There are two basic methods for acquiring U.S. citizenship at birth: by place of birth or by descent. The Fourteenth Amendment provides for so-called birthright citizenship (jus solis)—any person born in the U.S. is a citizen, regardless of the citizenship of his or her parents.
Congress has also acted to provide an additional means to acquire citizenship at birth—by descent (jus sanguinis). In many countries, especially those with legal systems based on Roman law, this is the exclusive method by which individuals can acquire citizenship at birth. In the U.S., however, this method of acquiring citizenship is only relevant to the status of babies who were born abroad and thus do not have the benefit of jus soli birthright citizenship. Because this means citizenship is not rooted in the Constitution, Congress has the authority to craft the terms on which it is offered (as long as, in doing so, it doesn’t violate other provisions of the Constitution such as the Equal Protection Clause).
Joanna L. Grossman,
Flag-Waving Gametes: Biology, Not Gestation or Parenting, Determines Whether Children Born Abroad Acquire Citizenship from U.S. Citizen Parents Verdict
Available at: https://scholarlycommons.law.hofstra.edu/faculty_scholarship/942