Unauthorized immigrants comprise slightly more than 4% of the adult population of the U.S., but because they are relatively young and have high birthrates, their children make up a much larger share of both the newborn population (8%) and the child population (7% of those younger than age 18) in this country.

These figures are based on data from the U.S. Census Bureau's March 2009 Current Population Survey, augmented with the Pew Hispanic Center's analysis of the demographic characteristics of the unauthorized immigrant population using a "residual estimation methodology" it has employed for the past five years.

The new Pew Hispanic analysis finds nearly four-in-five (79%) of the 5.1 million children (younger than age 18) of unauthorized immigrants were born in this country and therefore are U.S. citizens.

In total, 4 million U.S.-born children of unauthorized immigrant parents resided in this country in 2009, alongside 1.1 million foreign-born children of unauthorized immigrant parents.

The 14th Amendment to the U.S. Constitution, adopted in 1868, grants an automatic right to citizenship to anyone born in the U.S.

In recent weeks, a number of prominent elected officials have called for the repeal of birthright citizenship, which they argue serves as one of the magnets that attract undocumented immigrants to the United States.

A nationwide survey by the Pew Research Center for the People & the Press in June found that, by 56% to 41%, the public opposes changing this provision of the Constitution.

This report does not address the merits of the birthright citizenship debate. Rather, it analyzes the family structure and parenting status of unauthorized immigrants. A follow-up Pew Hispanic Center report, expected to be released in several weeks, will examine trends in the size of the unauthorized population and key demographic characteristics, including its geographic settlement patterns; its countries and regions of origin; and its economic circumstances.