Twins who were separated at birth, and who later married each other without knowing that they were brother and sister, have had their marriage annulled.

As babies they were adopted by different families and neither was told that they had a twin. Later in life, they met, fell in love, and married before discovering that they were siblings. They were granted an annulment at a special hearing at the High Court in London.

Marriages between people sharing blood, such as siblings, have been banned for “consanguinity” for many centuries, although modern genetic science is not precise about where to draw the line to avoid undesirable inbreeding.

A marriage between people who are too closely related isn’t valid. The law speaks of the “prohibited degrees of relationship” and two types of closeness can be measured. Consanguinity concerns people descended from the same stock or a common ancestor. Affinity concerns people related through marriage or civil partnership.

Historically, the law was that you couldn’t marry anyone within a certain number of degrees of proximity. The number of degrees at which marriage became permissible changed from seven degrees at one time to four degrees today. Now, marriages involving people related in the first three degrees are invalid but marriages in the fourth degree are okay.

How are degrees of relationship measured? The scheme is influenced by the Roman model which worked like this. Going down the generations like grandmother, mother, daughter, people are always one degree apart.

In general, if you want to know whether a relative is too close to marry, you count the spaces between each of you and your nearest common ancestor, then add together the two numbers. For instance, a brother and sister are two degrees apart because each is one degree from their common parent.

Marriage between people related in the third degree or less is prohibited. You can’t marry your mother’s sister or brother because you’re related in the third degree to them. The common ancestor is your grandmother, from whom you are two degrees away, and your aunt or uncle is one away — three degrees altogether.

First cousins are related in the fourth degree. Your common ancestor will be your grandmother. You have two degrees separating you from her, and so will your cousin (as will your mother’s sibling’s child). Added together, there are therefore four degrees of separation. In short, you can marry your first cousin.

The prohibited degrees were defined by Innocent III in 1215, recognised in an Act of Henry VIII in 1536 and are now in the Marriage Act 1949.

Specifically, the Marriage Act lists the invalid relationships and says a man can’t marry: his mother, adoptive mother, daughter, adoptive daughter, father's mother, mother's mother, son's daughter and so forth. A woman can’t marry: her father, adoptive father, son, adoptive son or former adoptive son, father's father and several others.

The principle that marriages of close non-blood relatives are unlawful was modified in Scotland by the Family Law (Scotland) Act 2005. It says that a person can now marry their mother-in-law or father-in-law where death or divorce has ended the original marriage.

The old principle was based on biblical lines, such as that in Leviticus 20:14 which says that if a “man takes his wife and her mother” all three shall be burnt alive. A law that would perhaps have given Dustin Hoffman second thoughts in The Graduate.

There was no particular science behind the old lists of prohibited degrees of affinity. According to one classic legal history, they were simply the “idle ingenuities” of men who liked drawing up tables and “doggerel hexameters” (worthless rhythmic lines).

Why did Scotland want to make special provision for a man to marry his mother-in-law or daughter-in-law, and a woman to marry her father-in-law or son-in-law, if death or divorce has ended the original relationship? The answer is that the Scottish Parliament took the view that “family law must be updated to ensure that it reflects the needs of all our people”.

Cupid can behave in curious ways, so diverse passions are legally permitted. Most things, in fact, except romantic love in the third degree.

Professor Gary Slapper is Director of the Centre for Law at The Open University

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