Whether bilingualism improves executive control, and how much of the apparent evidence for that 'bilingual advantage' is an artifact of selective publication, remains contested. de Bruin, Treccani, and Della Sala (2015) showed that conference abstracts supporting the bilingual-advantage theory were more than twice as likely to be published as abstracts challenging it, but could not decompose this bias into author non-submission versus editorial rejection, and could not see results that never reached a conference abstract at all. The problem is to quantify the relative contributions of these mechanisms to the outcome-dependent publication process in bilingualism-executive-control research, and - once unpublished null and mixed findings are fully incorporated - to establish the true magnitude of the bilingualism effect on executive functions and the boundary conditions under which it does and does not appear, including whether the higher task counts of outcome-challenging studies reflect selective reporting, Type I error inflation, or genuine task-specific modulation of the effect. Later meta-analyses and very large population studies have narrowed the magnitude question for unselected adult samples, but the mechanism decomposition and the boundary conditions remain open and actively contested.

Background

Executive functions (EFs) are the domain-general control processes that regulate thought and action: inhibiting dominant responses, shifting between task rules or mental sets, holding and updating information in working memory, and monitoring for conflict. They are commonly measured with conflict tasks such as the flanker and Simon tasks, in which a target must be identified despite irrelevant distractors, or with task-switching paradigms that measure the cost of changing rules. The 'bilingual advantage' hypothesis holds that a lifetime of managing two languages - selecting the intended language while suppressing the unintended one - acts as ongoing training for exactly these control processes, so that bilinguals should outperform otherwise comparable monolinguals on nonverbal EF tasks.

The hypothesis was influential, but it has always had a replication problem: many studies, especially large ones, find no difference. One diagnosis is publication bias - the tendency for studies reporting supportive, statistically significant results to reach journals more often than studies reporting null or contradictory results. Its best-known form is the 'file drawer' problem, in which authors never submit null results at all; editorial rejection of submitted null manuscripts is the complementary mechanism. Because meta-analyses can synthesize only what is published or otherwise recoverable, outcome-dependent publication can manufacture a field-wide 'effect' even if the underlying effect is absent or far smaller.

de Bruin, Treccani, and Della Sala (2015) tested this directly for the bilingualism literature with a conference-abstract tracking design. They located conference abstracts on bilingualism and executive control presented between 1999 and 2012, classified each abstract's outcome as supporting, mixed, or challenging the bilingual-advantage theory (104 abstracts in the analyzed set), and traced which studies were later published as journal articles. Supporting studies were most likely to be published, mixed studies less so, and challenging studies least (a later recounting of the dataset puts the rates at roughly 68% versus 29%); the gap was not explained by sample size, tasks used, or statistical power, and a funnel-plot test (a plot of effect size against study precision, whose asymmetry signals small-study or selective-publication effects) supported the presence of bias. Crucially, the study could not decompose the mechanism: a survey of authors about submission history drew too few responses to distinguish author non-submission from editorial rejection, and the design cannot observe negative results that were never even submitted as conference abstracts - so the measured bias may underestimate the problem. Two further loose ends were left open: abstracts challenging the advantage reported more tasks than supporting ones (selective reporting of the one positive task among many? inflated false positives - Type I errors - when only one task is tested? or genuine task-specific effects?), and the true magnitude of the bilingualism-EF effect once unpublished null and mixed findings are included remained unknown, as did the boundary conditions - the circumstances under which bilingualism does and does not affect executive functions.

The subsequent decade made the question sharper but not settled. Meta-analysis in adults found only very small advantages that vanish entirely after correction for publication bias, and very large population studies found no advantage; yet other quantitative syntheses still report advantages that depend on task and age, or argue that bilinguals outperform monolinguals more often than chance even if the average effect is tiny. The magnitude of the publication bias itself became disputed: a 2023 re-analysis of the original conference-abstract dataset argues that merging 'null' with 'negative' outcomes into one category, and excluding dissertations and other grey literature from what counts as 'publication', jointly overinflated the original bias estimate. Meanwhile the field's center of gravity moved toward asking under which sociolinguistic conditions any effects arise. What has not changed since 2015 is that no study has quantified, for this literature, how much of the outcome-dependent publication process is author non-submission versus editorial rejection versus attrition before studies reach even a conference abstract; that mechanism question has only been answered for a different, pre-registered literature (the TESS - Time-sharing Experiments for the Social Sciences - survey-experiment program in the social sciences, where author non-submission dominates).

Problem Statement

Determine the evidential value of the literature on bilingualism and executive control, in the specific form left unresolved by de Bruin, Treccani, and Della Sala (2015):

(1) Mechanism and magnitude of the outcome-dependent publication process. Decompose the observed publication-rate difference - conference abstracts supporting the bilingual-advantage theory were more than twice as likely to be published as abstracts challenging it - into the relative contributions of (i) author non-submission (the file-drawer problem), (ii) editorial rejection of submitted manuscripts reporting null or challenging results, and (iii) attrition upstream of the conference-abstract stage, where negative results may never be presented at all. Because the original study's author survey drew too few responses to distinguish the two submitted-side mechanisms, the decomposition must be reconstructed from what the recoverable record supports - the original conference-abstract cohort and its traceable publication outcomes, alongside registry, preprint, and forward-citation evidence - with the shares that cannot be observed directly (author non-submission, editorial rejection, and attrition before the conference-abstract stage) bounded by an explicit sensitivity analysis over their possible values, and with classification and grey-literature inclusion criteria robust enough to withstand the reclassification critiques later raised against the original analysis.

(2) True effect and its boundary conditions. Once the unpublished null and mixed findings so recovered are incorporated - alongside grey-literature outputs such as dissertations, whose exclusion from 'publication' is itself contested - estimate the true magnitude of any bilingualism effect on executive control and characterize the boundary conditions: the populations, ages, tasks, and language-use or sociolinguistic contexts in which bilingualism does and does not affect executive functions. This includes resolving why outcome-challenging conference abstracts reported more tasks than supporting ones: selective reporting of positive tasks, Type I error inflation when only one task is tested, or genuine task-specific modulation of the effect.

The two components are linked: the unpublished studies recovered under (1) are the input that corrects the effect estimate under (2), and the decomposition in (1) explains how, and at which stage, the published record is distorted. The decomposition in (1) covers the recoverable record, and the shares that cannot be observed directly are bounded by an explicit sensitivity analysis over their possible values. The effect estimate in (2) is obtained with and without the recovered unpublished, mixed, and grey-literature findings, and is interpreted through the boundary conditions - the populations, ages, tasks, and language-use or sociolinguistic contexts under which the effect does and does not appear. Both depend on the classification and grey-literature inclusion choices that later work showed to be consequential, and the sensitivity analysis over those choices establishes which conclusions survive them and which do not.

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