Historically, putative fathers have had fewer rights with regard to their children than either unwed mothers or married parents. Over the past several decades, putative fathers have used the Fourteenth Amendment to challenge the termination of their parental rights when the birth mothers relinquish their children for adoption. Nevertheless, States have almost complete discretion to determine the rights of a putative father at termination of parental rights or adoption proceedings. The U.S. Supreme Court has affirmed the constitutional protection of a putative father's parental rights when he has established a substantial relationship with his child. The Court defined a substantial relationship as the existence of a biological link between the child and putative father, and it defined the father's commitment to the responsibilities of parenthood as participating in the child's upbringing.