Legal Question in Family Law in California

The basic rule is that you are only legally responsible for your biological or adopted child.

There is, however, a special circumstances exception. Put simply, if not technically exact, if you intentionally act as if a spouse's child is your own, not just being their "dad," but holding yourself out to the world at large as their father, and particularly if you lead them to believe you are their father, AND their true father is not in their life at all, in short if there is no real difference between if you had adopted the child and not, a court MAY find it is equitable to impose some child support obligations on you if you and the mother split. If, however, you maintain the proper distinction, to the child and the world at large, that you are the stepfather and definitely not the child's biological or adoptive father, this exception would not be applied.