Researchers have found that bilingualism is associated with delayed onset of dementia symptoms. This story was covered in many North American news outlets, including the CBC:
Researchers at Toronto’s York University reviewed the hospital records of patients who were either monolingual or bilingual and who all had been diagnosed with dementia. A wide range of languages were spoken by the study participants.
They discovered that bilingual people were diagnosed three to four years after people who spoke one language....
a.) According to the statement above, what kind of study was conducted here? (Correlational or experimental?) What are the variables in the study? Were they measured or manipulated?
A comment by the lead research specifies a possible mediator for this relationship:
"Lifelong experience in managing attention to two languages reorganizes specific brain networks, creating a more effective basis for executive control and sustaining better cognitive performance throughout the lifespan," says Ellen Bialystok, lead author, in a release.
c) What kind of study would the researchers need to conduct in order to test this mediational hypothesis?
a) This is a correlational study. The two measured variables were whether each person was bilingual or not, and the age at which each person was diagnosed with dementia.
b) There is covariance here--bilingualism was associated with a later age at diagnosis. There is temporal precedence, too--we know that people were bilingual first, and diagnosed with dementia later. What about internal validity? It is possible that some other third variable is associated with both dementia onset and bilingualism. Perhaps education level or job status are third variables? In one part of the press release, we read that these particular third variables are not responsible. Consider this quote:
Interestingly, educational level and occupational status favoured the monolingual group, suggesting that those factors were not as critical in the development of dementia as linguistic ability, according to the study.
This information indicates that education and job status could not be the third variable, because although these two variables usually delay dementia onset, they were actually higher in the monolingual group. However, there may be other third variables, unmentioned here, that could be responsible for the relationship. Therefore, while we can predict that bilinguals will have a later onset of dementia, we cannot say for sure that bilingualism is the causal factor--the "active ingredient."
c) A study to test this mediational hypothesis would measure not only bilingualism and age at diagnosis, but would also measure brain networks involved in executive control. To support mediation, the results should show that bilinguals have richer such networks, and that such networks are associated with delayed dementia onset. Furthermore, the relationship between bilingualism and delayed onset of dementia should get weaker when the measure of brain networks is controlled for.