I have marriages between first cousins in my tree. I believe the concern was that there was double the chance of passing on any family health problems. It could also lead to mental & physical disabilities in their children.

Hello,
Basically, it was illegal, both in the eyes of the Church, and until the 20th century, by Civil Authorities. However, many Incumbents, even when they knew that there was a relatonship, closed their eyes. Often you will find such marriages - and there were hundreds of them - took place outside both the bride and groom's parishes. It became more prevalent in the later 19th century when couples did not need to marry in church.

In the eyes of the Roman Catholic church (and until comparatively recently, the Church of England) it is still illegal to marry an inlaw or a cousin. This has to do with the fact that it is seen that when a couple marry, their families are joined as one, and therefore the perception is that it is incestuous to marry a sister/brother-in-law/cousin. It has nothing to do with the possible dangers of inbreeding of bloodlines although 20th century people have thrown this into the argument. It is a theological point. The rules today are still laid out in the Book of Common Prayer and are a bit laxer than they were latterly.

Yours
Sellwood

I'm sure it happened - sometimes by accident , but probably more often with full knowledge.

Prohibited degrees
The prohibited Degrees - The marriage of first cousins is not unusual and been legal since the 16th century. However, marriages between people who were related in some other ways (known as the prohibited degrees) were forbidden by acts of Parliament and ecclesiastical law. A marriage between two people within a prohibited degree required (and still requires) a private act of Parliament authorising that marriage.

The prohibited degrees have varied over time, but are explained in detail in Chapman & Litton "Marriage Laws, Rites, Records & Customs". (Lovchin Publishing 1996).

The present position is set out in the Marriage Act 1949, as amended in 1986, but for most of the period that you will be researching, the relevant rules (reached in about 1560 and confirmed by church laws, known as Canons, in 1604) were listed in the Book of Common Prayer of 1662.

The prohibitions prevented someone marrying his or her:
(a) brother or sister (or their spouse);
(b) parent, grandparent, aunt, uncle, child or grandchild (or their spouse);
(c) niece or nephew (or their spouse);
(d) spouse's child, grandchild, parent, aunt, uncle or grandparent.

Statutes of 1907 and 1921 made an exception to the prohibition at (a) above, allowing people to marry the spouse of their brother or sister, if that brother or sister had died. Some further exceptions were made in 1931, 1949 and 1986 so that, for example, a man was allowed to marry his deceased wife's niece, aunt or widowed mother.

Notes are taken from "Ancestral Trails by Mark D Herber (ISBN 0-7509-1418-1)

http://en.wikipedia.org/wiki/Prohibited_degree_of_kinship

and discussed here

http://www.forum.forest-of-dean.net/index.php?mode=thread&id=12344#p12388

and a current view !
http://www.uk-civil-partnership-act-2004.co.uk/s-T

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Ἀριστοτέλης A Gloster & Hereford Boy in the Forest of Dean ><((((*>