According to Science Daily:
http://www.sciencedaily.com/releases/2010/09/100908132222.htm?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+sciencedaily+%28ScienceDaily%3A+Latest+Science+News%29Unauthorized Population Would Soar If Birthright Citizenship Repealed, Report Finds...
Of the estimated 4.7 million unauthorized immigrants who had been born in the United States as of 2050, 1 million would have two U.S.-born parents. The share of all U.S. children in 2050 who would be unauthorized would double, from 2 percent currently to 4 percent, under the proposed law.
Alternative scenarios that would limit citizenship beyond the proposed Birthright Citizenship Act -- for example, by denying U.S. citizenship to children who have one unauthorized immigrant parent -- would generate even higher unauthorized population estimates. The total unauthorized population would rise to 24 million in 2050 under a scenario in which citizenship is denied to U.S.-born children who have one unauthorized immigrant parent, even if the other parent were a U.S. citizen.
"What is less commonly understood in the current debate is that repeal of birthright citizenship would set in motion the creation of a self-perpetuating class of unauthorized immigrants," said the report's co-author, Michael Fix, senior vice president and director of studies at the Migration Policy Institute.
Under a scenario of denying birthright citizenship to children who have at least one authorized immigrant parent, by the third generation, 6.3 million U.S.-born people would be unauthorized despite having two U.S-born parents.
"This perpetuation of hereditary disadvantage based on the legal status of one's ancestors would be unprecedented in U.S. immigration law," Fix said.
The Migration Policy Institute is an independent, non-partisan think tank dedicated to analysis of the movement of people worldwide. Its report on birthright citizenship is available at www.migrationpolicy.org/pubs/BirthrightInsight-2010.pdf.