The current results build on and extend existing data on the relationship between bilingualism and diagnosis of AD in a number of ways. Most notably, in prior studies bilinguals were not well characterized in terms of degree of proficiency in each language. This left open several questions about the nature of the effect, and the possibility that some factor that correlated with bilingualism, but not specifically related to knowledge of two languages, was critical for introducing the observed effects. The current study used an objective measure of ability to produce picture names in each language, and showed that age of diagnosis of AD increased with increasing similarity of naming scores in each language. These data establish an explicit connection between knowledge of two languages and onset of AD (both measured as age-of-diagnosis and when we repeated our analyses using age of onset as the measure). Because all participants belonged to the same ethnic group (Hispanics in the USA) the current data reduce possible concerns that previously reported bilingual advantages were introduced by confounds inherent to comparing across language groups (i.e., bilinguals versus monolinguals).
In previous studies, bilinguals spoke a variety of different languages (Bialystok et al., 2007
; Craik et al., 2010
; Chertkow et al., 2010
; Kavé, Eyal, Shorek, Cohen-Mansfield, 2008
). The inclusion of a variety of bilingual types is a strength which implies that the benefits of bilingualism generalize broadly. On the other hand, it is very difficult to measure and compare proficiency levels across multiple languages. Thus, that approach left several questions unanswered with respect to how bilingual people must be before advantages begin to emerge. The current data provide additional clues as to the nature of bilingual effects, revealing them to be continuous in nature with greater benefits accruing with increasing proficiency levels.
Another important finding was that the benefit associated with bilingualism was robust only in bilinguals with low education level. This result implies an upper limit on the amount of benefit that bilingualism can confer for delaying diagnosis of AD. In bilinguals with low education level, the benefit can come from increasing bilingual language proficiency, but at higher levels of education the power of cognitive reserve for delaying AD is already at a maximum level and bilingualism does have any further effect. This interpretation increases confidence in prior conclusions of the cognitive advantages associated with bilingualism, and analogies between the effects of bilingualism and cognitive reserve (Bialystok et al., 2007
; Craik et al., 2010
Initial analyses suggested that the relationship between bilingual language proficiency and diagnosis of AD was present only for bilinguals who preferred to be tested in Spanish. This was a surprising result given that most Spanish-dominant bilinguals were not life-long bilinguals whereas English-dominant bilinguals had been bilingual since an early age. It might seem that this result resembles that of Chertkow et al, (2010)
who found a trend suggesting delay in age of onset of AD associated with bilingualism for native-French speakers but not for native-English speakers in Montreal. However, in that study education levels were relatively high (ranging from 10.3 to 12.8 in the native French groups; see Supplementary Materials in Chertkow et al.), and comparable for those who did versus didn’t show the bilingual advantage (i.e., French versus English natives). In addition, native-French study participants actually had higher SES than native-English study participants a puzzling result given that historically native-French speakers were disadvantaged in Montreal. In contrast, in the current study Spanish-dominant bilinguals who exhibited the advantage also tended to be less educated (5.7 years on average; see ), and many were immigrants. In this respect our results resemble more the immigrant participants studied in Chertkow et al. who also had low education levels (although in this case it was monolinguals who had low education levels, 6.3 years on average, versus bilinguals who had 10.5 years). Indeed in our study, analyses considering education as a continuous variable suggested that this was a critical difference between groups determining where the benefit of bilingualism is found (see ).
A number of limitations in the current report call for caution in any conclusions drawn. As in previous work bilingualism and education level were confounded with other factors (see ). For example, bilinguals with lower education levels had significantly lower DRS scores than bilinguals with high education levels. In previous studies, MMSE scores were used to measure the level of impairment at age-of-diagnosis and did not reveal any difference between bilinguals and monolinguals implying that bilingualism delays the onset but not the progression of AD (Bialystok et al., 2007
; Craik et al., 2010
). Consistent with this finding MMSE scores were not correlated with age-of-diagnosis (see ), and low and high education groups did not differ in MMSE scores at age-of-diagnosis (see ). However, the MMSE is a very brief measure, and it is possible that more sensitive measures of cognitive status like the DRS, and longitudinal data, will alter slightly the conclusions drawn to date with respect to disease progression.
Perhaps the most notable limitation in the current report is that although the results appear to be robust statistically, the number of bilinguals tested here is relatively small. A useful avenue to explore in further research is whether previously reported bilingual advantages remain, or if they are driven largely by participants with relatively lower education levels. Previously, an advantage was found comparing bilinguals to monolinguals even though bilinguals were significantly less educated than monolinguals (Craik et al., 2010
; but see Chertkow et al., 2010
). In one study, education level was not significant when included as a covariate in the comparison of bilinguals to monolinguals (Bialystok et al., 2007
). However, the inclusion of education as a covariate is not the same as specifying an interaction between education and bilingualism in a regression model. Our use of a continuous and objective measure of bilingualism in the current study, and a more rigorous approach to possible education effects, revealed relationships between these factors, and allowed significant effects to emerge despite the relatively small number of participants tested. Indeed although self-report and objective measures of degree of bilingualism are significantly correlated (see ; Gollan et al., in press
) self-report measures in the current study were not sufficiently sensitive to reveal the continuous nature of the effects of bilingualism.
A question that arises is why might there be an upper limit on the amount of cognitive reserve that can accumulate. Although our data do not provide an answer to this question, and our explanation of the interaction between bilingualism and education level is admittedly speculative at this point, the data we report bear striking resemblance to previous findings reported from the Nun Study. Mortimer and colleagues measured the relationship between brain size (inferred from head circumference) and dementia risk, and found a significant interaction such that larger brain size was associated with lower incidence of dementia but only in nuns with lower education level (Mortimer, Snowdon, & Markesbery, 2003
). This result supports the hypothesis that cognitive reserve is modulated by education level. The cross-study similarity could suggest that in addition to bilingualism increasing cognitive reserve, people who manage to become bilingual despite low education levels (or low SES) may also be better able to accumulate reserve. Additionally, this cross-study similarity provides converging evidence which lessens concerns about possible confounds in the current study (e.g., language-dominance), and possible problems with using onset and age of diagnosis or onset as the measure (incidence of AD was the outcome of interest in the Nun Study).
A practically important aspect of the current data is the extension of benefits associated with bilingualism to a relatively homogenous group of bilinguals, and to the most common type of bilingual in the USA. Previous studies included a majority of bilinguals who were immigrants from Europe (especially Eastern Europe speaking Yiddish, Polish, Hungarian and Romanian (Bialystok et al., 2007
; Chertkow et al., 2010
; Craik et al., 2010
); one study did not report the individual languages spoken by participants (Kavé et al., 2008
), but the majority originated from Israel and Europe/America). As pointed out by Chertkow et al., (2010)
people who lived in Europe during World War II likely had very different life experiences including possible risk factors for dementia. The present data suggest that this particular set of life experiences, and also early-bilingualism, are not necessary to find an effect of bilingualism on dementia onset.