Until relatively recently, determining the paternity of a child was not easy to do with certainty. The law created rules to avoid confusion and thus there was, and still is, a rebuttable presumption that the child of a married woman is the child of her husband. This presumption could be rebutted by the husband only if he could prove he was unable to have conceived the child (sterility, away from the home during the time of conception, etc.). Now, the law retains this presumption, but only DNA evidence is allowed to rebut the presumption and there are strict constraints on how to proceed in disestablishing paternity.
Recently, the Indiana appellate court ruled that the husband of a woman who conceived a child with another man during their marriage (while the husband was in prison) was still the legal father of the child despite his wishes to disestablish paternity. This case has unique facts in that the husband was aware that this child was not his when it was born and proceeded to sign the birth certificate and carry on as the father for twelve years. Only upon the divorce of the parties did the husband then seek to disestablish paternity in order to avoid paying child support. The court found that the husband remained the legal father. The court prohibited him from admitting evidence that he was not the biological father as he had accepted this legal presumption for many years, prevented his wife from contacting the biological father for support at the time of birth many years before (allowing the statute of limitations to run), and induced the wife to act on his promises to be the father of the child. So while DNA testing today allows paternity to be established, it does not always allow it to be disestablished.