In Pennsylvania, paternity by estopple is a doctrine that functions to prohibit a person from denying paternity due to his conduct. Estopple can also be used to prevent a third party from making a claim that he is actually the father. The doctrine of estopple centers around the idea that the best interest of the child is the most important factor when it comes to paternity and custody law. The court does not want to force the child to start a relationship with a stranger instead of continuing the relationship with the father the child has know all along.
Estopple does not necessarily mean that the parties needed to be married when the child was born. All that matters is that dad acted like dad for at least part of the child's life. Once this relationship if formed, the father cannot just claim that he is no longer the father. Also, a third party cannot attack the relationship that the father has already formed with the child. Similarly, if mother thinks a third party is the father of the child, she cannot file for support against a third party, because the husband is presumed to be the father.
Estopple can be overcome if the father can show fraud or misrepresentation on the part of the mother. This may occur if the mother blatantly tells a man he is the father, even though she knows that is not true. Another example may be where a mother omits the fact that there is a possibility someone else is the biological father of the child.
If a father has reason to believe he is not actually the biological father of the child, he must stop acting like the father to be able to overcome the estopple doctrine. For example, if mother tells the presumed father that he is not the biological father of the child, but he continues to act like the father by spending time with the child, bonding with the child, letting the child call him "dad" etc., then he cannot decide down the road that he wants to challenge paternity. The doctrine of estopple would prevent denial of paternity for a man who holds himself out to be dad, even though he knows or has reason to know that he is not.