Tuan Anh Nguyen was born in Vietnam to an American father and a Vietnamese mother who were not married. He moved to the United States with his father and became a legal permanent resident of the U.S. at age six, but his father did not attempt to establish any claim of U.S. citizenship for the boy. At age 22, Nguyen pleaded guilty to sexual assault; this made him subject to deportation based on his criminal record.

Nguyen’s father obtained evidence of parentage in an attempt to have his son recognized as a U.S. citizen, but his efforts were rejected by the Immigration and Naturalization Service (INS) because 8 U.S.C. § 1409 required any such evidence to have been presented before the child’s 18th birthday. Nguyen—together with his father—mounted a court challenge to the law, claiming that 8 U.S.C. § 1409 was unconstitutionally discriminatory because it imposed stricter requirements for a foreign-born illegitimate child of an American father than would have applied if his American parent had been his mother.

The Supreme Court rejected Nguyen’s arguments and upheld the law denying him citizenship, holding by a 5–4 majority that 8 U.S.C. § 1409 was consistent with the equal protection principle, applied through the Due Process Clause of the Fifth Amendment to the Constitution.

First, the Court noted that whereas a mother’s biological relationship to her child is easily verified and documented, the same cannot be said of the father.

Second, the Court concluded that the law was designed “to ensure that the child and citizen parent have some demonstrated opportunity to develop… a relationship… that consists of the real, everyday ties that provide a connection between child and citizen parent and, in turn, the United States”—something that was inherent in the case of an American mother and her child, but not inevitable in the case of a single father.

Even though Nguyen’s father had submitted DNA evidence proving the father-son relationship, the Court noted that “scientific proof of biological paternity does nothing, by itself, to ensure contact between father and child during the child’s minority”. In the end, the Court held that Congress was “well within its authority in refusing, absent proof of at least the opportunity for the development of a relationship between citizen parent and child, to commit this country to embracing a child as a citizen”.

The dissent (written by Associate Justice Sandra Day O’Connor) concluded that the INS “[had] not shown an exceedingly persuasive justification for the sex-based classification… because it [had] failed to establish at least that the classification substantially relate[d] to the achievement of important government objectives”, and on that basis the minority would have ruled in Nguyen’s favor.