A data-driven approach to the issue of ”catching up with monolinguals”

BACKGROUND: Multilingual children in monolingual education systems are typically assessed against monolingual norms, complicating differentiation between normal multilingual development and genuine language disorders. When language experience is distributed across multiple languages, clinicians and educators struggle to determine whether multilingual learners need routine support or specialist intervention for developmental language disorders.AIMS: This study identified: (1) the cumulative societal language (SL) exposure threshold beyond which multilingual children perform within monolingual proficiency ranges, and (2) when cumulative SL experience no longer predicts SL proficiency.METHODS: We examined multilingual learners (ages 5–9) across three societal languages (English, French, Dutch), assessing morphosyntax via sentence repetition and receptive vocabulary (breadth and depth). Binary partition and broken stick regression identified exposure-outcome thresholds while controlling for environmental and cognitive factors. Cumulative SL exposure was measured in month-equivalents.RESULTS: For morphosyntax, multilingual children required minimum 59 month-equivalent cumulative SL exposure to reach monolingual performance ranges. Diminishing returns occurred after approximately 57 months (4 years), beyond which morphosyntactic outcomes were no longer strongly exposure-constrained. For vocabulary breadth, no clear threshold emerged; significant differences persisted even at 94 month-equivalent exposure, with outcomes predicted more by cognitive factors and language experience richness than cumulative exposure. Vocabulary depth showed a threshold at 91 months, though sample limitations prevent firm conclusions.CONCLUSIONS: Results demonstrate domain-specificity: morphosyntax and vocabulary show different exposure-outcome dynamics. Multilingual children require approximately 93 month-equivalent cumulative SL exposure for full morphosyntactic competence comparable to monolinguals. Before 57 months cumulative exposure, difficulties are developmentally expected; specialist referral should occur if concerns persist beyond this threshold. These findings provide research-informed guidance for contextualizing assessment expectations based on multilingual backgrounds, moving from deficit-based comparisons toward recognizing expected developmental trajectories.