PATERNITY FRAUD

Paternity fraud is the pretense that a man is the legitimate and biological father of a child. It usually occurs when a woman deliberately misidentifies a man as the biological and legitimate father of her child. This fraudulent act led the man to believe a biological connection with a child of another man and sometimes remain undetected for several years. In most cases, women are often aware that the presumed biological father is not genetically linked with the child. A DNA expert from Lagos University Teaching Hospital claimed that 30% of the Nigerian men who went to paternity testing centers were not the biological fathers of the children in their custody. However, samples drawn from paternity testing laboratories are not representative of a given general population and are many times more likely to contain instances of paternity fraud than a random sample from the populations from which they are drawn. Similarly, two large samples from paternity testing laboratories drawn from Sweden and from a white American group from Los Angeles found paternity fraud rates of 38.7% and 24.9% respectively, although the rates in the general populations are much lower. The major causes of paternity fraud in Nigeria had been attributed to "infidelity, adultery, and increase in sexual recklessness among Nigerian couples" together with "poor family planning". Mix-ups in maternity hospitals are an additional factor in failed paternity tests: in these cases, maternity tests will fail too.

[Source: Wikipedia]. 

Most times men are not sure how to handle DNA test results that prove that a child doesn't belong to them. This is what I think a man should handle it; If a man finds out his wife cheated on him and the kids ain't his, he should see it in the same light as when he gets a girl pregnant outside his marriage. The real father of the children should be informed and made to take responsibility for them.

If a man comes home with a baby from outside the woman takes it as her husband's child. Yes, she may not treat the child well but it's her husband's own. So if a woman brings a child home, it's not her husband's own yes, it's not his responsibility yes. But he should just take the child the same he would take any child she had out of wedlock.

He should forgive her if he can and move on with the marriage. 

If he can't forgive her then obviously he has to get a divorce and move on with his life. 

In most parts of Igbo land, a woman's bride price makes every child she gives birth to including the ones she had out of wedlock legitimate children of her husband. Sometimes, when the family of a man senses his inability to impregnate a woman due to impotency or any kind of deformities either physical or mental, they would marry a girl who has a child outside wedlock or is carrying an unwanted pregnancy for him in other to make the child(ren) his. Most times the woman is free to get pregnant outside and bring the pregnancy home to her husband. This way the man can have more children to bear his name.

In some cases, a widow is allowed to get pregnant after the death of her husband and the child would still be considered a legitimate child of her husband. Even widows who were unable to give birth to children for their husbands can pay the bride price of a girl carrying a wanted pregnancy on behalf of their dead husbands and the children become  legitimate children of their husbands. 

I do not know the cultural belief of other tribes, but considering the concept of adoption, the legitimacy and ownership of a child is solely dependant on the individuals involved and their readiness to take full responsibility for the child in question.