England's Statute of Merton (1235) stated, regarding illegitimacy: "He is a bastard that is born before the marriage of his parents." This definition also applied to situations when a child's parents could not marry, as when one or both were already married or when the relationship was incestuous.
The Poor Law of 1576 formed the basis of English bastardy law. Its purpose was to punish a bastard child's mother and putative father, and to relieve the parish from the cost of supporting mother and child. "By an act of 1576 (18 Elizabeth C. 3), it was ordered that bastards should be supported by their putative fathers, though bastardy orders in the quarter sessions date from before this date. If the genitor could be found, then he was put under very great pressure to accept responsibility and to maintain the child."
Under English law, a bastard could not inherit real property and could not be legitimized by the subsequent marriage of father to mother. There was one exception: when his father subsequently married his mother, and an older illegitimate son (a "bastard eignè") took possession of his father's lands after his death, he would pass the land on to his own heirs on his death, as if his possession of the land had been retroactively converted into true ownership. A younger non-bastard brother (a "mulier puisnè") would have no claim to the land.
There were many "natural children" of Scotland's monarchy granted positions which founded prominent families. In the 14th century, Robert II of Scotland gifted one his illegitimate sons estates in Bute, founding the Stewarts of Bute, and similarly a natural son of Robert III of Scotland was ancestral to the Shaw Stewarts of Greenock.
In Scots law an illegitimate child, a "natural son" or "natural daughter", would be legitimated by the subsequent marriage of his parents, provided they were free to marry at the date of the conception. The Legitimation (Scotland) Act 1968 extended legitimation by the subsequent marriage of the parents to children conceived when their parents were not free to marry, but this was repealed in 2006 by the amendment of section 1 of the Law Reform (Parent and Child) (Scotland) Act 1986 (as amended in 2006) which abolished the status of illegitimacy stating that "(1) No person whose status is governed by Scots law shall be illegitimate ...".
The Legitimacy Act 1926 of England and Wales legitimized the birth of a child if the parents subsequently married each other, provided that they had not been married to someone else in the meantime. The Legitimacy Act 1959 extended the legitimization even if the parents had married others in the meantime and applied it to putative marriages which the parents incorrectly believed were valid. Neither the 1926 nor 1959 Acts changed the laws of succession to the British throne and succession to peerage and baronetcy titles. In Scotland children legitimated by the subsequent marriage of their parents have always been entitled to succeed to peerages and baronetcies and The Legitimation (Scotland) Act 1968 extended this right to children conceived when their parents were not free to marry. The Family Law Reform Act 1969 (c. 46) allowed a bastard to inherit on the intestacy of his parents. In canon and in civil law, the offspring of putative marriages have also been considered legitimate.
Since 2003 in England and Wales, 2002 in Northern Ireland and 2006 in Scotland, an unmarried father has parental responsibility if he is listed on the birth certificate.
In the United States, in the early 1970s a series of Supreme Court decisions held that most common-law disabilities imposed upon illegitimacy were invalid as violations of the Equal Protection Clause of the Fourteenth Amendment to the United States Constitution. Still, children born out of wedlock may not be eligible for certain federal benefits (e.g., automatic naturalization when the father becomes a US citizen) unless the child has been legitimized in the appropriate jurisdiction.
Many other countries have legislatively abolished any legal disabilities of a child born out of wedlock.
In France, legal reforms regarding illegitimacy began in the 1970s, but it was only in the 21st century that the principle of equality was fully upheld (through Act no. 2002-305 of 4 March 2002, removing mention of "illegitimacy" — filiation légitime and filiation naturelle; and through law no. 2009-61 of 16 January 2009). In 2001, France was forced by the European Court of Human Rights to change several laws that were deemed discriminatory, and in 2013 the Court ruled that these changes must also be applied to children born before 2001.
In some countries, the family law itself explicitly states that there must be equality between the children born outside and inside marriage: in Bulgaria, for example, the new 2009 Family Code lists "equality of the born during the matrimony, out of matrimony and of the adopted children" as one of the principles of family law.
The European Convention on the Legal Status of Children Born out of Wedlock came into force in 1978. Countries which ratify it must ensure that children born outside marriage are provided with legal rights as stipulated in the text of this Convention. The Convention was ratified by the UK in 1981 and by Ireland in 1988.
In later years, the inheritance rights of many illegitimate children have improved, and the change of laws have allowed them to inherit properties. More recently, the laws of England have been changed to allow illegitimate children to inherit entailed property, over their legitimate brothers and sisters.