Some people eat, sleep and chew gum, I do genealogy and write...

Monday, July 20, 2015

Who can you marry? Forbidden Marriage Laws

A list of forbidden marriages was drawn up by the Church of England in 1560. The list may not be surprising, but it is extensive. You can see a summary of the scheme on the Genetic and Quantitive Aspects of Genealogy website article, "Forbidden Marriage Laws of the United Kingdom." Many people would have a difficult time figuring out the relationships in this set of charts. Notwithstanding this difficulty, most of the prohibited relationships are so ingrained in our collective society as to seem obvious, but the real question for genealogists comes when the marriages seem to modify or ignore the restrictions. Restrictions such as the ones codified in the United Kingdom exist in nearly every country of the world.

In addressing this subject it is difficult to separate fact from the sensational and the situation would appear to become even more difficult in the future.

As long as I can remember, I have been aware of a restriction concerning marriage between first cousins, that is, people who share a common grandparent. As far as I was aware, the restriction did not extend one step further to include second cousins or those who shared a common great-grandparent. As I got interested in genealogy, I came to realize that my own parents were second cousins. In addition, my great-grandfather had two wives who were cousins, so his two sets of children were not only half-siblings but also cousins. The common belief about such marriages was that the unions would produce genetic problems in the offspring, a fact that is beginning to be examined by actual scientific evidence. For example, here is a statement on the subject by the National Genetics and Genomics Education Centre of the National School of Healthcare Science in the United Kingdom:
Cousin marriage
For every non-consanguineous couple there is a 1 in 40 (2.5%) chance that each child will suffer a severe problem, such as a congenital malformation or learning disabilities. A consanguineous couple is at increased risk for both autosomal recessive disorders and several congenital malformations, this increased risk being due to a common mutation in an autosomal recessive gene from a shared ancestor. For a couple who are first cousins in a family where there is no other consanguinity, their overall risk is about 1 in 20 (5%). This risk is therefore double that of the general population. The additional risk for second cousins is 1% (overall risk 3.5%). If there have been several other cousin marriages in the family, the risk may reach 1 in 10 and the risks may need to be calculated from the degree of relationship. If someone in the family is already known to have an autosomal recessive disorder, then risks for this disorder can be accurately calculated. It is sensible to offer carrier testing for autosomal recessive traits known to be common in the couple's particular ethnic group (for example cystic fibrosis in Northern Europeans, Tay-Sachs disease in Ashkenazi Jews, thalassaemia in people from the Mediterranean, the Middle East, South-East Asian and the Indian sub-continent and sickle cell disease in people from Africa, the Caribbean, the Eastern Mediterranean, Middle East and Asia). Couples should also be offered detailed ultrasound scanning in pregnancy, in addition to routine antenatal care.
Estimates worldwide concerning cousin marriages are difficult to obtain, but in many cultures, despite the risks, such unions are not only permitted but encouraged.

One practical effect of such marriages on a family's pedigree is the phenomena of "pedigree collapse." I have written about this previously, but not from the point of view of which marriages are allow or disallowed in a particular culture. The issue of permitted marriages is so wrapped up in traditions that it is almost impossible to find objective discussions on the subject.

The New York Times published an infographic showing the laws for or against first cousin marriages here. Throughout the United States, with the decision of the Supreme Court in Obergefell v. Hodges, the whole issue has reached an entirely new level of examination.

From the standpoint of genealogical research, knowledge of the laws, both civil and ecclesiastical, concerning marriage patterns in a community may help solve some very difficult questions. There are so many articles and books on this subject that listing even a few is an overwhelming challenge. I suggest focusing on a particular time period and location for establishing any historical context.


  1. The NY Times map you linked to is for at least fairly ~current~ legislation. Ohio, for instance, used to allow 1st-cousin marriages until early in the 19th century.

  2. I wonder what Relative Finder folks would think of all this, since it shows, at least in the LDS community, that we are almost all cousins, quite a few steps removed, thank heavens.

  3. First cousin marriages were somewhat common in my family in northern Ohio. My great-grandparents marriage record in 1874 requires "that said parties are not nearer kin than first cousins," meaning that first cousin marriages were legal at that time. However, my grandparents marriage record in 1907 requires "that said parties are not nearer than second cousins." Sometime between 1874 and 1907 the law changed. I suspect it was before 1899 because my northern Ohio first-cousin family members went to Detroit to get married in that year.