Despite a meteoric rise, results in the cognitive science of bilingualism present with significant inconsistency. In parallel, there is a striking absence of an ecologically valid theory within bilingualism research. How should one interpret the totality of available data that can pull in opposing directions? To proceed, we need to identify which practices impede progression. Hitherto, we bring to the fore an undiscussed practice, contextualizing how it impacts the ability to embed the available results into an overarching theory. We suggest that a stacking the deck fallacy – the tendency to engage differently with available evidence, directing focus to specific sub-samples – hampers theory formation. We put forth a proposal for making progress, building on the premise that what is lacking in the field is a unifying perspective that reconciles seemingly contradictory results. We suggest that the necessary shift of perspective towards progress crucially entails linking the notions of spectrum and trade-off.

Being bilingual confers certain behavioral adaptations. The two ongoing discussions that surround their occurrence concern the type of the effect and its origin. The former can be analyzed in terms of three outcomes: positive, negative, and null. The status of the latter is less clear. While many studies recognize some interaction of cognitive factors with social factors such as socio-economic status and sociolinguistic prestige, these observations are often made in passing and lack critical detail. Consequently, it has not been yet determined what degree of the reported bilingualism-related behavioral adaptations derive primarily from sociolinguistic factors. This systematic PRISMA-based review addresses this question. Analyzing the results of 368 studies, we find that 73.41% of the 267 studies that report bilingual adaptations attribute them either to sociolinguistic factors alone or to the interaction of sociolinguistic factors with cognitive factors. Linking the two debates, type of effect and origin of effect, this systematic review finds a previously unreported correlation: Studies that find evidence for bilingual disadvantages are more likely to claim a sociolinguistic origin, while studies that report bilingual advantages are more likely to link their findings to a cognitive origin. We discuss these results and present the key components of a sociolinguistic theory of the origin of bilingual effects.

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