren't experiencing dementia in their older age, while the autobiographies with lower idea density could be a very early sign of the disease.
It's fascinating to me that all of the 678 of the sisters involved in the study agreed to donate a small piece of their brains to it after their death, showing their willingness to be investigated when there is good reason for it.