Data synthesis

The first of these studies was conducted by Bialystok et al. in 2007. They used medical notes and interviews to investigate whether the onset age of dementia and the first appointment to a memory clinic in Canada significantly differed between monolinguals and bilinguals. Individuals were defined as bilingual if they had “spent the majority of their lives, at least since early adulthood, regularly using at least two languages” (Bialystok et al., 2007, p. 460). Bilinguals were diagnosed with the disorder and experienced symptom onset significantly later than monolinguals. These associations were present despite heterogeneous factors between groups which should have benefitted the monolinguals. The monolingual group had on average received a higher level of formal education (12.4 years, compared to 10.8 years in the bilingual group), a factor which has been suggested to increase cognitive reserve and potentially delay the onset of dementia (Bennett et al., 2003; Meng & D’Arcy, 2012; Sattler et al., 2012). In addition, a much higher number of the bilingual group were immigrants (87%, compared to 14% of the monolinguals). As many of these immigrants arrived from Europe after World War II, it is likely they were exposed to more stressful life events than the non-immigrants, which is a known risk factor for dementia (Chertkow et al., 2010; Gollan, Salmon, Montoya, & Galasko, 2011; Guzmán-Vélez & Tranel, 2015; Machado et al., 2014). Nevertheless, despite these factors, a significant difference was present between groups, leading the authors to conclude that bilingualism markedly delays the development of dementia (Bialystok et al., 2007).

Critics, however, argue that the higher levels of immigrants within the bilingual group may have actually favoured the bilinguals (Craik, Bialystok, & Freedman, 2010; Guzmán-Vélez & Tranel, 2015). Although stressful, migration requires learning and adaption to a new environment, an experience which is cognitively stimulating (Guzmán-Vélez & Tranel, 2015). Migration may therefore positively impact upon cognitive functioning, potentially increasing cognitive reserve and delaying the development of dementia (Guzmán-Vélez & Tranel, 2015). Given the significantly higher percentage of immigrants in the bilingual group, it has been suggested that the beneficial effect of migration on cognitive reserve may at least partially account for the differences observed between the monolinguals and bilinguals by Bialystok and colleagues (Guzmán-Vélez & Tranel, 2015).

Nevertheless, bilingualism has been reported to delay dementia even after potentially confounding variables such as immigration and education status have been controlled for (Alladi et al., 2013; Craik et al., 2010). Craik and colleagues (2010) compared the age of symptom onset and diagnosis in monolinguals and bilinguals diagnosed with Probable AD in Canada. Individuals in the bilingual group were, on average, diagnosed with dementia 4.3 years later than individuals in the monolingual group, whilst also reporting onset of symptoms 5.1 years later. Groups did not differ in occupation and cognitive level, and the monolingual group had received more formal education, leading the authors to conclude that the differences observed between groups could be attributed to bilingualism.

The results were corroborated by Alladi et al. (2013), who compared retrospective reports of symptom onset for all types of dementia in monolinguals and bilinguals who were all born and raised in India. Significant differences between groups were observed for several types of dementia, including AD, frontotemporal dementia and vascular dementia. Across all dementia types, bilinguals were, on average, 4.5 years older at the age of onset of dementia compared to monolinguals. These associations were found independent of potentially confounding variables such as education status, occupation, and gender.

However, inconsistencies are present within the literature, with some studies reporting no significant difference in the age of symptom onset and the age of diagnosis between monolinguals and bilinguals (Chertkow et al., 2010; Lawton, Gasquioine, & Weimer, 2015; Yeung, St. John, Menec, & Tyas, 2014; Zahodne, Schofield, Farrell, Stern, & Manly, 2014). Zahodne and colleagues (2014) conducted a 23-year prospective study, following Hispanics living in the USA. Participants were all fluent in Spanish, and were defined as bilingual if they reported speaking English “very well”, “well” or “not well.” They were defined as monolingual if they reported not speaking English at all. Although bilingualism was associated with initial higher scores on measures of memory and executive function, no significant differences were observed in the rate of cognitive decline or the proportion of participants diagnosed with dementia (after controlling for potentially confounding variables such as education level, gender, and age of enrollment). Similar results were also found by Yeung et al. (2014) and Lawton et al. (2015) who reported no significant association between bilingualism and the risk of dementia development.

Further refuting the relationship, Chertkow et al. (2010) found no significant differences in the age of symptom onset or dementia diagnosis in monolinguals and bilinguals. However, protective effects of multilingualism were reported, with individuals speaking three or more languages diagnosed with dementia significantly later than monolinguals. A significant interaction between bilingualism and immigration status was also reported. In immigrants, bilingualism was protective, delaying dementia diagnosis by almost five years. However, in non-immigrants, speaking two languages was actually burdensome, with bilinguals diagnosed with dementia, on average, 2.6 years earlier than monolinguals.

Thus, based on the aforementioned studies, the relationship between bilingualism and the development of dementia is inconclusive. Although some studies have reported that bilingualism is protective, others have found no significant differences between groups or even a detrimental effect. Caution should therefore be taken when making recommendations on whether bilingualism delays the onset of dementia.

However, there have been large differences in the definitions of bilingualism used between studies (e.g., language proficiency required and the age of acquisition of the second language), which may potentially explain some of the inconsistent findings yielded. Furthermore, studies have employed different study designs, which may have also impacted the results found. The next section will therefore evaluate whether these factors are likely to be important, and whether they could potentially explain some of the inconsistent findings reported in this research area thus far.

Language proficiency

An important factor which may influence the relationship between bilingualism and the development of dementia is the level of proficiency in the second language (Antoniou, Gunasekera, & Wong, 2013; Gollan et al., 2011; Kavé, Eyal, Shorek, & Cohen-Mansfield, 2008; Kaushanskaya & Prior, 2015; Zied et al., 2004). For instance, Zied and colleagues (2004) reported that balanced bilinguals (who use both languages proficiently) performed significantly better on a Stroop task, measuring executive function, than unbalanced bilinguals (who were less proficient in one language). As increased levels of executive control may mediate the relationship between bilingualism and delayed dementia development, it would follow that the levels of language proficiency are likely to impact on whether bilingualism delays the onset of dementia (Abutalebi, Guidi, et al., 2015; Bialystok, 2011).

The level of proficiency required for participants to be classified as bilingual has varied substantially between studies, which may explain some of the inconsistent findings reported within the research area. Indeed, in studies which have required participants to be highly proficient in their second language, bilingualism has been associated with delayed onset of symptoms and diagnosis (Bialystok et al., 2007; Craik et al., 2010; Woumans et al., 2015). However, in studies which have used more liberal definitions, non-significant differences have been reported (Zahodne et al., 2014). Taken together, these studies suggest that bilingualism may only be protective if individuals are highly proficient in both languages. However, this suggestion is speculative and based on a limited number of studies. Furthermore, some studies which have used strict proficiency criteria (e.g., Chertkow et al., 2010) have not found significant difference between monolingual and bilingual groups, suggesting that language proficiency cannot be the only factor responsible for the inconsistent findings reported.

Age of acquisition of the second language

Another factor proposed to impact upon whether bilingualism delays onset of dementia is the age of acquisition of the second language (Zahodne et al., 2014). Given that studies have used differing age of acquisition criteria, this may potentially explain some of the inconsistent findings reported. In many studies reporting that bilingualism delays the onset and diagnosis of dementia, participants have been lifelong bilinguals (Bialystok et al., 2007; Craik et al., 2010). Conversely, in studies where more liberal definitions of bilingualism have been used, no significant differences have been reported (Lawton et al., 2015; Zahodne et al., 2014). In the study conducted by Zahodne et al. (2014), most participants acquired their second language during adulthood, whilst in the study conducted by Lawton et al. (2015), no data on age of acquisition was collected. Nevertheless, some studies which have recruited lifelong bilinguals have found no significant differences between groups (Chertkow et al., 2010), suggesting that age of acquisition alone cannot fully account for differences in findings reported between studies.

Study design

Prospective studies which have followed healthy, community-dwelling adults for a number of years have typically found that bilingualism is not associated with delayed onset or diagnosis of dementia (Lawton et al., 2015; Yeung et al., 2014; Zahodne et al., 2014). Conversely, retrospective studies which have investigated age of onset or diagnosis in individuals already diagnosed with the disease have typically reported that bilingualism delays the development of dementia (Alladi et al., 2013; Bialystok et al., 2007; Craik et al., 2010; Gollan et al., 2011; Woumans et al., 2015). Generally, prospective studies are preferred to retrospective studies as they are less affected by potential confounds such as recall bias, where individuals recall details incorrectly (Guzmán-Vélez & Tranel, 2015). However, as Bialystok et al. suggested (2007), there is no reason why bilinguals would be more likely to recall information differently to monolinguals. Furthermore, age of diagnosis was determined by medical records in these studies (Alladi et al., 2013; Bialystok et al., 2007; Craik et al., 2010; Gollan et al., 2011; Woumans et al., 2015), which are unlikely to contain false information. Nevertheless, further research is needed to explore why strikingly different findings have been reported using prospective and retrospective studies.