Bilingualism is not a rare experience; more than half of the world’s population speaks more than one language (
Grosjean 2010). Over the past decades, a large body of work on bilingualism has suggested that being bilingual brings a number of advantages. One such benefit, as proposed by the bilingual cognitive advantage, is that bilingual individuals may have greater cognitive skills than monolinguals (e.g.,
Adesope et al. 2010;
Bialystok 2017;
Gunnerud et al. 2020). More specifically, a large number of studies have documented that bilingual individuals have greater executive functioning skills than their monolingual peers. Executive functioning includes controlled and coordinated mental processes required for daily tasks, such as planning and attentional control (
Arizmendi et al. 2018), and is multifaceted, including inhibitory control, attentional flexibility, working memory, and regulation. The basis for the bilingual cognitive advantage is believed to be due to bilinguals’ experience of selectively attending to one of multiple jointly-activated languages, while inhibiting the others, in order to prevent interference from any non-target language (
Bialystok 2017;
Bialystok et al. 2010). Importantly, it appears that there is no general cognitive advantage for bilinguals, as confirmed by two recent meta-analyses (
Gunnerud et al. 2020;
Lowe et al. 2021). Instead, if a bilingual cognitive advantage does exist, it is likely domain-specific.
1.1. Inhibitory Control
Two domains of inhibitory control exist: cold inhibition and hot inhibition. Cold inhibition, also referred to as attentional inhibition, refers to the ability to suppress attentional shifts to external stimuli (
Arizmendi et al. 2018) and is typically measured with conflict tasks that require participants to inhibit distracting stimuli to successfully complete the task (e.g., Stroop tasks, Flanker task, Attention Network Task; see
Lowe et al. 2021). Among children, the bulk of past research suggests that attentional inhibition may be stronger among bilingual children. For example, throughout early childhood bilinguals have been found to outperform monolinguals on various conflict tasks, such as the Flanker task (
Carlson and Meltzoff 2008;
Sorge et al. 2016). Furthermore, the link between bilingualism and attentional inhibition may develop early in life.
Poulin-Dubois et al. (
2011) found that 24-month-old bilinguals displayed better performance than monolingual participants on a child-appropriate Stroop task. While the extant literature on attentional inhibition reports evidence for a bilingual advantage, note that there is low convergent validity between tasks, so that the manifestation of inhibitory control may differ across tasks (
Bialystok and Craik 2022;
Poarch and Krott 2019;
Poarch and van Hell 2019). This may lead to significant differences between monolinguals and bilinguals on one task, but not another.
Alternatively, hot inhibition refers to response suppression and is typically measured using delay tasks, such as delay of gratification tasks (e.g.,
Mischel et al. 1989). Few studies have assessed the differences in response inhibition between monolingual and bilingual children; indeed, a meta-analysis by
Gunnerud et al. (
2020) reported that language effects for response inhibition were inconclusive as too few studies were available for analysis. In contrast, a more recent meta-analysis by
Lowe et al. (
2021) indicated that bilinguals aged 3- to 17-years-old had greater response inhibition than monolinguals, although this effect may be partially explained by bilingual children typically having higher socioeconomic status (SES). Nevertheless, some research has failed to find a bilingual advantage in response inhibition. For example, while
Carlson and Meltzoff (
2008) reported an advantage in attentional inhibition, they failed to find one for response inhibition as measured through delay tasks. Additionally,
Crivello et al. (
2016) found a monolingual advantage on a gift delay task at 31 months of age. Finally, a recent review noted that monolinguals and bilinguals tend to have similar performance on tasks measuring response inhibition (
Bialystok and Craik 2022). Thus, further research is needed to determine whether a bilingual advantage in hot inhibition does exist during early childhood, and whether this advantage is due to SES.
As the present study is interested in hot inhibition, it expanded upon
Lowe et al.’s (
2021) meta-analysis in three important ways. First, SES was controlled for. Second, we aimed to assess whether response inhibition is affected by language status and language exposure around 2 years of age. Finally, unlike all studies testing response inhibition in bilinguals, the present study used a parental questionnaire to measure response inhibition.
1.2. Attentional Flexibility
The ability to shift focus from one stimulus to another, termed attentional flexibility (
Calcott and Berkman 2015), is thought to be central to the bilingual cognitive advantage (
Bialystok 2017;
Bialystok and Craik 2022). The ability to manage multiple language systems and efficiently switch between the two is imperative to bilinguals’ communication, and it is believed that attentional flexibility skills gained in linguistic contexts can be transferred to non-linguistic scenarios, thus aiding in everyday tasks. Indeed, a link between language status and attentional flexibility has been noted, even among preverbal infants. For example,
Kovács and Mehler (
2009) reported that, compared to monolinguals, 6- to 7-month-old bilinguals were better able to redirect their attention to a target cue during an implicit anticipatory looking task. Recently,
Comishen et al. (
2019) replicated Kovács and Mehler’s findings, further supporting the notion that attentional flexibility may be stronger among bilingual infants. Overall, these findings suggest that bilingual advantages in attentional flexibility are not limited to linguistic tasks and that these advantages emerge early in development.
While the above findings have highlighted the evidence for a bilingual advantage in attentional flexibility, not all studies have succeeded in finding this effect. For example,
Poulin-Dubois et al. (
2021) found no difference between monolingual and bilingual 17-month-olds on a delayed response task. Additionally, many recent studies have failed to demonstrate that bilingual infants outperform monolingual peers on anticipatory looking tasks (
D’Souza et al. 2020;
Ibanez-Lillo et al. 2010;
Kalashnikova et al. 2020;
Tsui and Fennell 2019). Finally, two recent meta-analyses have failed to report a language status effect on attentional flexibility throughout childhood (
Gunnerud et al. 2020;
Lowe et al. 2021). It remains possible that differences in attentional flexibility do not develop between monolinguals and bilinguals during childhood.
1.3. Working Memory
A third facet of executive functioning is working memory, a component of short-term memory that allows individuals to maintain and manipulate information during cognitive processing (
Baddeley 2000). Greater second language proficiency may be associated with stronger working memory (
Grundy and Timmer 2016;
Linck et al. 2014). Specifically, a meta-analysis by
Grundy and Timmer (
2016) reported that a bilingual advantage in working memory existed across development and that this effect was larger among children, suggesting that differential working memory skills between monolinguals and bilinguals develop early in life. Additionally, by 5 to 7 years of age,
Morales et al. (
2012) reported that bilingual children have greater working memory skills on a visuospatial task compared to their monolingual peers; however, these results should be interpreted with caution, as the bilingual advantage was most evident on tasks that required the use of additional executive functions and did not purely measure working memory. Regardless, the bilingual advantage has been found for visuospatial working memory across adulthood.
Luo et al. (
2013) noted that among younger and older adults, and on simple and complex tasks, bilinguals performed better on visuospatial tasks than monolingual adults. However, the opposite was found for verbal working memory tasks, where a monolingual advantage was instead discovered. Overall, it appears that bilingual children may outperform monolinguals on working memory tasks, specifically those assessing visuospatial working memory. However, it is possible that this advantage may be influenced by the use of additional executive functions during working memory tasks.
Indeed, ambiguous evidence for a bilingual advantage in working memory is prevalent in the literature, with many studies failing to report a significant language effect. Recent studies have found that working memory likely develops similarly between monolingual and bilingual infants (
Brito et al. 2021;
Brito et al. 2014;
Bialystok 2017;
Poulin-Dubois et al. 2021). Specifically, no statistically significant differences in performance on multiple location tasks were reported between bilinguals and monolinguals aged 17 months (
Poulin-Dubois et al. 2021), 18 months (
Brito et al. 2021) nor 24 months (
Brito et al. 2014). Even during childhood there is no clear evidence for a bilingual advantage; previous studies have reported that among children aged 6 to 12 years, there is no effect of language status on working memory task performance (
Bonifacci et al. 2011;
Engel de Abreu 2011). Finally, among younger and older adults, past studies have found similar performance on working memory tasks between monolingual and bilingual adults (
Bialystok et al. 2008;
Ratiu and Azuma 2015). It is possible that monolingual and bilingual individuals have similar working memory skills, and differences between the two groups may be due to assessments requiring the use of additional executive functions. Instead of solely relying on in-lab measures of cognitive skills, perhaps researchers should also aim to utilize parental reports. While the results from parental assessments may also be influenced by intrusions of other executive functions, when both forms of assessment are included in the literature, a better understanding of bilingual cognition may be achieved. While in-lab assessments bring the advantage of strong experimental control, parental reports offer greater ecological validity.
1.5. What Explains the Mixed Evidence on the Bilingual Cognitive Advantage?
A number of explanations have been offered for the mixed evidence concerning the bilingual cognitive advantage.
Lowe et al. (
2021) indicated that if this advantage does exist during childhood, it may be small and variable. The bilingual cognitive advantage may also be partially due to confounding variables, such as SES (
Bialystok and Craik 2022;
Brito and Noble 2018;
Calvo and Bialystok 2014;
Lowe et al. 2021). However, even when controlling for SES, there is evidence for cognitive advantages among bilingual children (for a review, see
Bialystok and Craik 2022). It is also possible that variability within the bilingual group may contribute to the mixed evidence. For example, language exposure has been linked to performance on executive function tasks, where bilinguals who have greater non-dominant language exposure perform better on a Stroop task than bilinguals with less non-dominant language exposure (
Poulin-Dubois et al. 2011). It is argued that bilinguals with greater non-dominant language exposure may have more practice switching between and inhibiting language systems, thus leading to greater cognitive skills.
Past evidence also suggests that bilingual children whose vocabulary contains a greater proportion of cross language synonyms, or translation equivalents (TEs), may have greater executive functioning (
Crivello et al. 2016). TEs refer to concepts that have similar meanings in different languages (e.g.,
dog in English and
chien in French). TEs can be used as a measure of how effectively individuals can switch between multiple languages, as bilinguals must manage each language system by selectively inhibiting one language to activate another (
Bialystok et al. 2010;
Meuter and Allport 1999). Thus, individuals who switch between language systems more often tend to have a higher proportion of TEs in their vocabulary (
Genesee and Nicoladis 2006;
Pearson et al. 1997). Bilinguals with a higher TE proportion may have more opportunities for inhibiting one language while another is activated. Additionally, as language systems may be independently activated (
Singh 2014), bilinguals may need to frequently switch between multiple language systems, and thus gain strong inhibition and attention skills (
Patterson and Pearson 2004). Supporting this idea, a longitudinal study used TEs as an index of children’s ability to code-switch and found that bilingual toddlers who acquired more TEs between the ages of 24 and 31 months had greater inhibition skills by 31 months than those who acquired fewer TEs during the same period (
Crivello et al. 2016). Further,
Prior and Gollan (
2011) found that bilingual young adults who frequently switched between their known languages exhibited greater ease in task switching compared with monolinguals; this difference did not hold for bilinguals who code-switched less frequently. Thus, gains in attentional flexibility and inhibition occur when bilinguals switch between two language systems more frequently. The present study used TEs as a proxy for code-switching experience. Additionally, because older children naturally have larger vocabularies and produce more TEs than younger children (
Tsui et al. 2021), the present study used a proportional TE measure to control for overall vocabulary size.
A further explanation for the mixed results is that cognitive advantages may be task-specific (
de Bruin et al. 2015;
Paap 2019;
Poarch and Krott 2019;
van den Noort et al. 2019;
Ware et al. 2020). Past research has typically measured various executive functions using in-lab assessments (e.g., gift delay tasks, Flanker Inhibitory Control and Attention Task, and the Dimensional Card Sort Task); however, this comes with limitations. Among infants and young children in particular, executive functioning can be difficult to assess in tasks administered in the laboratory where the aim is to measure specific skills in isolation. During the first few years of life executive functioning skills are mildly unstable and children may display poor performance on individual tasks measured at a single timepoint (
Hendry et al. 2016). Additionally, administering a battery of executive function tasks may result in high attrition rates, as children may lose focus throughout the testing session (
Poulin-Dubois et al. 2021).
Willoughby et al. (
2010) further outlined limitations of assessing childhood executive functioning using in-lab tasks. One issue is that many tasks use dichotomous scoring (e.g., pass/fail); this scoring technique provides limited information of individual differences in task performance. Further, in-lab assessments are often difficult to administer and may not be consistent between labs, as experimenters must be trained to administer tasks, assessments often lack formally written training documents, and task materials are inconsistent between labs. Another problem is that specific cognitive tasks may not purely measure executive functioning; instead, these tasks may also measure verbal comprehension. If toddlers have low comprehension skills, they may misunderstand verbal instructions and perform poorly on the task (
Burgess 1997;
Hendry et al. 2016;
Miyake and Friedman 2012). In order to minimize these measurement issues, executive functioning can instead be assessed through parental reports, which provide both broad and detailed information of children’s behavior over a longer time period (
Rothbart and Muaro 1990). Moreover, parental reports can be easily completed in online formats (e.g., Wordbank;
Frank et al. 2016), allowing for remote testing. Given the methodological challenges of testing executive functions in very young children, the present study used a recently created parental report, the Early Executive Functions Questionnaire (EEFQ;
Hendry and Holmboe 2021). The EEFQ has four sub-scales that respectively measure response inhibition, attentional flexibility, working memory, and regulation. Three of the sub-scales (inhibitory control, attentional flexibility, and working memory) are additionally loaded into a global cognitive measure, Cognitive Executive Function (CEF).
1.6. The Present Study
The present study aimed to fill a number of gaps in the extant literature on the emergence of a bilingual cognitive advantage in early childhood. First, given the limited research conducted to assess the bilingual cognitive advantage during the second year of life, it was designed to provide further insight into the development of executive functioning in multilingual and monolingual toddlers. Second, this study assessed differences in executive functioning by analyzing the impact of language exposure on a continuum. Finally, past research has consistently assessed the cognitive advantage with in-lab assessments. To our knowledge, no study to date has measured executive functioning among multilinguals with a questionnaire-based assessment. Thus, the main goal of the present study was to assess differences in executive functioning between monolingual and multilingual toddlers using the EEFQ. Three hypotheses were tested. First, when comparing samples of monolingual and multilingual children, it was predicted that multilinguals would show higher executive functioning than monolinguals in the following domains: inhibitory control, attentional flexibility, and regulation. However, as there is limited evidence to suggest that children exposed to multiple languages have greater working memory than monolinguals, we expected this cognitive skill to be similar between both groups. Second, among all participants, we expected that those with greater non-dominant language exposure would exhibit stronger inhibitory control, attentional flexibility, and regulation than those with less non-dominant language exposure. However, no such effect was predicted for working memory. Third, given that TEs provide opportunities for language switching, multilinguals with a larger proportion of TEs should have greater inhibitory control, attentional flexibility, and regulation than participants with a lower proportion of TEs.