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This study aimed to explore the phenomenon of birthdate misregistration, using birth data from 45,226,875 Polish citizens, that is, all those born 1900–2000 and registered in Poland’s Universal Electronic System for Registration of the Population (PESEL). I transformed the data into a daily series of births, detrended by dividing each value by the daily average for the relevant year. Next, I selected the dates with the highest deviations based on the coefficients of the linear regression model with dummy variables. Finally, I estimated the size of the phenomenon in subsequent years by comparing the numbers of births on selected dates to their expected values. This paper is the first to document the specificity, scale, duration, and probable causes of birthdate misregistration in Poland in the twentieth century.

Are the birthdates of our ancestors real? Date of birth misregistration in twentieth-century Poland