Adding a citizenship question to the census reduces the participation of people who aren’t U.S. citizens, particularly those from Latin American countries, according to a new research paper that comes as Republicans in Congress are pushing to add such a question to the census form.

Noncitizens who pay taxes but are ineligible to have a Social Security number are less likely to fill out the census questionnaire or more likely to give incomplete answers on the form if there is a citizenship question, potentially exacerbating undercounts of some groups, according to the paper released this summer by researchers at the U.S. Census Bureau and the University of Kansas.

Other groups were less sensitive to the addition of a citizenship question, such as U.S.-born Hispanic residents and noncitizens who weren’t from Latin America, the study said.

The paper comes as Republican lawmakers in Congress push to require a citizenship question on the questionnaire for the once-a-decade census. Their aim is to exclude people who aren’t citizens from the count that helps determine political power and the distribution of federal funds in the United States. The 14th Amendment requires that all people are counted in the census, not just citizens.

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In May, the GOP-led House passed a bill that would eliminate noncitizens from the tally gathered during a census and used to decide how many House seats and Electoral College votes each state gets. The bill is unlikely to pass the Democratic-controlled Senate. Separately, the House in coming weeks is to consider an appropriations bill containing similar language seeking to omit people in the country illegally from the count used to redraw political districts.

During debate earlier this month at a House appropriations committee meeting, Democratic U.S. Rep. Grace Meng of New York described the efforts to exclude people in the country illegally as “an extreme proposal” that would detract from the accuracy of the census.

“Pretending that noncitizens don’t live in our communities would only limit the crucial work of the Census Bureau and take resources away from areas that need them the most,” Meng said.

But Republican U.S. Rep. Andrew Clyde of Georgia argued that including people in the country illegally gives state and local governments an incentive to attract noncitizens so that they can have bigger populations and more political power.

“Every noncitizen that is included actually takes away from citizens’ ability to determine who their representatives are,” Clyde said.

The next national head count is in 2030.

In their paper, the Census Bureau and Kansas researchers revisited a study assessing the impact of a citizenship question on a 2019 trial survey that was conducted by the Census Bureau ahead of the 2020 census.

The trial survey was conducted by the Census Bureau as the Trump administration unsuccessfully attempted to add a citizenship question to the 2020 head count’s questionnaire. Experts feared a citizenship question would scare off Hispanics and immigrants from participating in the 2020 census, whether they were in the country legally or not. Years earlier, a Republican redistricting expert had written that using citizen voting-age population instead of the total population for the purpose of redrawing of congressional and legislative districts could be advantageous to Republicans and non-Hispanic whites.

The citizenship question was blocked by the Supreme Court in 2019.

As part of the trial survey, test questionnaires were sent by the Census Bureau to 480,000 households across the U.S. Half of the questionnaires had a citizenship question and the other half didn’t. Preliminary results showed that adding a citizenship question to the 2020 Census wouldn’t have had an impact on overall response rates, even though earlier studies had suggested its inclusion would reduce participation among Hispanics, immigrants and noncitizens. Later analysis showed it would have made a difference in bilingual neighborhoods that had substantial numbers of non-citizens, Hispanics and Asians.

Instead of focusing on census tracts, which encompass neighborhoods as in the 2019 study, the new study narrowed the focus to individual households, using administrative records.

“The inclusion of a citizenship question increases the undercount of households with noncitizens,” the researchers concluded.

During the 2020 census, the Black population had a net undercount of 3.3%, while it was almost 5% for Hispanics and 5.6% for American Indians and Native Alaskans living on reservations. The non-Hispanic white population had a net overcount of 1.6%, and Asians had a net overcount of 2.6%, according to the 2020 census results.

The once-a-decade head count determines how many congressional seats and Electoral College votes each state gets. It also guides the distribution of $2.8 trillion in annual federal spending.

The research paper was produced by the bureau’s Center for Economic Studies, whose papers typically haven’t undergone the review given to other Census Bureau publications. The opinions are those of the researchers and not the statistical agency, according to the bureau.

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