Bias in fertility estimation for recent migrants

The total fertility rate, used naively, is biased in how it estimates the fertility of recent migrants. The reason is as follows (explained step by step):

• Many females have their children in the few years immediately after migrating. They postpone children until they have migrated, and stop having children after the few years immediately after migration. A major driver here is marriages between male natives of the country and female migrants.
• In particular, this means that if a country has recently become a lot more welcoming of migrants than it was in the past, recent migrants are overrepresented in the migrant count at every age. Thus, all the age-specific fertility rates for migrants are inflated due to dominance by recent migrants. The total fertility rate is therefore also an overestimate.
• Even if there hasn't been a dramatic increase in migration, the age-specific fertility rates are still an overestimate, because the years spent by migrants prior to migrating, where they have low fertility, are not included in the denominator when computing age-specific fertility rates. For instance, the people who migrate at age 21 and then have children add to the fertility in that age, but the people who migrate later (say, at age 25) aren't being counted in the denominator for people at age 21.

References

• Fertility Among Immigrant Women in France: New Data, a New Approach by Laurent Toulemon, Population Association of America Proceedings, (Year 2006): ^{Ungated PDFMore info}