MPI-Penn State Methodology for Projections of Birthright Citizenship Change to U.S. Unauthorized Population
May 2025
Using broadly accepted demographic techniques, analysts from the Migration Policy Institute (MPI) and Penn State’s Population Research Institute projected the size of the U.S. population that is comprised of unauthorized immigrants and temporary visa holders, as well as the number of their U.S.-born children and subsequent generations from 2025 to 2075 for two scenarios:
(1) No change: Birthright citizenship remains in place as is.
(2) Repeal: Birthright citizenship would be denied for children born to parents who are unauthorized immigrants, temporary visa holders, or a combination of the two. (In other words, automatic birthright citizenship would be reserved for children born to parents who are U.S. citizens or lawful permanent residents [LPRs, also known as green-card holders].) For the purpose of this analysis, affected parents who are unauthorized immigrants or temporary visa holders (for example international students or H-1B holders) are referred to here as “non-birthright” parents.
The two scenarios differ in how citizenship status is assigned to children. For the “no change” scenario, all children born in the United States are considered citizens (currently only a tiny fraction of births on U.S. soil, chiefly the children of foreign diplomats, do not receive U.S. citizenship at birth). For the “repeal” scenario, children of non-birthright parents are deemed “unauthorized.”
The projections modeled here apply the repeal scenario prospectively to births estimated to occur from 2025 through 2075, considering all U.S.-born children prior to that period to be U.S. citizens. Additionally, the repeal scenario is applied only to children with two non-birthright parents. If at least one parent is a U.S. citizen or has a green card, the child is considered a U.S. citizen.
In all other respects, the scenarios are the same. Both assume steady-state demographic behaviors for unauthorized immigrants and temporary visa holders, meaning that their annual rates of in-migration and out-migration, mortality, and fertility observed since 2017 are assumed to continue at the same levels in the future. Holding these factors constant across the two scenarios helps isolate the effects of repeal. That said, it is possible that immigrants would be less likely to come to the United States illegally or more likely to leave the country if the repeal of birthright citizenship takes place, so the MPI-Penn State researchers also considered more complex scenarios in which repeal is accompanied by changes in migration (see below).
The researchers used the cohort-component method to project the size and characteristics of the population that would be deemed “unauthorized” in the future (comprised of both the unauthorized immigrant population as well as those born in the United States who would not receive U.S. citizenship at birth). This well-established method starts with a “baseline” population as it is currently observed (the unauthorized immigrant population as well as the temporary visa-holding population), broken down into age and sex groupings. It then extrapolates what the “unauthorized” population’s size would be in future years if this population comprised of unauthorized immigrants and the U.S. born not receiving U.S. citizenship at birth adhered to a given set of age-specific fertility, mortality, and migration rates. To do this, the method first multiplies the baseline population by these rates to calculate the expected number of births, deaths, and migrations expected that year. The expected population size for the following year is then obtained by projecting from the baseline population, i.e., adding the expected number of births and new immigrants, subtracting the expected number of deaths and out-migrants, and aging the surviving population. This is repeated for each year across the entire projection period.
As with all population projections, these rest on data inputs and assumptions, which are described below.
Baseline Population: The projections work begins with MPI’s mid-2023 estimates of the size and national-origins composition of the unauthorized immigrant population, the most recent year available. According to these estimates, as of mid-2023, there were 13.7 million unauthorized immigrants in the United States and an additional 2.4 million temporary visa holders. (The latter includes a small number of resettled refugees and asylees who had yet to obtain LPR status.)
Fertility: The researchers used the 2018–22 panels from the U.S. Census Bureau’s Survey of Income and Program Participation (SIPP) to obtain information on fertility for women by nativity, ethnicity, and age. The analysts assumed that unauthorized immigrant and temporary visa-holding women have the same age- and group-specific fertility rates as observed in the SIPP among noncitizen women without lawful permanent residence. Given the executive order signed by President Donald Trump on January 20, 2025 would deny U.S. citizenship at birth to children with two non-birthright parents, only births to non-birthright women who have a non-birthright partner or spouse were counted.
Mortality: The projections assume that the “unauthorized” population (comprised of unauthorized immigrants and the U.S. born who would not receive U.S. citizenship at birth) has the same sex- and age-specific mortality rates as the overall foreign-born population in 2023. The projections are relatively insensitive to assumptions about mortality.
Migration: The models here assume that a fixed number of unauthorized immigrants and temporary visa holders will move to the United States each year during the projection period, with the number of new arrivals matching the average annual numbers (by age, sex, and country of birth) observed between 2017–19 (selected because these years were a period of moderate levels of immigration, not as low as during the COVID-19 pandemic in 2020 and 2021 but not as high as during record border arrivals of 2022–23). The researchers also assumed that these groups will depart from the country or legalize at the same rates (by age, sex, country of birth, and duration of residence) as observed in 2017–19. Finally, they assumed that U.S.-born children of non-birthright immigrant parents will emigrate at the same rate as unauthorized immigrants, and that the U.S.-born children of U.S.-born parents lacking citizenship do not emigrate.
Differing Assumptions About Migration
The MPI-Penn State projections rest on the assumption that unauthorized immigrants will continue to behave as they do now: they and their descendants will continue to have children and die at the same rates as they do today, and the foreign-born unauthorized population will continue to arrive and depart at the same rate as it did in the late 2010s.
But how would the estimates change if different assumptions were made?
One hypothetical possibility is that repeal of automatic birthright citizenship for all U.S.-born babies, in combination with increased border enforcement and ramped-up deportations, would reduce illegal migration. To account for this possibility, the analysts examined the likely outcome under an extreme scenario in which all future unauthorized immigration stopped as of 2025 (but migration of those with temporary immigrant visas continued) and return migration rates for both unauthorized migrants and temporary visa holders doubled. Doubling the return migration rate would require an enormous allocation of resources to enforcement and deportation operations, but would come close to the Trump administration’s goal of deporting 1 million people per year.
Even in this circumstance, the change to birthright citizenship would still lead to a larger unauthorized population than if birthright citizenship remained unchanged. This would be due to U.S. births to non-birthright parents and grandparents. There would be 2.5 million such births over the next two decades, with an estimated 1.3 million of these still living in the country by 2045 (after accounting for emigration and deaths, as well as the fact some could gain legal status).