It can be difficult for a father to prove his right over his biological child. At the same time, it can also be a very happy time in the father's life.
In the event that the child is born to two married people, or even during the course of a longstanding relationship, the father might sign all of the necessary documents to acknowledge paternity because he believes that he is the father of the child. However, there have been cases in which it has been discovered later on that the mother lied about the paternity of the child and the father named on the documents is not the biological father.
Paternity fraud has, much like all issues relating to paternity, become a subject of debate recently. Historically, establishing paternity was very important for the mother and the child, as a child born without the father's name would have been considered illegitimate. That child would have been shunned by society. However, the law recognized the negative implications of the child being considered illegitimate and it made paternity binding under the Mansfield Rule, which says that the paternity was acknowledged by the father voluntarily. However, in the past, genetic testing did not exist yet, so paternity could not be proved scientifically.
Now, fathers' rights advocates are fighting for the right to nullify paternity on the grounds of paternity fraud. Under Arizona law, the right to disestablish claims to paternity is based under the principle of evidence where there must be "clear and convincing evidence" to unequivocally refute the paternity already established. Although the law does not explicitly call forth scientific and DNA evidence, most attorneys rely heavily on such tests to refute paternity. In Arizona, however, nullifying paternity still lies with the discretion of the judge in court, who needs to evaluate all of the evidence to determine if paternity can be truly refuted.
Source: AAML.org, "If the genes don't fit: an overview of paternity disestablishment statutes," June 21, 2011