The Immigration and Nationality Act of 1952 provided different rules for when a U.S.-citizen father could transmit citizenship to his child born out of wedlock than for when a U.S.-citizen mother could transmit citizenship to her child born out of wedlock. The rules for paternal transmission were far more stringent. In the context of an equal protection claim being evaluated under intermediate scrutiny, the Government proffered two interests (ensuring a sufficient connection between citizen children and the United States and avoiding statelessness), but the Second Circuit found that the statute was not substantially related to these or any other actual and important government interests. The court then severed the more onerous provision applied to fathers and deemed that the less stringent requirement for mothers would apply to both parents.
The Second Circuit's decision now creates a split with the Ninth Circuit, who affirmed the constitutionality of statute's differential treatment of parental transmission of citizenship.
The full text of Morales-Santana v. Lynch can be found here: http://www.ca2.uscourts.gov/decisions/isysquery/485dec25-f79a-4061-94c0-6aaecad5562c/1/doc/11-1252_opn.pdf#xml=http://www.ca2.uscourts.gov/decisions/isysquery/485dec25-f79a-4061-94c0-6aaecad5562c/1/hilite/