REVIEW ARTICLE

Clear Theories Are Needed to Interpret

Differences: Perspectives on the
Bilingual Advantage Debate

Angela de Bruin1,2, Anthony Steven Dick3, and Manuel Carreiras2,4,5

an open access journal
1
Department of Psychology, University of York, York, United Kingdom
2
Basque Center on Cognition, Brain and Language (BCBL), Donostia-San Sebastián, Spain
3
Department of Psychology, Florida International University, Miami, FL, United States
4
University of the Basque Country, Bilbao, Spain
5
Ikerbasque, Basque Foundation for Science, Bilbao, Spain

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Keywords: bilingual advantage, executive control, bilingualism, inhibition, language control, brain
plasticity

ABSTRACT
The heated debate regarding bilingual cognitive advantages remains ongoing. While there are
many studies supporting positive cognitive effects of bilingualism, recent meta-analyses have
concluded that there is no consistent evidence for a bilingual advantage. In this article we focus
on several theoretical concerns. First, we discuss changes in theoretical frameworks, which
have led to the development of insufficiently clear theories and hypotheses that are difficult to
falsify. Next, we discuss the development of looking at bilingual experiences and the need to
Citation: de Bruin, A., Dick, A. S., &
Carreiras, M. (2021). Clear theories are better understand language control. Last, we argue that the move from behavioural studies to a
needed to interpret differences:
Perspectives on the bilingual
focus on brain plasticity is not going to solve the debate on cognitive effects, especially not
advantage debate. Neurobiology of when brain changes are interpreted in the absence of behavioural differences. Clearer theories
Language, 2(4), 433–451. https://doi.org
/10.1162/nol_a_00038 on both behavioural and neural effects of bilingualism are needed. However, to achieve this, a
solid understanding of both bilingualism and executive functions is needed first.
DOI:
https://doi.org/10.1162/nol_a_00038

Supporting Information:
https://doi.org/10.1162/nol_a_00038

Received: 11 November 2020

Accepted: 5 April 2021

Competing Interests: The authors have INTRODUCTION

declared that no competing interests
exist. Speaking more than one language is advantageous in this multilingual and highly intercon-
Corresponding Author:
nected world. Multilinguals can communicate directly with speakers of other languages and
Manuel Carreiras have access to other cultures, among other benefits. Apart from these communication advan-
m.carreiras@bcbl.eu
tages, bilingualism has been claimed to improve other cognitive domains, in particular exec-
Handling Editor: utive functioning (e.g., Bialystok et al., 2004). The underlying idea is that executive functions
Karen Emmorey
would be used and developed more strongly in bilinguals than in monolinguals. Bilinguals are
said to acquire better inhibitory control and monitoring skills than monolinguals because they
Copyright: © 2021 need to inhibit the irrelevant language, monitor the surroundings, and resolve conflicting in-
Massachusetts Institute of Technology formation (e.g., Bialystok et al., 2004). In addition, bilinguals who frequently switch between
Published under a Creative Commons
Attribution 4.0 International languages in daily life may have better task-switching skills (e.g., Prior & Gollan, 2011).
(CC BY 4.0) license
Furthermore, cognitive benefits of bilingualism have been linked to delays in the onset of dis-
eases such as dementia and to less decline associated with healthy aging, with possible im-
plications for public health (Bialystok et al., 2016; Perani et al., 2017). This attractive idea
The MIT Press
Perspectives on the bilingual advantage debate

Bilingual advantage: termed bilingual advantage (e.g., Kroll & Bialystok, 2013) has been quickly adopted by the
Better performance (reaction times mass media publishing headlines and statements such as: “Bilingual children and adults ex-
or/and accuracy) of bilinguals as
perience significant health benefits” (Huffington Post, 2013); “Bilingual students advance fas-
compared to monolinguals in
cognitive tasks. ter in middle school” (Rahhal, 2018).
Despite the popularity of this topic in the media and the emphasis on societal benefits, the
evidence for enhanced cognitive functioning in bilinguals is also widely questioned. In this
article, we will start with a brief overview of the evidence for a bilingual advantage. (Note that
we will be using the term bilingual advantage to refer to cognitive advantages.) We will focus
on bilingualism and cognition in healthy participants. This has been the focus of many recent
systematic reviews and meta-analyses (e.g., Antoniou, 2019; Bialystok, 2017; Donnelly et al.,
2019; Hilchey et al., 2015; Lehtonen et al., 2018; Paap, 2019). Critically, we will focus on

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several theoretical issues that are important to consider to move the debate forward. First,
we discuss the continuous changes in theoretical frameworks and issues when testing these
frameworks. Next, we discuss how differences in executive functioning have been associated
with individual differences between bilinguals. Although this is a promising way forward, we
argue that this can only be examined with a solid understanding of individual differences in
language control. Third, recent years have seen an increase in neuroimaging studies compar-
ing bilinguals and monolinguals on executive control tasks. We argue that, while these neuro-
imaging studies are interesting in their own respect, they cannot solve the bilingual advantage
debate without behavioural evidence and without clear predictions about the specific brain
regions and brain circuits that should reveal bilingual-monolingual differences in executive
control. For this field to progress, more specific theories and hypotheses need to be formed
regarding the behavioural and neural relationship between bilingualism and executive control.
We will posit that two developments are needed to achieve this. First, to develop theories,
a better understanding of bilingualism and bilingual language control is needed. Second,
multi-lab studies with clear preregistered hypotheses are needed to reliably interpret the data
across different types of bilinguals.

BEHAVIOURAL COMPARISONS BETWEEN BILINGUALS AND MONOLINGUALS

Several early studies comparing bilinguals and monolinguals showed enhanced performance
in bilinguals on various tasks assumed to measure executive functioning (e.g., Bialystok et al.,
2004; Bialystok et al., 2008; Bialystok & Martin, 2004; Costa et al., 2009; Costa et al., 2008).
In contrast, there are also many studies reporting no differences between bilinguals and mono-
linguals on executive control tasks (e.g., Antón et al., 2014; Antón et al., 2016; Duñabeitia
et al., 2014; Gathercole et al., 2014; Paap & Greenberg, 2013), especially in studies using
larger sample sizes (e.g., Antón et al., 2019; Dick, Garcia et al., 2019; Duñabeitia et al.,
2014; Nichols et al., 2020; Paap & Greenberg, 2013).
A more comprehensive overview of the literature on this topic has been provided in recent
systematic reviews (e.g., Antoniou, 2019; Bialystok, 2017; van den Noort et al., 2019) and
meta-analyses (e.g., Donnelly et al., 2019; Grundy, 2020; Gunnerud et al., 2020; Lehtonen
et al., 2018; Paap, 2019). While there is both evidence for and against a bilingual cognitive
advantage, recent meta-analyses taking into account publication bias (e.g., Lehtonen et al.,
2018) conclude that there is no strong or consistent evidence for enhanced executive func-
tioning in bilinguals. Similar conclusions were reached in a meta-analysis on children
(Gunnerud et al., 2020), although a small but significant effect on switching was found.
These reviews and meta-analyses also attempted to go beyond a “yes/no” answer, acknowl-
edging that studies differ in, amongst others, the type of bilinguals tested and the type of

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Perspectives on the bilingual advantage debate

executive control tasks used. These are all variables that can potentially modulate perfor-
mance of both bilinguals and monolinguals on executive control tasks. Currently, however,
there is no consistent evidence across studies for a modulating role of, for example, specific
bilingual experiences or the type of task used (Lehtonen et al., 2018).
In addition, it is frequently claimed that differences may not be captured in young adults
because they perform at ceiling, but that cognitive development and decline should allow for
effects of bilingualism to occur in children and older adults (e.g., Kroll & Bialystok, 2013;
Grundy, 2020). Meta-analyses, however, have found comparable (null) results in different
age groups (e.g., younger and older adults, Lehtonen et al., 2018; children and young adults,
Donnelly et al., 2019), suggesting that evidence for a bilingual advantage is limited not only in
young adults, but also in children and (healthy) older adults. Interestingly, it has also been

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proposed that the effects of bilingualism on attentional processes could be observed already
during infants’ first year of life (Comishen et al., 2019; Kovács & Mehler, 2009). However, data
from a recent Registered Report (Kalashnikova et al., 2021), show that bilingual and monolin-
gual infants’ performance did not differ in attentional control.
In sum, systematic reviews and meta-analyses thus suggest that a cognitive bilingual advan-
tage is at best small and may only exist in very specific circumstances or for specific types of
bilinguals. We will discuss three recent developments that have been proposed as future
avenues for research on bilingualism and executive control. First, we will consider changes
in theoretical frameworks. Second, we will discuss the role of individual bilingual experiences.
Third, we will review recent neuroimaging research used to examine the potential relationship
between bilingualism and executive control.

THEORETICAL FRAMEWORK AND DATA INTERPRETATION

Much past and recent work on bilingual-monolingual differences focuses on inhibition costs.
Studies looking at inhibitory control typically include conditions with incongruent trials (pre-
senting information that interferes with the expected response) and congruent trials (presenting
information that is compatible with the expected response). Participants usually need more
time to respond to incongruent than congruent conditions, a difference that is taken to reflect
an inhibition cost. Some studies observe bilingual-monolingual differences on these inhibition
costs (e.g., Pelham & Abrams, 2014) while others do not (e.g., Paap & Greenberg, 2013).
Others find that within the study, effects may be task dependent (e.g., Woumans et al., 2015,
showed a larger monolingual inhibition cost on one task but longer monolingual overall reac-
tion times [RTs] on another task). The initial framework regarding bilingualism and executive
control focused on inhibition. According to the initial hypothesis focusing on inhibition, a
bilingual constantly needs to inhibit one of their languages in daily life, which should lead to
non-verbal inhibitory control advantages (e.g., Bialystok et al., 2004). Theories focusing on
inhibition would posit that bilingual-monolingual differences should occur on measures of
inhibition costs, such as the difference between incongruent and congruent trials described
above. This offers a testable hypothesis.
One major challenge when looking at inhibition costs, however, is that tasks reporting
these costs often show low correlations (cf. Paap & Greenberg, 2013; Rouder et al., 2019).
The idea that inhibition is a unitary construct has therefore been criticised (Rey-Mermet
et al., 2018), raising the point that tasks might not measure the underlying construct of inhi-
bition but rather the “highly task-specific ability to resolve the interference arising in that task”
(Rey-Mermet et al., 2018, p. 515). Following this account, it could be argued that the mixed
results in the literature on bilingualism might reflect task-dependent effects, with bilinguals

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Perspectives on the bilingual advantage debate

having an advantage on a certain type of task-specific interference resolution. If this is true,

certain tasks should stand out (e.g., a Simon task might be more likely to reveal a bilingual-
monolingual difference than, e.g., a flanker task). However, current comparisons across stud-
ies (e.g., Lehtonen et al., 2018) do not show consistent evidence for such differences between
inhibition tasks. Furthermore, based on our current understanding of these different tasks, it is
unclear exactly which types of task-specific interference resolution would be most likely to be
affected by bilingualism (cf., e.g., Blumenfeld & Marian, 2014, and Paap et al., 2019). To be
able to formulate any theories or hypotheses that take into consideration task-specific types of
inhibition/interference resolution, we need a much better understanding of what these differ-
ent tasks actually measure.
A second explanation of low correlations between inhibition tasks is that they measure the

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same underlying construct but that task outcomes are influenced by task-specific features that
add noise. This issue could, at least to some extent, be overcome through analyses using latent
variables (cf. Friedman, 2016; Rouder et al., 2019). If there is an underlying construct, this
allows researchers to examine whether there is a bilingual-monolingual difference on that con-
struct as measured through multiple tasks rather than on one specific task that might be influ-
enced by task-specific features. Similar approaches can be used for other constructs of interest
that have been tested in relation to bilingual-monolingual differences (e.g., task switching,
Prior & Gollan, 2011; working memory, e.g., Engel de Abreu, 2011).
While the initial research (and much of the recent research) focuses on specific aspects of
executive function, such as inhibition or shifting, the hypotheses regarding bilingualism and
executive control are undergoing constant changes. This is driven by studies finding group
differences on overall RTs in conflict tasks (e.g., Emmorey et al., 2008), without a difference
in inhibition costs (but cf. Bialystok et al., 2004). New theories were therefore proposed that
focused on an advantage in conflict monitoring rather than inhibition, possibly related to a
bilingual’s need to monitor the circumstances to select the current target language (e.g.,
Costa et al., 2009). These theories can offer testable hypotheses if they include a baseline con-
dition. For instance, accounts on conflict monitoring posit that a bilingual advantage should
only occur in conditions involving both conflict and non-conflict trials (i.e., requiring more
conflict monitoring; cf. Costa et al., 2009). Such advantage would not be expected in condi-
tions that do not require conflict monitoring. However, studies finding overall RT differences
between bilinguals and monolinguals on conflict tasks do not always include a baseline con-
dition. Without such a baseline condition, it is difficult to interpret whether RT differences are
really due to conflict monitoring. For example, it might be that one group is faster in general.
Adding a baseline task without conflict could show whether bilingual-monolingual RT differ-
ences are specific to conflict monitoring (RT differences in the conflict but not in the baseline
task) or related to differences in general processing speed (RT differences in all tasks, even
those without conflict). We therefore recommend the inclusion of simple baseline conditions
without conflict. For example, a flanker task could include a baseline condition in which par-
ticipants simply respond to one arrow presented in the centre of the screen. RTs (and potential
group differences) in this baseline task can then be compared to the flanker task requiring con-
flict monitoring through presentation of congruent and incongruent (conflict) trials.
In recent years, a more holistic approach has been advocated (e.g., Kroll & Bialystok, 2013)
that focuses on executive functions as a whole rather than specific subcomponents such as in-
hibition or switching. Different terms have been used, including enhanced cognitive flexibility
(Kroll & Bialystok, 2013) and executive attention (Bialystok, 2017). This executive attention
system is described as a continuous, central, domain-general system in which memory and
attention interact to allow for complex cognition (Bialystok, 2017). The danger with some

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Perspectives on the bilingual advantage debate

newer frameworks, however, is that they become unprofitably vague. That is, executive atten-
tion is seen as a continuous mechanism that is involved in all sorts of complex cognition.
Differences in executive attention can occur on a wide range of tasks and measures without
a clear theory or hypotheses as to when and where these effects should be observed (cf.
Hartsuiker, 2015; Laine & Lehtonen, 2018).
This poses problems for studies that test bilinguals and monolinguals on different tasks, that
look at conflict costs and overall RTs, and that measure RTs and accuracy, and that then find
a bilingual-monolingual difference on one measure only. This type of research should either
be hypothesis-driven or should avoid drawing conclusions about individual tasks. When there
is a clear theory and hypothesis, task-specific patterns can be interpreted. For example, a
researcher might be interested in assessing the role of verbal versus nonverbal stimuli based

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on the hypothesis that bilinguals experience language interference in tasks using verbal mate-
rials. A bilingual disadvantage on the verbal but not on the nonverbal task could then be ex-
plained following their hypothesis. When such theory or hypothesis is not present (e.g., when
a Simon and flanker task are included, without a clear hypothesis about potential task differ-
ences), the danger is that any difference on any task would be interpreted as evidence for a
bilingual advantage, with a focus on the study’s positive findings. Without a clear theory or
hypothesis about what different tasks/measures indicate, how they differ or compare, and
how they might show different patterns in bilinguals and monolinguals, we should be careful
that we do not focus too much on or overinterpret the measure that shows a positive finding.
In the absence of clear hypotheses about task-specific effects, latent-variable analyses
(cf. Friedman, 2016; Rouder et al., 2019) might be preferable. This avoids having to generate
post hoc explanations for task differences that might be the result of noise added by task-
specific features.

INDIVIDUAL DIFFERENCES BETWEEN BILINGUALS

In addition to tasks potentially influencing bilingual-monolingual differences, the type of bi-
linguals tested might play an important role. This potential influence of individual differences
in bilingualism has been the focus of recent research. One advantage of this approach is the
move away from presenting bilinguals and monolinguals as homogenous groups that are the
same across populations and studies. Comparing a group of bilinguals to a group of monolin-
guals inevitably requires the researchers to define where one group ends and the other starts.
While the definition of a monolingual might seem straightforward (e.g., a person who can only
speak one language), this is often not as easy as it seems. “Monolinguals” sometimes include
participants who have learned and/or have some proficiency in another language (e.g., Paap &
Greenberg, 2013). Furthermore, dialect users are often classified as monolinguals (cf. Kirk
et al., 2018) and monolinguals living in a linguistically diverse environment might differ from
those living in more monolingual environments (cf. Bice & Kroll, 2019). Therefore, it might not
(always) be possible to make a clear categorical distinction between bilinguals and monolin-
guals. Furthermore, a comparison between one group of bilinguals and one group of mono-
linguals creates the suggestion that there are no individual differences within those groups.
Within bilinguals there are many individual differences (e.g., proficiency, age of acquisition,
use, switching) and one’s personal language experiences have been argued to influence lan-
guage control and consequently executive functions. These individual differences can and
need to be studied in relation to executive functions. This can be done by comparing well-
defined groups of bilinguals (e.g., a group of bilinguals with a high proficiency level in both
languages versus a group with a lower proficiency in their second language) and/or by treating
bilingualism and bilingual experiences (e.g., proficiency) as a continuum. Several language

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Perspectives on the bilingual advantage debate

experiences have been studied in relation to executive function, including age of acquisition
(e.g., Luk, De Sa, & Bialystok, 2011); proficiency (e.g., Singh & Mishra, 2013); and modality
(e.g., Emmorey et al., 2008). Recent frameworks focus on language use and switching (cf.
Blanco-Elorrieta & Pylkkänen, 2018). Green and Abutalebi’s (2013) Adaptive Control
Hypothesis focuses on how a bilingual’s language use can shape both language control and
executive functions, depending on the language environment they find themselves in. For ex-
ample, a bilingual who spends much time in more controlled dual-language environments that
require them to switch languages in response to cues (e.g., interlocutors) might have more
need for and develop interference suppression and goal maintenance more strongly than a
bilingual who can freely switch with other bilinguals who speak the same languages. The re-
search comparing different types of language switchers has shown mixed effects (e.g., Paap
et al., 2017; Prior & Gollan, 2011; Verreyt et al., 2016). To some extent mixed results might

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be due to the way language switching is measured and the type of switchers that are com-
pared. For example, looking at frequency of switching might not consider that bilinguals
who switch frequently might do so very differently. Thus, in addition to considering switching
frequently, type of switching (e.g., in response to external cues versus free dense code switch-
ing) needs to be considered (Green & Abutalebi, 2013).
Crucially, though, the hypothesis that certain types of bilinguals might be more likely to
show cognitive advantages than others is based on language control depending on bilingual
experiences. The Adaptive Control Hypothesis (Green & Abutalebi, 2013) describes how these
experiences might moderate both language control and executive control, but there is very
little empirical work to assess influences on language control. Influences on executive control
depend on understanding language control. To understand transfer of training (from language
to executive control), we first need to understand the training itself (i.e., language control). To
facilitate this, we need more detailed descriptions and measures of bilingual participants to
examine the influence of bilingual experiences within and across studies (cf. de Bruin,
2019; Surrain & Luk, 2019). Recent work has started to suggest that language control might
be shaped by the interactional context (e.g., Blanco-Elorrieta & Pylkkänen, 2017; de Bruin
et al., 2018). There is also some emerging work comparing different types of bilinguals on,
for example, the processing of code switches (e.g., Beatty-Martínez & Dussias, 2017).
However, far more research is needed to understand how language experiences can shape
language control before we can form more exact theories about which language experiences
might shape (components of ) executive control.

BRAIN PLASTICITY
In addition to behavioural studies, there is now an increasing number of studies focusing on
brain differences between bilinguals and monolinguals (cf. Bialystok, 2017; Grundy et al.,
2017; Vınerte & Sabourin, 2019). Other studies have shown that similar brain regions might
be involved in language control and inhibitory control (cf. Abutalebi & Green, 2007) or in
language and task switching (De Baene et al., 2015), which has been taken to suggest a close
relationship between language and executive control. However, while many brain differences
have been observed between bilinguals and monolinguals, there is currently no consistent
evidence that specific control-related regions show bilingual-monolingual differences across
studies (see García-Pentón et al., 2016). We do not aim to provide an exhaustive review of the
neuroimaging literature here. What we do review is designed to show that the same issues
described above for behavioural research also apply to neuroimaging studies. In addition,
we will highlight additional challenges that need to be addressed to be able to interpret

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Perspectives on the bilingual advantage debate

EEG: neuroimaging studies. We will first analyse recent work assessing EEG, followed by structural
Electroencephalography is a MRI and functional MRI (fMRI) research on bilingualism and executive functions.
technique to record electrical activity
of the brain with the electrodes
placed along the scalp. Electrophysiological Evidence

EEG (Electroencephalography) provides a rich and complex brain measure to investigate the
potential influence of bilingualism on other cognitive processes such as executive control. The
MRI:
event related potentials (ERPs) that result from time locking the EEG signal to a particular event
Magnetic Resonance Imaging is an
imaging technique used in radiology offer latency, amplitude, polarity, and topography as potential variables. Interestingly, some of
to form pictures of the anatomy and these variables, like latency, are appropriate to capture early cognitive effects and the time
the physiological processes of the course of different processes given its high temporal resolution. Nonetheless, differences
body. between monolinguals and bilinguals in ERP amplitudes in the several different components

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(e.g., N200, P300, N400, ERN, etc.) allow different interpretations depending on whether
an amplitude increase or decrease is associated with more efficient cognitive processes. At
this point, ERP evidence for or against cognitive processing benefits for bilinguals versus
monolinguals is limited and mixed (see Cespón & Carreiras, 2020).
As an overall strategy, Cespón and Carreiras (2020) reviewed the effects of tasks used to
measure executive functions (e.g., Simon, flanker, Stroop tasks) on the latency and amplitude
of different ERP components (N200, P300, N400/N450, ERN). This allowed them to establish
hypotheses about whether an increase or decrease in amplitude was related to more or less
efficient cognitive processes independently of bilingualism. Taking this knowledge into
account they hypothesized how bilingualism is expected to modulate effects (e.g., latency,
amplitude) of a particular component if a bilingual advantage were present. Based on this
review per ERP component, they offer two main recommendations for the way differences in
latency and amplitude (e.g., larger or smaller amplitude) should be interpreted when com-
paring bilinguals and monolinguals on these tasks. First, only specific differences in ERP latency
and/or amplitude can be used to support claims regarding enhanced executive-control effi-
ciency in bilinguals. Cespón and Carreiras (2020) reviewed literature on executive control
assessing different ERP components and formulated hypotheses for bilingual-monolingual differ-
ences and interpretations depending on the direction of a difference (i.e., how bilingualism will
modulate the established latency/amplitude effects and the corresponding interpretation). These
hypotheses can guide new ERP research on bilingualism and executive control, allowing
researchers to establish a priori hypotheses based on previous work linking specific changes in
ERPs to more or less efficient cognitive processes.
Second, in their review Cespón and Carreiras (2020) recommend that researchers carry out
correlational analyses of ERP amplitude with behavioural performance. This is important to
establish whether potential ERP differences form a direct reflection of bilingual advantages/
disadvantages. When there is a correlation between the behavioural difference and the ERP
fMRI: difference (e.g., a smaller behavioural cost in combination with a decreased amplitude for
Functional Magnetic Resonance bilinguals), this supports the interpretation that there is an advantage for bilinguals on this
Imaging measures brain activity by task. When such correlation is not observed (or when there is an ERP difference without a
detecting changes associated with
behavioural difference), it suggests that bilinguals and monolinguals might differ in the way
blood flow.
they process information but without a directly associated bilingual advantage.

DWI: Magnetic Resonance Imaging Evidence

Diffusion-weighted Imaging is the
use of specific MRI software that Functional magnetic resonance imaging (fMRI) and structural MRI (including diffusion-weighted
generates images from the diffusion imaging (DWI) measures and morphologic measures recovered from T1- and T2-weighted
of water molecules. MRI scans), as well as related functional imaging methods such as functional near-infrared

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Perspectives on the bilingual advantage debate

spectroscopy (fNIRS) and magnetoencephalography (MEG), have been extremely useful for
Brain plasticity: understanding brain plasticity and development, and have been informative for mapping
The ability of neural networks in the the functional specialization of different brain regions. Several of these methods have also
brain to change and reorganize, such
been used to examine brain differences in structure or function that result from bilingual ex-
as when learning a new ability like a
second language. perience as it relates to differences in brain networks associated with executive function (see
Bialystok et al., 2012; Costa & Sebastián-Gallés, 2014; García-Pentón et al., 2016; Grundy
et al., 2017; Pliatsikas & Luk, 2016 for reviews). Rather than giving a review of the fMRI
literature, we will discuss the key issues affecting fMRI research on bilingualism and executive
function.

Theoretical framework

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Most of the MRI studies on bilingual-monolingual differences or differences between bilin-
guals in executive function have framed their investigations within the context of cognitive
and neural models such as the Inhibitory Control model (Green, 1998), and more recent
modifications formulated in the Adaptive Control Hypothesis (Green & Abutalebi, 2013), or
the bilingual anterior-to-posterior and subcortical shift (BAPSS) model (Grundy et al., 2017).
Theoretical models are critically important for prediction and interpretation of activation
differences across groups of bilinguals and monolinguals. This is because neurobiological
models of language and executive function in monolingual individuals are themselves com-
plicated, and there is debate within each subfield about which grey matter regions and white
matter tracts are critical to networks underpinning each process. In the field of language neu-
robiology, for example, neural networks for speech production and speech perception are
only partially overlapping, and the issue gets more complicated at higher levels of language
processing. Thus, at the sentential and discourse levels, much more of the brain, on both hemi-
spheres, is recruited, especially in situations where syntactic and semantic constructions are
more complex, and in cases where pragmatics come into play (Hagoort, 2019). When these
latter linguistic processes are brought to bear, it is reasonable to argue that higher-level control
processes might be recruited to navigate the additional semantic, syntactic, and pragmatic
challenges of communicating in more than one language. Indeed, such demands may not
be universal, but may emerge only in specific contexts or situations—that is, any advantages
emerge not from being bilingual, but from the ways in which languages need to be controlled
in situational contexts (e.g., Blanco-Elorrieta & Pylkkänen, 2018). The question at the neuro-
biological level, though, is what this might look like in terms of changes in regional activity
and network dynamics. Thus, some sort of framing model is needed to interpret any potential
differences.
The Adaptive Control hypothesis is designed to address these issues, as its central goal is to
“identify a set of language control processes that support conversation in different interactional
contexts, articulate the relative demands of these contexts on these processes, and spell out the
neural bases of adaptive changes” (Green & Abutalebi, 2013, p. 516). In different interactional
contexts, potentially competing linguistic representations generated across languages may
emerge and require resolution at multiple levels and timepoints in the process of producing
and understanding language. Control processes such as goal maintenance and interference
control (i.e., conflict monitoring and interference suppression) are likely to be brought to bear
in such situations, and in some situations other processes such as selective response inhibition
and task engagement and disengagement might be required. A network of brain regions is
proposed to implement these component processes, mainly during language production
(Green & Abutalebi, 2013). These include bilateral inferior frontal gyrus, inferior parietal, insula,
dorsal striatum, thalamus, right cerebellum, anterior cingulate cortex, and pre-supplementary

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Figure 1. Brain regions comprising the Adaptive Control Model, based on Green and Abutalebi (2013). Cortical and subcortical brain regions
are proposed to be involved in language control and language switching, and in implementing various control processes proposed by the
Adaptive Control Model. LH = Left Hemisphere; RH = Right Hemisphere.

motor area (pre-SMA; see Figure 1). The expectation, from the perspective of the Adaptive
Control model, is that adaptive effects should be expressed in these regions that mediate con-
trol demands in bilingual contexts where these specific demands are high.
A complementary neural model that has also been proposed is the BAPSS (Grundy et al.,
2017). The central tenet of this model is that, for nonverbal executive tasks, “bilingualism is
associated with a model of efficient brain recruitment” in the form of less recruitment of “fron-
tal and executive regions” and greater recruitment of “posterior/subcortical regions” (p. 190).
However, other than naming the dorsolateral prefrontal cortices and anterior cingulate corti-
ces, the model is rather vague on what are the “basal ganglia and posterior regions.” In con-
trast to the Adaptive Control Model, which does specify a number of specific regional
expectations, the BAPSS model is vague to the point where a number of findings can still
fit within the model. As such, it does provide a framework for investigation, but further
refinement of the specific regions proposed to be affected by bilingual experience would
improve its usefulness.
Several fMRI studies have observed monolingual-bilingual differences in the regions
described in these models (with some of these studies published before and forming the basis
of these models). Studies focusing on cortical thickness or regional volume have reported dif-
ferences between monolinguals and bilinguals at the whole-brain level (Grogan et al., 2012;
Klein et al., 2014; Mechelli et al., 2004; Pliatsikas et al., 2014; Ressel et al., 2012; Zou et al.,
2012), and some report the difference in brain regions associated with language
control/executive control in, for example, the Adaptive Control Hypothesis (e.g., left inferior
parietal lobule in Mechelli et al., 2004; also see Abutalebi et al., 2015; left caudate nucleus in
Klein et al., 2014; Zou et al., 2012). Investigations based on region of interest more reliably
find differences in such regions (Abutalebi et al., 2015) and in others (e.g., auditory cortex;
Ressel et al., 2012), although several studies also report no reliable differences between

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monolingual and bilingual groups when the groups are examined at the whole-brain level
(Gold et al., 2013; Grogan et al., 2012; Ressel et al., 2012).
Similar studies examining differences in white matter diffusion properties, whether in white
matter regions or in specifically defined fibre pathways, have also been reported. Such findings
are, in some ways, even more difficult to interpret than regional morphometric differences with
respect to how they relate to bilingual executive function advantages. This is because the na-
ture of structural connections in white matter is difficult to ascribe to singular functions. First,
although there is an emerging consensus regarding which fibre pathways comprise a “lan-
guage connectome” (Dick et al., 2014), there is not a one-to-one mapping between function
and pathway, and some pathways are associated with both language and executive function
(Dick, Garic et al., 2019). Second, the white matter of the cortex is overwhelmingly populated
with crossing-fibres from multiple dissociable projections. Thus, regional differences in white

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matter may indicate structural differences in multiple crossing fibre pathways, or at least it may
be difficult to ascertain which pathway is contributing to the differences. Interpretation of
group differences in white matter are therefore a cautious exercise.
Considered alongside these important caveats, there are several findings of note in the lit-
erature looking at structural differences (see García-Pentón et al., 2016, for review). It is en-
couraging to note that reliable differences are found in white matter and fibre pathways
associated with processing language in monolingual populations (e.g., inferior fronto-occipital
fasciculus (IFOF), inferior longitudinal fasciculus (ILF), uncinate fasciculus (UF), and superior
longitudinal fasciculus (SLF); Cummine & Boliek, 2013; Gold et al., 2013; Luk, Bialystok et al.,
2011; Mohades et al., 2012; Mohades et al., 2015; Pliatsikas et al., 2015). But although at first
pass these differences seem encouraging, it is problematic that in some cases the same metric
under study (e.g., fractional anisotropy measuring directional diffusion of water in white mat-
ter) is higher in the bilingual group, and in other cases the metric is higher in the monolingual
group. Sometimes these contradictory effects occur in the same fibre pathways. For example,
Gold et al. (2013) show increased white matter in the IFOF for monolinguals while Luk,
Bialystok et al. (2011) show a decrease. Thus, as in the morphologic and ERP literature, incon-
sistency overwhelms a clear and concise neurobiological story. Some methodological issues
explaining such contradictions are likely to be unique to research on bilingual populations
(e.g., the definition of bilingualism, or the age-group under study). Others may be specific to
research using DWI (e.g., the post-processing pipeline, or the acquisition parameters of the
scan), or to both (e.g., age is a methodological confound for both bilingual research and
DWI). However, at present the research on structural differences offers an ambiguous and
inconsistent story about the neurobiology of bilingualism more generally (García-Pentón
et al., 2016), and does not strongly and reliably overlap with expectations from neurocognitive
models of the bilingual advantage for executive function.
fMRI studies have been used to examine whether activation dynamics in brain regions as-
sociated with executive function during non-language tasks are different across bilingual and
monolingual groups. In an often-cited and focused review, Pliatsikas and Luk (2016) examined
a corpus of such studies and concluded, based on this evidence, that bilingual experience has
consequences for brain activity in domain-general executive control regions beyond language
processing. In some cases, the reviewed studies indeed report bilingual-monolingual differ-
ences in areas associated with executive control, including the right caudate nucleus
(Mohades et al., 2014); anterior cingulate cortex (Garbin et al., 2010; Mohades et al.,
2014); and left parietal lobule (Ansaldo et al., 2015, although no direct comparison between
groups was provided; Luk et al., 2010). In addition, studies have identified differences in bi-
linguals and monolinguals in many other regions (e.g., in the bilateral cerebellum, bilateral

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superior temporal gyri, left supramarginal gyrus, bilateral postcentral gyri, and bilateral precu-
neus; Luk et al., 2010) that would not have been predicted by either the Adaptive Control
model or the BAPPS model. It is not always clear, therefore, how differences between bilin-
guals and monolinguals reflect the functioning of brain networks involved in domain-general
executive function. Furthermore, and similar to ERP studies, it is sometimes unclear whether
an increase or decrease in these regions would reflect more efficient processing in bilinguals,
and sometimes both directions have been observed (cf. Garbin et al., 2010 vs. Mohades et al.,
2014).

fMRI and behavioural data

The interpretation of the direction of activation differences becomes even more difficult when

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the findings are either not accompanied by behavioural differences or accompanied by a
behavioural disadvantage for the bilingual group. For example, Mohades et al. (2014) scanned
two groups of bilingual children and a group of monolingual children while they performed a
Simon task, and a Stroop task, in the MRI scanner. In prior behavioural studies, bilingual chil-
dren show smaller switch costs and faster RTs on these tasks (Bialystok et al., 2004; Coderre &
van Heuven, 2014). But in the study by Mohades and colleagues, the two bilingual groups ac-
tually showed significantly worse performance than the monolingual participants. Furthermore,
the brain differences were not related directly to behavioural performance. This, coupled with
the fact that the bilingual children actually performed worse on the tasks based on behavioural
measurements, calls into question the degree to which this study provides neuroimaging
support for a bilingual executive function advantage in Simon and Stroop tasks. This study is
cited as evidence in favor of the BAPSS model because it shows that children over-recruit
regions that are later engaged more efficiently by bilingual adults (Grundy et al., 2017). But
given the performance difference, with bilinguals performing more poorly, it is difficult to fit
this study within a model trying to explain bilingual advantages in executive function. This is
a good illustration of the problem of an under-specified neurobiological theory, because essen-
tially all findings fit the model.
Other studies find brain differences without behavioural differences (e.g., Ansaldo et al.,
2015; Luk et al., 2010). Despite the absence of behavioural differences, the latter study by
Luk et al. is cited to support the conclusion that “response inhibition and response selection
are distinguishable but related processes; and the recruitment of the more distributed network
for response selection by bilinguals suggests that they can rely on this network for interference
suppression more successfully than monolinguals” (Pliatsikas & Luk, 2016, p. 700). The lack of
a significant group difference might be due to the small sample size (n = 10), but regardless,
the lack of a behavioural difference makes the neuroimaging data difficult to interpret. Unlike
some studies reviewed above, the authors did examine the association between behavioural
performance and brain activation. But again, there was no statistically significant relationship
identified. Despite this, the authors reported that the pattern of brain-behaviour associations
was different between the groups.
Similar problems plague other studies, even those conducted more recently. For exam-
ple, a more recent study by DeLuca et al. (2020) using the flanker paradigm in an fMRI
study of bilinguals also found no behavioural difference—that is, measures of bilingual
language experience did not modulate behavioural performance on the task. In other
studies using task-switching paradigms, group differences in response time were not statis-
tically reliable, or direct comparison of activation differences was not statistically reliable
across the groups, or both (Garbin et al., 2010; Rodríguez-Pujadas et al., 2013). An exception

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to this is the study by Gold et al. (2013), which did report a bilingual advantage for task switch-
ing, although this only occurred for an older bilingual group, but not for a younger bilingual
group tested on the same task. Activation differences were found in left middle and inferior
frontal gyrus, and anterior cingulate cortical regions predicted by the Adaptive Control model,
but only for the older group. In terms of its overlap with the regions proposed by the Adaptive
Control Model, this might provide the strongest combination of behavioural and brain differ-
ences, but it still provides only partial support for the predictions of the model because the
finding only occurs in the older group. Thus, there is simultaneously evidence for and against
the model predictions.

Reverse inference

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The link between behaviour and brain is arguably critical for understanding the source of po-
tential cognitive benefits of bilingualism, but research groups differ in terms of how they view
the utility of behavioural data. Some have suggested that the lack of behavioural differences is
an advantage, and argue that “equivalent performance in the two groups allows meaningful
interpretation of the differences in functional neural correlates without the possible confound
of behavioral differences” (Luk et al., 2010, p. 355). De Luca et al. (2020) suggest that
neuroimaging data are more “granular” and by implication more sensitive and reliable, and
Grundy et al. (2017) make a complementary argument, stating that, when investigating the
bilingual advantage, matching behaviour allows brain differences to be discussed in the ab-
sence of a behavioural confound.
However, the opposite is arguably true. First, neuroimaging data tend to be noisier and less
sensitive than behavioural data (e.g., ADHD-200 consortium, Brown et al., 2012). Second,
and more importantly, we argue that the behavioural difference is not a confound, but rather
it is a condition on which the study is predicated. A coherent model of how bilingual expe-
rience shapes the specialization of neural regions involved in domain general executive func-
tion should at a minimum be able to relate performance differences to regional activity
proposed to be involved in the executive function processes of interest. In fact, any model
claiming a behavioural advantage for executive function in bilinguals should be able to show
evidence of such an advantage in the sample under investigation. As an analogy, a pharma-
ceutical company would not be providing convincing evidence of a new performance en-
hancing drug if they failed to show that it enhances performance, even if they could
reliably show differences in activation patterns in brain circuitry between treated and non-
treated participants.
In summary, although the conclusion of some research groups (e.g., DeLuca et al., 2020;
Grundy et al., 2017; Pliatsikas & Luk, 2016) is that bilingual experience has consequences for
neural processing during executive control tasks, the evidence is inconsistent, not always sup-
ported by behavioural differences or correlations with behavioural patterns, and suffers from
critical threats to internal validity. One of the biggest threats to internal validity, which applies
to both the fMRI and the structural imaging findings, is the strong reliance on what Poldrack
(2006) called “reverse inference.” Put simply, this is the process of reasoning backwards from
the presence of some brain activation or structural difference to the engagement of or differ-
ence in a particular cognitive function.
Why is reverse inference reasoning a threat to internal validity? At a definitional level, in-
ternal validity is about causal inferences, or more specifically that any observed covariation
between A and B reflects a causal relationship (Shadish et al., 2002). Because it is a charac-
teristic of a knowledge claim (Shadish, 1995), it is not inherent to research design, but the

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degree to which a claim has high internal validity is tightly bound to research design. Thus, to
support valid inference of causality, the research design must maximize three tenets of internal
validity: (1) A must precede B in time; (2) A must covary with B (which is implied), and (3) no
other explanations for the relationship are plausible.
When reverse inference reasoning is employed, it is often done without regard to design,
and in most cases, it violates at least two of the tenets described above, and sometimes all
three. Further, as Poldrack (2006) notes, fundamentally, reverse inference is a deductively in-
valid line of argument; that is, it is akin to the logical fallacy of affirmation of the consequent.
(Affirmation of the consequent is a formal logical fallacy. It takes the form “IF P, THEN Q. Q,
THEREFORE P.” For example, “IF an animal is a cat, THEN it has a tail. My dog has a tail,
THEREFORE it is a cat.”) It begins with the assumption of a one-to-one mapping between brain
activity / activity differences and a specific behaviour, taking the argument form, “If cognitive

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process A is engaged, then brain area B is active.” Then it applies the logical fallacy, “If brain
area B is active, cognitive process A is engaged” (i.e., affirmation of the consequent). If the
association between A and B were exclusive, this would not necessarily be invalid. That is,
when B occurs IF AND ONLY IF A occurs, there is the establishment of temporal precedence
and covariance, and a mitigation of alternative explanations for the association. But for brain
imaging data, this is rarely, if ever, the case. Even in primary cortical areas, exclusivity is not
established (e.g., activity in primary visual cortex does not imply the person is seeing anything
physically in the environment because visual cortex can show activity during visual halluci-
nations in the absence of sensory input; Pajani et al., 2015). In regions associated with higher-
order cognitive processes, selectivity of activation associated with a particular process is very
difficult to establish (Poldrack, 2006).
Despite this shortcoming, in the set of neuroimaging studies we reviewed, the logic of re-
verse inference was predominant. Thus, when behavioural differences are not established or
even tested, or the results are in the opposite direction from what was expected (i.e., bilinguals
perform worse), or when there is no tested or established association between brain and be-
haviour, there cannot be a valid statement about how fMRI or structural MRI data relate to
cognitive benefits of bilingualism. At best, using this line of reasoning is useful as a statement
of probability (Poldrack, 2006), but noting activation or structural differences in a region can-
not, by itself and without a link to behaviour, provide strong evidence that a cognitive process
is at work.

Individual differences in language experiences

Assessing whether there are structural or functional brain differences between bilinguals and
monolinguals is an interesting question on its own. Indeed, there is a large amount of literature
focusing on differences between bilinguals and monolinguals without making claims regard-
ing behavioural consequences or bilingual advantages. Recent frameworks have been pro-
posed to describe and understand how bilingualism might change brain structure and
functions (e.g., Pliatsikas, 2019), including a focus on different types of language learners
and bilinguals. Empirical work has started to examine how different bilingual experiences
can shape brain structure and function. For example, DeLuca et al. (2019) studied how L2
age of acquisition, L2 length of immersion, L2 use at home, and L2 use in social settings re-
lated to brain structure and connectivity. Their results showed relationships between the var-
ious language experiences examined and structure and connectivity measures, highlighting
the importance of studying individual differences in future research. Other recent studies have
also started to focus on individual differences between bilinguals in fMRI studies on executive

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control (e.g., Claussenius-Kalman et al., 2020; Del Maschio et al., 2020; DeLuca et al., 2020;
Gullifer et al., 2018). These studies will be of great use to further develop theories on neural
changes in response to language learning and/or bilingualism (e.g., Pliatsikas, 2019). In line
with similar developments in behavioural studies, these are very important and promising
pathways. However, similar to behavioural research, we do not know enough about how in-
dividual differences modulate language control in the brain to formulate concise hypotheses
about how these individual differences might modulate the neural correlates of executive con-
trol. Both behavioural and neuroimaging research on individual differences in language con-
trol is pivotal if we want to understand how bilingualism might modulate the structure and
function of the brain in relation to non-linguistic control.

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MOVING FORWARD
There is a clear and undisputable advantage that comes with bilingualism: being bilingual and
being able to communicate and connect with more people. Bilingualism and bilingual edu-
cation should therefore be promoted, regardless of the putative cognitive consequences. In this
review we focused on key issues affecting behavioural and neuroimaging research on bilin-
gualism and executive control: the need for falsifiable theories and the need for a better un-
derstanding of bilingual language control. The development of vague theoretical frameworks
and loose interpretation of behavioural data mean that existing theories are becoming more
difficult to falsify. We are not arguing that changes in theoretical frameworks or hypotheses are
problematic. To advance as a field, new data should stimulate the formulation of new hypoth-
eses and theories need to be updated based on new findings. The increasing emphasis on
individual bilingual experiences, moving away from comparisons between bilinguals and
monolinguals as groups without clear definitions of their language profile, is also laudable.
However, the research into bilingual language control and the relationship with individual
bilingual experiences is still in its infancy. Understanding this relationship is crucial for the
argument that language experiences might be related to executive control. Before we can form
clear hypotheses, we first need to clarify the link between language control and language ex-
periences. Until we know more about bilingualism, bilingual language control, and structural
and functional networks related to bilingualism, theories and hypotheses will remain vague.
This means theories can be supported regardless of their results and cannot be falsified. For
both behavioural and neuroimaging research on this topic to develop and improve, more pre-
cise theories on bilingual experiences and language control (as well as the relationship with
cognitive control) are needed.
To move forward, especially with a focus on individual differences between bilinguals, we
need more multi-lab collaborations in which researchers a priori agree on (and pre-register)
the type of participants, measures of interest, data collection, data analysis, and data interpre-
tation. Such a priori agreement will help with the development of clear, testable hypotheses
and can help to overcome the multiple interpretations that are possible when, for example,
different tasks show different findings. Such a collaboration across research groups will allow
for a more unbiased interpretation of data, which in turn can help to formulate clearer theories.
Moreover, multi-lab collaborations are especially important if we want to compare different
types of bilingualism. To directly compare different bilinguals and the role of individual expe-
riences, access is needed to multiple bilingual populations who complete the exact same study
and are analysed in exactly the same way.
It is not a solution to just keep adding data. For the research field to progress, we need to
take a step back. We need to study the cognitive and neural mechanisms underlying language

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Perspectives on the bilingual advantage debate

control. This knowledge can then be used to formulate clearer theories and hypotheses, which
in turn need to be tested in collaborations across research groups.

ACKNOWLEDGMENTS
The first author received funding from the European Union’s Horizon 2020 research and in-
novation programme under the Marie Skłodowska-Curie grant agreement number 743691.
The last author received funding from the Basque Government (2018–2021 BERC), the
Agencia Estatal de Investigacion: The Severo Ochoa Programme for Centres/ Units of
Excellence (SEV-2015-490) and grant (RTI2018-093547-B-I00).

FUNDING INFORMATION

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Angela de Bruin, H2020 Marie Skłodowska-Curie Actions (https://dx.doi.org/10.13039
/100010665), Award ID: 743691. Manuel Carreiras, Eusko Jaurlaritza (https://dx.doi.org/10
.13039/501100003086), Award ID: 2018-2021 BERC. Manuel Carreiras, Agencia Estatal de
Investigación (https://dx.doi.org/10.13039/501100011033), Award ID: SEV-2015-490.
Manuel Carreiras, Agencia Estatal de Investigación (https://dx.doi.org/10.13039
/501100011033), Award ID: RTI2018-093547-B-I00.

AUTHOR CONTRIBUTIONS
Angela de Bruin: Conceptualization: Equal; Writing – original draft: Equal; Writing – review &
editing: Equal. Anthony Dick: Conceptualization: Equal; Writing – original draft: Equal;
Writing – review & editing: Equal. Manuel Carreiras: Conceptualization: Equal; Writing – original
draft: Equal; Writing – review & editing: Equal.

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