Between Guesstimates and Estimates? A Case Study of Unregistered and Irregular Populations in Belgium

Sylvie Gadeyne , Vrije Universiteit Brussel
johan Surkyn, Vrije Universiteit Brussel
Pieter-Paul Verhaeghe, Vrije Universiteit Brussel

Measuring, modeling, and forecasting migration remains a persistent challenge due to inconsistent definitions and methodological limitations. Central issues include determining what constitutes a migratory boundary and how long a person must reside elsewhere to be considered a migrant. While regular migration is already complex to quantify, irregular migration is even harder to capture. This latter form of mobility has become a focal point of public debate, with terms such as irregular migrants, unregistered residents, or undocumented migrants commonly referring to individuals without legal residence permits—often facing the threat of deportation if identified. Reliable data on irregular migration are rare. According to UN OHCHR and ILO estimates from the late 2010s, migrants represented about 3.5% of the global population, with 15–20% living without documentation. Within Europe, the estimated number of irregular migrants in 2008 ranged from 1.9 to 3.8 million, though the EU-funded Clandestino Project later criticized these figures for their limited precision and methodological weaknesses. To address these shortcomings, this study introduces an alternative estimation technique grounded in mortality extrapolation, using Belgian death certificate data to infer the size of the irregular population. Applying this method, we estimate that around 490,000 “mobile” foreign residents were present in Belgium in 2016. Of these, approximately 329,000 were living without a regular legal status. The majority—around 217,000—were EU citizens, while roughly 112,000 were non-EU nationals lacking legal residency, diplomatic status, or pending asylum claims.

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 Presented in Session 92. Flash Session International Migration