In the case of Tuan Anh Nguyen and Joseph Boulais v. Immigration and Naturalization Service, 2000, the U.S Supreme Court ruled on a matter concerning immigration law and gender discrimination. The petitioners were a father (Boulais) and his biological son (Nguyen), who was born in Vietnam to an unwed couple. When they moved to the United States, Boulais failed to meet certain statutory requirements necessary for his son's automatic citizenship because he did not legitimize him before he turned 18 years old as required by law for children born out of wedlock abroad with American fathers. After being convicted of sexual assault in Texas, Nguyen faced deportation due to lack of US citizenship despite having lived most of his life there under care from his father. The pair sued INS arguing that laws requiring different treatment based on gender violated equal protection principles embedded within Fifth Amendment's Due Process Clause; if roles were reversed - if it had been an American mother instead - their child would have received automatic citizenship without needing legitimization. However, the Supreme Court upheld this distinction between mothers and fathers by a 5-4 majority vote stating that such differentiation is justified given undeniable biological differences between sexes during childbirth process which makes establishing maternity more straightforward than paternity.

In the dissenting opinion for Tuan Anh Nguyen and Joseph Boulais v. Immigration and Naturalization Service, Justice O'Connor disagreed with the majority's decision to uphold a law that imposed different requirements on mothers and fathers seeking citizenship for their children born out of wedlock abroad. She argued that this distinction was based on outdated stereotypes about gender roles in parenting, which violated equal protection principles. Furthermore, she contended that there were less discriminatory alternatives available to achieve the government's interest in ensuring a biological relationship between parent and child. For instance, requiring both parents to prove a genetic connection or allowing evidence of real parental relationships would be fairer methods without resorting to sex discrimination.