A more thorough explanation:

Fiallo v. Bell is a legal case that was decided by the United States Supreme Court in 1977. The case dealt with a challenge to a section of the Immigration and Nationality Act of 1952 that granted special preference immigration status to certain family members of United States citizens or lawful permanent residents.

Fiallo v. Bell was a Supreme Court case that upheld the constitutionality of a section of the Immigration and Nationality Act of 1952 that granted special preference immigration status to legitimate children and their parents, or illegitimate children and their mothers, while excluding illegitimate children and their fathers.

For example, if a United States citizen had a child with a woman who was not his wife, that child would be considered illegitimate. Under the Immigration and Nationality Act of 1952, that child's mother would be eligible for special preference immigration status, but the child's father would not be. The Supreme Court upheld this distinction as constitutional.

The case of Fiallo v. Bell dealt with the question of whether it was constitutional for Congress to grant special preference immigration status to certain family members of United States citizens or lawful permanent residents, while excluding others. The Supreme Court held that Congress had the power to make such distinctions, and that it was not the role of the judiciary to second-guess those decisions. The Court also held that the distinction between legitimate and illegitimate children, and between mothers and fathers, was rational and served a legitimate government interest in preventing immigration fraud.