Anomalous distributions of birthdates across days of the month: An analysis using Spanish statistical records

This study investigates birthdate patterns in a context of well-established civil registers and intensive migration inflows. Leveraging extensive Spanish microdata on residential variation flows and the Spanish Municipality Register, this research reveals new facets of the distributions of immigrants’ birthdates across days of the month that differ significantly from those of non-migrants. The registered days of the month for birthdates are categorized into six distinct types based on the assumption that the anomalous distributions of birthdates will display rounding or simplifying patterns (digit preferences). The investigation reveals important anomalies in the distribution of birthdates that are much more pronounced for immigrants. A notable concentration of recorded birthdates is confirmed within all the designated types, contrasted by an under-recording of births on the remaining days of the month. These anomalies depend primarily on migrants’ country of origin and age group. The paper ends by proposing some recommendations for mitigating the anomalies.