Multilingualism – a Contradictory Finding?

I'm always annoyed when, just after I've written a blog about a new finding, reports of a study with contradictory findings are published. A couple of weeks ago I summarized a study reporting that bilingual seniors were able to delay onset of the cognitive symptoms of Alzheimer's, compared with monolingual people. Now a presentation at the Alzheimer's Disease International Conference has described results from the Nun Study that indicate no protective effect of multilingualism against Alzheimer's disease.

The Nun Study includes 678 nuns aged 75 or older from the School Sisters of Notre Dame. These ladies were highly educated – over 90% were teachers and a few were immigrants to the US. The average age of death was roughly 90 years. Assessments were first made between 1991 and 1993, and 12 times subsequently; clinical dementia was diagnosed using the standard CERAD (Consortium to Establish a Registry for Alzheimer's Disease) battery of neuropsychological tests together with impairment of activities of daily living. Neuropathology (postmortem exam of brains) used the CERAD and the NIA-Reagan criteria to diagnose Alzheimer’s.

Multilingualism, which was ascertained by questioning the subjects, was defined as speaking 2 or more languages fluently.  It was not significantly associated with Alzheimer’s, whether the latter was defined by the CERAD battery of tests or NIA-Reagan pathology diagnostic criteria.  Various classifications of the nuns, based on the CERAD tests and the pathology exams, were all found to have equal or very similar percentages of subjects claiming that they were multilingual. The investigators concluded: “Regardless of the neuropathological criterion used, multilingualism was not significantly associated with Alzheimer’s disease”. 

Is there an obvious reason for the difference in results between this study and the Canadian one I reported a few weeks ago?  Well, the Toronto study used the time of onset of Alzheimer symptoms to compare two populations, while the Nun study compared the frequency of multilingualism in women diagnosed with the disease, both in life and postmortem.  And regarding multilingualism, there were a couple of important differences in the Nun study.  There was no evidence of the proficiency of the self-reported multilingualism, and it’s not very likely that the second language as used much in the sisterhood environment; also, the socialization among nuns is likely to have been much less than in the subjects of the Canadian study.  I feel better now – both studies can be considered ‘correct’, without major contradictions . . .