Paternity may be determined scientifically, such as through DNA testing and by the Presumption of Paternity, where a child is born during or of an intact marriage.

Paternity by Estoppel occurs where a Father holds a child out to be his own regardless of a biological relationship between the Father and the child.  It also occurs when a Mother holds someone out to be the child’s father even though he may not be the biological father. “Holding the child out to be his own” can be demonstrated by evidence indicating that the “father” is spending time with the child, the child calls the man his/her Dad, or the “father” has represented to others that the child is his own child. In this situation, the “Father” is estopped or prevented from denying paternity in legal proceedings.  One may be “estopped” or told ‘you cannot go back on your word’ where the father’s and mother’s conduct continues to establish a parental relationship.  The parties hold the child out to the community as “theirs” and the father supports and fosters the relationship with that child.  Once estoppel is established, genetic testing may not be admitted to contradict it.  This is because the courts desire permanency for the child in the relationship with father.

The doctrine of paternity by estoppel is most often applied in child support cases to either preclude a man who has held the child out as his own from avoiding support of the child after his relationship with the child’s mother has ended or to preclude a mother “who held one man out as her child’s father from seeking support from another man” at a later time. The Pennsylvania Supreme Court has stated: “Those who mislead a child as to the identity of his or her natural father, cannot then turn around and disprove their own fiction to the detriment of the child.”

The doctrine of paternity by estoppel can also be raised in an effort to preclude a biological father from asserting his parental rights over a child.

In cases where the father denies that the child is his, he may not be able to ask that a DNA test be performed if he made prior statements admitting to being the father.  Paternity may also be inferred from a persons conduct indicating that he is the father.  This can be sufficient for the court to enter a support order.

In situations where a man denies that he is the father, the Mother may not be able to request a DNA test when she has held someone else out to the world to be the child’s father and that person has acted as the child’s father.

In paternity by estoppel cases, the focus often is on whether the putative father has failed to timely exert his parental claim. If the putative father acquiesces in the fiction that someone else is the father of a child, the doctrine of estoppel may be invoked and will have a higher probability of precluding the putative father from assuming his parental rights in such an instance.

Most states have been moving away from the doctrie of paternity by estoppel.  The doctrine of paternity by estoppel was recent addressed in the Pennsylvania Superior Court child custody case of T.E.B. v. C.A.B. v. P.D.K. Jr., 74 A.3d 170 (Pa. Super. 2013). The court concluded: “In summary, paternity by estoppel continues to pertain in Pennsylvania, but it will apply only where it can be shown, on a developed record, that it is in the best interests of the involved child.”