Most of the marriage and incest laws originated in Tudor times in the U.K. and were influenced by King Henry VIII who wanted to marry a distant cousin. Laws forbidding cousin marriages were removed from the statute book and never replaced. In the case of single first and half first cousins the level of inbreeding is sufficiently low to avoid any major problems, but in the case of double first cousins and some other double relationships, the inbreeding levels are a cause for concern. I have tentatively suggested that the coefficient of relationship between parents should never exceed 1/8. However, even this limit should be used with caution if there has been a history of harmful genetic conditions in the family.

It is hard to be dogmatic on this subject when suggesting new guidelines, because different populations have experienced different genetic histories and ideally should be treated differently. For example, small genetically-isolated communities, which have undergone intense inbreeding in the past, and have been partially cleansed of harmful recessive genes, are better able to cope with higher levels of inbreeding than larger random-breeding populations, which may still carry a considerable burden of harmful genes. However, national or state laws should be simple and treat everyone equally.

Table 1 classifies all the common relationships according to whether marriage is currently allowed in the UK and whether or not those permitted marriages are advisable. It also separates those relationships which are legally incestuous from those which are not. Table 2 gives the coefficients of relationship for all the common types of cousins. The first degree relationships listed in both tables are those where the full sibs connecting those relatives are replaced by identical twins.

The present situation with nominal (non-blood) relatives, is that all in-laws and step relatives can now marry. A few of them, however, are still subject to certain restrictions. Those step relatives listed here in the previous monograph, can only marry if they are both over 21, and provided the younger person (when under the age of 18) never lived under the same roof as the older person and was not treated as a child of the older person's family. Those in-laws, also specified on here, can only marry if they are both over 21, and any former spouses must be dead. Adopted children can not marry their adoptive parents under any circumstance, but adopted children of the same adoptive parents can marry provided they are not blood relatives on the above forbidden list.

Table 1 Acceptable and Non-acceptable Levels of Inbreeding in Human Populations *

Between Relatives
Resulting Level
Of Inbreeding
1. Parent - Child 0.50.25
2. Full Sibs0.50.25
3. Half Sibs0.250.125
4. Grandfather - Granddaughter 0.250.125
Non-permitted Marriages (Not incestuous) 
5. Grandmother - Grandson 0.250.125
6. Uncle - Niece and Aunt - Nephew (First Degree) 0.50.25
7. Uncle - Niece and Aunt - Nephew 0.250.125
8. Half Uncle - Half Niece and Half Aunt - Half Nephew0.1250.0625
Permitted Inbreeding (Unwise)  
9. Double First Cousins (First Degree)0.50.25
10. Double First Cousins0.250.125
11. Double First Cousins Once Removed (First Degree)0.250.125
12. Single First Cousins (First Degree)0.250.125
Permitted Inbreeding (Acceptable) 
13. Double First Cousins Once Removed0.1250.0625
14. Single First Cousins Once Removed (First Degree)0.1250.0625
15. Single First Cousins0.1250.0625
16. Double Half First Cousins 0.1250.0625
17. Double Second Cousins (First Degree)0.1250.0625
18 Single Half First Cousins 0.06250.03125

* Excluded are all the other rare double relationships described in the main text.

Table 2 Coefficients of Relationship Between Various Degrees of Cousins


(Once Removed)

(Twice Removed)
1. Double 1st Cousins (1st Degree)1/21/41/8
2. Double 1st Cousins1/41/81/16
3. Single 1st Cousins (1st Degree)1/41/81/16
4. Single 1st Cousins1/81/161/32
5. Double Half 1st Cousins1/81/161/32
6. Double 2nd Cousins (1st Degree)1/81/161/32
7. Double 2nd Cousins1/161/321/64
8. Single Half 1st Cousins1/161/321/64
9. Single 2nd Cousins (1st Degree)1/161/321/64
10. Single 2nd Cousins1/321/641/128
11. Double Half 2nd Cousins1/321/641/128
12. Double 3rd Cousins (1st Degree)1/321/641/128
13. Double 3rd Cousins1/641/1281/256
14. Single Half 2nd Cousins1/641/1281/256
15. Single 3rd Cousins (1st Degree)1/641/1281/256
16. Single 3rd Cousins1/1281/2561/512
17. Double Half 3rd Cousins1/1281/2561/512
18. Double 4th Cousins (1st Degree)1/1281/2561/512
19. Double 4th Cousins1/2561/5121/1024
20. Single Half 3rd Cousins1/2561/5121/1024
21. Single 4th Cousins (1st Degree)1/2561/5121/1024

Note:- Half Cousins can not have a first degree form.