The inability to establish legal filiation (the right and responsibility of parenthood) over an illegitimate child arises in certain countries where filiation of children born out of wedlock is left to the father's discretion. Maternal affiliation is not automatically established by the sole fact of giving birth to the child; and without the acceptance of the father, the mother may not be legally permitted to assume filiation over the child. In other countries, the establishment of paternal affiliation of children born as a result of adulterous or incestuous associations is not allowed. Adoption raises similar issues over legitimacy in many countries.
This problem occurs particularly in countries where the law provides only one status for parents, whether married or not, regarding rights and responsibilities. The difference between legitimate and illegitimate children regarding legal status then rests on filiation.