The "presumption of legitimacy" is a common law rule of evidence that states that a child born within the subsistence of a marriage is deemed to be the child of the husband.
Early history
Paternity (law) has been considered a historically important issue in determining the rules of succession. Illegitimate children were originally debarred from inheriting a monarchy, title, nobility or property.
References
Presumption of legitimacy Wikipedia(Text) CC BY-SA