The provision of the Fourteenth Amendment to the American Constitution that appears to (see Posner’s discussion for why I say “appears to”) guarantee all children born in the United States automatic citizenship applies not only to children born to parents who are in this country illegally, but also to students, tourists, and others who are in the country on temporary visas. Yet most of the anger felt about giving citizenship to all children born here is directed at children of illegal immigrants.
This is partly due to the illegality of the presence of their parents in the US, and partly to the evidence that some women cross the Mexico-US border, or come from other countries, just to have their children become Americans. The hostility is also fueled by the apparently correct belief that children born of illegal parents are a significant contributor to the total number of births in the United States. The Pew Hispanic Center estimates that about 8% of all children born in the US in 2008 have parents who are in the country illegally. This is about double the estimated percent of the US population that are here illegally. Although it is not possible to know how accurate these estimates are, one would expect Mexican immigrants to have relatively many children, even if the children did not automatically become citizens, since these immigrants tend to be younger and less educated than the typical American. Extensive studies would be necessary before one could know how many of the children of illegal immigrants were born here because of the guarantee of American citizenship.