Courts may set a higher threshold for fathers than mothers when deciding the citizenship of children born overseas and out of wedlock, the Supreme Court ruled Monday. 

In a Texas case involving a man born in Vietnam to an American father and Vietnamese mother, the court ruled narrowly that setting such separate standards does not violate the equal protection clause of the Constitution, given a mother's natural role in childbirth. 

"To fail to acknowledge even our most basic biological differences — such as the fact that a mother must be present at birth but the father need not be — risks making the guarantee of equal protection superficial," said the majority opinion by Justice Anthony Kennedy. 

The Texas father, Joseph Boulais and his son, Tuan Ahn Nguyen, had argued the law amounted to "sex-based stereotyping" and unlawful discrimination against men. 

Government lawyers countered that Congress has broad authority to decide who is entitled to U.S. citizenship. 

Although the case was filed by two men challenging a law they said was unfair to men, the case was closely watched by women's rights groups for signs of how the current court views issues of gender equality. 

In 1996, when the justices ruled that Virginia Military Institute must admit women as students or give up its state funding, the justices said there must be an "exceedingly persuasive justification" for any government action based on gender. 

The ruling upheld a federal appeals court. Kennedy was joined in the majority by Chief Justice William Rehnquist and Justices John Paul Stevens, Antonin Scalia and Clarence Thomas. The two female justices, Sandra Day O'Connor and Ruth Bader Ginsburg dissented, along with Justices David Souter and Stephen Breyer. 

Federal immigration law automatically gives citizenship to children in Tuan's circumstance if their mother is an American and has previously lived in this country for at least a year. 

But if the father is American, the child can be considered a U.S. citizen only if the father legalized the relationship through a court order or sworn statement by the time the child turned 18, and if the father agreed in writing to support the child until adulthood. 

Kennedy said that calling these rules stereotypes would "obscure ... prejudices that are real." 

"The difference between men and women in relation to the birth process is a real one, and the principle of equal protection does not forbid Congress to address the problem at hand in a manner specific to each gender," he wrote. 

O'Connor, in the dissent, said the majority did not follow prior rulings in which the court applied a more careful analysis of gender-based equal-protection cases. 

Tuan was born in Vietnam in 1969 and abandoned by his mother at birth. Boulais brought him to the United States in 1975 and raised him in Texas. 

In 1992, Nguyen pleaded guilty in state court to two charges of sexually assaulting a child and was sentenced to eight years in prison. 

The Immigration and Naturalization Service then began deportation proceedings, contending Nguyen was an alien convicted of an aggravated felony. 

Nguyen argued he should be considered a U.S. citizen because of his father's citizenship. In 1998, he obtained a Texas court ruling that declared Boulais to be his father. 

Immigration authorities, however, said the court order came too late and ordered him deported. The 5th U.S. Circuit Court of Appeals also ruled against Nguyen, saying a mother "will obviously be with the child at its birth" but fathers can be required to do more to demonstrate a biological relationship.