Justice O'Connor argued that the majority recited heightened scrutiny but did not actually apply it. Under the Court's sex-discrimination cases, the Government bears the demanding burden of providing an exceedingly persuasive, genuine justification for a classification. A court may not invent post hoc interests for the statute or uphold sex-based rules through the more deferential reasoning associated with rational-basis review.
The majority's first asserted interest—verifying a biological parent-child relationship—did not substantially justify § 1409(a)(4). Section 1409(a)(1) already required clear and convincing proof of a blood relationship, and DNA evidence offered highly reliable proof. The additional pre-18 legitimation, acknowledgment, or adjudication requirement added little to proving biology, especially because a sworn acknowledgment may prove little and an adjudication could occur over the father's objection.
The second asserted interest—ensuring an opportunity for a meaningful relationship between the citizen parent and child—was likewise too attenuated. A father and child may have both an opportunity for, and an actual, close relationship without formalizing paternity before the child turns 18, as Nguyen and Boulais did. Conversely, a formal adjudication need not establish any relationship. The statute therefore did not substantially advance either an actual relationship or the mere opportunity for one.
Justice O'Connor emphasized that readily available sex-neutral alternatives exposed the statute's poor fit. Congress could require proof of parentage from either parent, presence at or knowledge of birth, or actual contact between the citizen parent and child. Treating a mother as sufficiently connected merely because she gave birth while requiring an additional act from a father who was present at birth treated similarly situated parents differently solely because of sex.
In the dissent's view, the law also reflected a historical regime that assigned responsibility for nonmarital children to mothers while allowing fathers to avoid it. The majority's reasoning risked validating the stereotype that mothers will form caring relationships with children while fathers may not. Justice O'Connor would have held § 1409(a)(4) unconstitutional, severed the offending provision under the INA's general severability clause, and rejected the claim that immigration-law deference displaced ordinary heightened scrutiny in a case about citizenship at birth.