TIME.com: Health -- The Nun Study So he and Mortimer, along with University of Kansas psychologist Susan Kemper, began analyzing the autobiographies for evidence of such extra capacity. Kemper, an expert on the effects of aging on language usage, had earlier shown that "idea density" — the number of discrete ideas per 10 written words — was a good marker of educational level, vocabulary and general knowledge. Grammatical complexity, meanwhile, was an indicator of how well memory was functioning.