For quite some time now, research has been conducted on the cognitive impact of bilingualism in young children. In early work on bilingualism, the acquisition of languages other than the native one was considered a risk factor for verbal and non-verbal cognitive development. Studies reported that bilinguals performed worse relative to their monolingual peers on a variety of variables, ranging from smaller vocabulary sizes (Smith, 1949) to impaired general intelligence (Darcy, 1946). However, in a seminal paper, Peal and Lambert (1962) reported that 10-year-old bilinguals actually outperformed their monolingual peers on tests of intellectual reasoning. The result was later confirmed by Ben-Zeev (1977) for 5- to 8-year-olds, and, in fact, a bulk of research spread over the past 20 years has actually shown that bilingualism might foster cognition rather than impede it. As such, in a longitudinal study with 5-year-olds, Woumans et al. (2016) found that only bilinguals improved significantly on intelligence over a period of one year. An extensive branch of research has focused on the effect of bilingualism on executive functioning (EF), which is to be understood as covering a broad range of cognitive functions that are used to control and regulate actions and thought. Around the turning of the centuries, a consensus was reached that a bilingual's languages are always simultaneously active and interacting (Hermans et al., 1998; Costa et al., 2000; Duyck, 2005; Brysbaert and Duyck, 2010), resulting in constant cognitive conflict for the bilingual (Green, 1998; Woumans et al., 2016). It became a common research question whether this conflict, as a kind of cognitive exercise, has repercussions outside of the verbal domain. Because language control, and therefore resolution of language conflict, relies on executive functions, this has gradually led to an almost exclusive focus on advanced executive functioning as a by-product of bilingualism, moving away from the former focus on general intelligence.

Several studies have pointed to beneficial effects of bilingualism on executive functioning in children. To illustrate, Kovács and Mehler (2009) revealed through three eye-tracking studies with 7-month-old infants that they outperformed matched monolinguals on cognitive control abilities. Kang and Lust (2019) found that bilingual language proficiency in 8-year-old children was a predictor for their EF performance. Tran et al. (2019) detected a similar bilingualism effect on cognitive control processes measuring selective attention, switching, and inhibition in a longitudinal study with 3- to 4-year-olds. In a large-scale study (N = 18,200) with children aged 5 to 7, bilingualism was found to moderate the effects of socio-economic status (SES) by ameliorating the detrimental consequences of low-SES on EF (Hartanto et al., 2019).

These beneficial effects have, however, proven difficult to systematically reproduce, resulting in an ongoing profound debate on the existence and scope of the bilingual advantage. Both for EF and for intelligence, very diverging results have been reported. As such, Dick et al. (2019) found no evidence for a bilingual executive control advantage in a large sample (N = 4,524) of 9- to 10-year-olds who were tested for inhibitory control, attention, task switching, and cognitive flexibility. Similar results were obtained in a study by Jaekel et al. (2019) on bilingual Turkish immigrant children aged 5 to 15 years. Equally, no effect of bilingualism on tasks of inhibition, updating, and shifting, i.e., components of EF, were found in a study by Arizmendi et al. (2018) among 7- to 9-year-olds. A meta-analysis by Gunnerud et al. (2020) targeting children aged 18 and under gave little support for a bilingual advantage in overall EF.

Those outcomes are in line with what has been identified in bilingual advantage literature targeting adult populations. In a recent large-scale study with 11,000 participants, bilinguals showed no advantage over monolinguals in a battery of 12 EF tasks (Nichols et al., 2020). In fact, findings on bilingual cognitive advantage effects have been contested altogether, starting with the hallmark criticism study of Paap et al. (2015). Issues that have been raised in studies skeptical of bilingual advantages include the observation of and publication bias for frequent null results, insufficient sample sizes, and the use of questionable research methods. Or, it is claimed that participants would be inadequately matched on background variables and any significant differences in performance may well reflect task-specific mechanisms instead of domain-free executive functioning abilities (Paap et al., 2015; Laine and Lehtonen, 2018). The criticism was further corroborated by a meta-analysis indicating no systematic support for the benefits in cognitive control functions associated with bilingualism (Lehtonen et al., 2018). Nevertheless, the meta-analysis by De Bruin et al. (2015) showed a significant bilingual advantage effect across published studies, although the simultaneously observed publication bias for bilingual advantages received more attention and fueled the doubt about the bilingual advantage. Moreover, in his own meta-analysis, Grundy (2020) argued “that there are several reasons, often overlooked, that lead to failed replications, and that when group differences do appear on EF tasks, despite these issues, performance favors bilinguals far more often than monolinguals” (p. 177), supporting his claim with Bayesian analysis of 167 independent studies that resulted in a Bayes Factor of BF10 = 2.91 × 10⁸, classified as “decisive” evidence.

Taken altogether, the strongest evidence for a bilingual advantage seems to come from studies targeting very young children and aging adults (Bialystok et al., 2005; Woumans et al., 2015), suggesting that bilingualism mainly impacts the sensitive periods of cognitive development and cognitive decline. Development of the cognitive control (CC) system is one the most essential processes in childhood (Diamond, 2002), evolving rapidly, especially between the ages of three to six (Best and Miller, 2010). Beneficial effects of bilingualism are reported in children from birth up to the age of six (Martin-Rhee and Bialystok, 2008; Kovács and Mehler, 2009; Morales et al., 2013; Crivello et al., 2016), but it appears that null effects arise more frequently in children over the age of six (Martin-Rhee and Bialystok, 2008; Abdelgafar and Moawad, 2015). As such, it may be that the acquisition of a second language (L2) in addition to acquisition of the first (L1) accelerates cognitive development during the critical period, but that afterwards the monolinguals catch up again with their bilingual peers. Likewise, studies into cognitive decline and its relation to bilingualism tend to show a temporary bilingual advantage (e.g., Abutalebi et al., 2015). As such, the onset of dementia, for instance, is found to be delayed by ~4 to 5.5 years in bilinguals (Craik et al., 2010; Alladi et al., 2013; Woumans et al., 2015). This temporary nature of differences is why, in the present review focusing on children, we have differentiated between studies targeting children younger than 6 years old (critical) and studies targeting children between the ages of 6 and 12 (post-critical). In doing so, we aimed to give an overview of previous research reporting on bilingual advantage effects in young children, shedding light on the validity and robustness of possible cognitive effects of L2 acquisition. Specifically, this review considers research on inhibition and switching, two constructs which are frequently related to bilingualism as the concept of speaking two languages in itself requires inhibition of the non-spoken language and switching between languages. Particular attention is paid to the experimental tasks that have been applied and the persistence of possible effects throughout critical and post-critical periods for cognitive development.

All articles considered in our analysis were retrieved from Web of Science through two systematic searches. The searches “((ALL = (bilingual advantage)) AND ALL = (children)) AND ALL = (executive control)” and “((ALL = (immersion)) AND ALL = (cognitive control)) AND ALL = (children)” resulted in 281 and 49 hits, respectively. All hits were automatically filtered on “Document type: Article” and “Web of Science categories: Psychology Experimental or Linguistics,” reducing the total number of hits to 189. During manual filtering, articles targeting age groups older than 12 were excluded and the sample was limited to articles looking into the constructs of switching and inhibition. Articles with a focus that did not entail a direct comparison between bilinguals or second language learners, and monolinguals were also removed, resulting in a final sample of 58 references.

From each article, age range, number of participants, targeted measures and tasks applied were extracted. It was indicated for each task if a significant bilingual advantage was detected. The results of this process are summarized in Table 1. In the naming of the measures and tasks, the terminology applied by the original author was kept. Articles were visually split up in the table according to the age range they were reporting on, in an attempt to visualize the persistence of any possible effects through the critical and post-critical period for cognitive development. The threshold for the transition from critical to post-critical was set at the age of 6, as the cognitive control system is especially evolving rapidly between the ages of three and six (Best and Miller, 2010).

Both research into measures associated with inhibition and research into measures associated with switching was considered in the present review article. As could be expected, the overview includes mixed results for virtually every measure and every task at hand. However, the main aim of this analysis is to detect trends throughout these mixed results in both grouping them and discussing them individually. One way of grouping them is to consider them according to age range, as was done for the visual representation in Table 1.

Moreover, grouping was also done in interpreting the diverse terminology applied in pinpointing measures. We argue a great deal of different tasks and labels really come down to measuring two major constructs in executive functioning: inhibition and switching. Inhibition includes, among others, measures that have previously been called response inhibition, interference suppression, inhibitory control, and conflict resolution. Switching includes measures such as shifting, task switching and cognitive flexibility. Within the larger constructs of inhibition and switching, different tasks are individually discussed.

Quite a lot of research has been conducted on the effect of bilingualism on inhibition and switching in children. Although findings of bilingual advantages on such cognitive control measures have dominated the research field for quite some time now, a debate is still raging on when, how, why, and even if these advantages appear. The present review set out to distinguish between bilinguals pre and post the critical age of development in order to determine whether age is a possible modulator of the effect. Our review included 58 articles on the topic and covered a total of 125 tasks.

It appears that in general, more research has been conducted on the construct of inhibition (90 tasks) than on the construct of switching (40). One possible explanation for this apparent focus of research is that all bilinguals constantly need the ability to inhibit input from the non-used language, while not all bilinguals have to switch very often (e.g., when the use of either language is restricted to different contexts). It should be noted, however, that inhibition was also defined as a broader construct than switching, including both response inhibition and interference suppression. Overall, a great deal of variation was present in the outcomes of these tasks, which strongly relates to the criticism uttered on frequent null results and the failure to reproduce bilingual effects (Paap et al., 2015). Moreover, variation in the current review was also omnipresent in terminology applied by different authors. Measures of inhibition were, among others, called “response inhibition,” “inhibitory control,” “conflict inhibition,” and “conflict resolution,” without defining resemblances and differences between any of those concepts. There appeared to be no consensus on which measures EF consists of exactly, nor what tasks can be used for measuring them, as was previously also indicated by among others Morra et al. (2018). Simon tasks (Simon and Rudell, 1967), for example, were applied to evaluate interference suppression (Baralt and Mahoney, 2020) as well as conflict resolution (Poarch and van Hell, 2012). The DCCS task was used to tap into switching (Simonis et al., 2020), inhibitory control (Escobar et al., 2018), and attentional control (Kalashnikova and Mattock, 2014). In considering the somewhat broader concepts of inhibition and switching, we tried to accommodate for these issues. As for different tasks applied to different age ranges, measures of Stroop, Simon, and Flanker were all administered among children of both critical and post-critical age, whereas parent/teacher questionnaires, gift delay tasks, and tapping tasks were employed solely among children under the age of 6 and stop-signal tasks only among older children.

Looking at tasks of inhibition, it was noted that Stroop- and Flanker-like tasks led to a bilingual advantage relatively more often in younger children than they did in children between the ages of 6 and 12. Interestingly, the opposite was true for Simon-like tasks, where older children showed a bilingual advantage relatively more often. This seeming lack of convergent validity between different tasks is in line with the mixed findings in research on the subject (e.g., Ross and Melinger, 2017; Poarch and van Hell, 2019). However, it should be noted that in one study targeting the critical group (Martin-Rhee and Bialystok, 2008), a delay was inserted in two versions of the Simon Task (Simon and Rudell, 1967) leading to null results on both occasions. If we were to exclude those measures, the balance would already be slightly modified and lead to a bilingual advantage being detected 3 out of 6 times in the critical age group (i.e., a 50% detection rate as opposed to the 60% detection rate in the post-critical group). Lee et al. (2013) already reported that between the ages of 6 to 15, inhibition costs reduced rapidly on the Flanker Task whereas they remained present and relatively stable on the Simon Task. This might explain why there is a higher bilingual advantage rate for post-critical age groups on the Simon Task than on the Flanker Task; there is simply more room for a bilingual advantage to exist. Overall, null results on tasks of inhibition were more frequently reported than results of a bilingual advantage, in addition to the likely presence of a publication bias (cf. De Bruin et al., 2015), which already favors alternative over null results. Our findings therefore strengthen the pleas for caution and skepticism made by Paap et al. (2015).

In tasks measuring switching, we were able to document a nearly exclusive focus on DCCS-like tasks. Twenty-four out of 40 tasks were of this type, other tasks being employed three times or less. The results for most tasks were mixed. A bilingual advantage for switching was found relatively more often for the younger age group (63%) than for the older age group (33%). The difference was entirely driven by the results for DCCS-like tasks, on which children aged younger than 6 showed a bilingual advantage on 78% of the tasks, whereas children between the ages of 6 and 12 demonstrated one in 33% of the cases. It has been shown in the general literature on DCCS that, while at the age of 3 most children exhibit a pattern of inflexibility, by the age of 5 most children switch when they are instructed to do so (Zelazo, 2006). Our findings suggest that this switching ability arises earlier in bilingual children, resulting in a bilingual advantage during the critical period which tends to disappear in post-critical age groups. This could be influenced by the constant switches bilinguals make between their languages, as previous research has also indicated language switching to be a key determinant for bilingual advantages in CC processes (Verreyt et al., 2016).

The Opposite Worlds task (Manly et al., 2001) was applied only twice but showed a bilingual advantage on both occasions. Although the task is evidently connected to the DCCS Task, they are different in that the DCCS Task requires participants to respond to two visible cues whereas the Opposite Worlds task requires ignoring the visible cue in favor of the instruction. Furthermore, while most tasks were applied throughout childhood, the Multilocation Task (Zelazo et al., 1998) and questionnaires were only used among younger children, and the Global/Local Task (Navon, 1977) was only used with older children. For the latter, this can easily be explained as basic reading ability is required to complete the task. However, there is no clear indication as to why the Multilocation Task has never been applied with older children. Teacher/parent questionnaires seem to be used when researchers anticipate that improved CC cannot be observed in behavior yet, notwithstanding the experimental results that were gathered by Kovács and Mehler (2009) with participants as young as 7 months old. Nonetheless, we feel like the more comprehensive view these questionnaires offer might also prove useful in older age groups, that is, taking into account the possible susceptibility to parent/teacher bias.

In all, both on tasks measuring inhibition and switching, bilingual advantages were detected more frequently in the critical age group (inhibition: 56 vs. 37%; switching: 63 vs. 33%). Especially in tasks that were frequently applied, such as Stroop-like tasks, Flanker-like tasks, Go/No-go tasks, and DCCS-like tasks. There was a substantial difference in bilingual advantage detection rates, favoring the critical age group over the post-critical one. Furthermore, we found that across age groups the insertion of delay in a given task influenced outcomes greatly. Whenever response was delayed, bilingual advantages rarely emerged in the studies under scrutiny. In gift delay tasks measuring inhibition, five studies showed null results whereas only one study established a significant effect. The Multilocation Task (Zelazo et al., 1998) likewise led to null results on both occasions where it was used. Interestingly, this task required a 10-s delay before participants could answer the question at hand. These descriptive results deserve more attention in future research. Still, the most prominent finding of this review is that if young bilinguals show an advantage in EF over monolinguals, it tends to partially disappear as they grow older. Crucially, there is wide agreement in the monolingual literature that the age span from three to six is critical for CC development (Best and Miller, 2010; Chevalier et al., 2012; Lucenet and Blaye, 2014). Hence, there seems to be an overlap in the timeframe in which the bilingual advantage is mostly observed and the sensitive period for CC development. Our results are in line with what we hypothesized, namely that bilingualism might accelerate CC development, but that this is only a temporary effect and monolinguals manage to catch up at a later stage. As we already stated, similar findings are reported in literature on the relation between bilingualism and cognitive decline, providing more support for the overlap between periods of crucial CC evolution and periods in which bilingual advantages can be detected.

As a parting statement, it should be noted that the format of a systematic review restricts us to presenting descriptives, whereas an added meta-analysis could lead to more conclusive insights on the existence of cognitive advantages in EF for bilingual children. Moreover, several studies in the present review include not only native speakers, but also children who were exposed to another language slightly later on, both through high and low exposure. An analysis of the difference between these groups was beyond the scope of this review but might have influenced outcomes. However, the current review can prove extremely useful within the research field as it has shed light on terminological issues and frequently applied tasks in addition to providing a concise overview of research on the bilingual advantage in switching and inhibition in children. Moreover, it has differentiated between critical and post-critical age groups and, in doing so, was able to draw links between timeframes in which a bilingual advantage emerges and periods that are crucial for EF development in children.

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

NP and EW conceptualized the study. NP conducted the search on Web of Science, drew up the table, and drafted the first manuscript. Both WD and EW have presented valuable feedback counting on their profound expertise in the subject at hand. All authors contributed to the article and approved the submitted version.