Is it really true that all Icelanders are descended from Jón Arason, the last Catholic bishop of Iceland, who was executed in 1550?

Answer

The original question was the following:

It is often said that all Icelanders are descended from Jón Arason, the last Catholic bishop of Iceland, who was executed in 1550. Is this really true? And, if so, what evidence is there to support it?

From the point of view of statistical population studies, the simplest way to approach this question is to consider the number of potential ancestors any individual might have. If we take as an example someone born in around 1970, their two parents would presumably have been born somewhere around 1940, their four grandparents perhaps some time around 1910, and their great-grandparents – eight in total – around 1880. Allowing 30 years for a generation, we can project this back in time. The table below calculates the number of forefathers of any individual alive today, and shows that there would have been 8192 who were born in around 1580. It should of course be said that the true number of ancestors would be rather lower, since people who were related might have married and had children. This factor, however, is limited by the fact that up until 1780 marriages between third cousins (common great-great-grandparent(s)) were forbidden by consanguinity laws and up until 1560 between fourth cousins (common great-great-great-grandparent(s)).

1. 1970: 1 4. 1880: 8 7. 1790: 64 10. 1700: 512 13. 1610: 4096

2. 1940: 2 5. 1850: 16 8. 1760: 128 11. 1670: 1024 14. 1580: 8192

3. 1910: 4 6. 1820: 32 9. 1730: 256 12. 1640: 2048

1. 1580: 1 4. 1490: 8 7. 1400: 64 10. 1310: 512 13. 1220: 4096

2. 1550: 2 5. 1460: 16 8. 1370: 128 11. 1280: 1024 14. 1190: 8192

3. 1520: 4 6. 1430: 32 9. 1340: 256 12. 1250: 2048

1. 1190: 1 4. 1100: 8 7. 1010: 64 10. 920: 512 13. 830: 4096	2. 1160: 2 5. 1070: 16 8. 980: 128 11. 890: 1024 (This takes us back to the time of the original settlements.)	3. 1130: 4 6. 1040: 32 9. 950: 256 12. 860: 2048

Jón Arason was born in 1484. A modern Icelander would thus have had around 8192×8 = 65,536 ancestors living at that time. This means that it is overwhelmingly likely that every individual modern Icelander is a descendant of everyone who was alive at the time and had any significant number of descendants, and often in several different ways. The likelihood is increased by the fact that Jón Arason himself had many children, who themselves had many children. In addition, the population of Iceland in 1484 was probably less than 65,356, and many of these people produced no children. So there is every chance that all modern Icelanders are descended, in more than one way, from a fertile family line like that fathered by Jón Arason.

The further back we go in time, the probability of people being descended from particular historical characters becomes ever greater. For example, at the time of Snorri Sturluson, who was born in 1179, our potential number of ancestors comes out as 67 million. So, patently, we are all descended from Snorri Sturluson in many different ways.

But not all Icelanders produced descendants. A fair number would have died before reaching childbearing age, and many more spent their entire, or almost all their, lives in a state of what has been called “enforced celibacy”, i.e. they spent their childbearing years as indentured labourers and as such were generally not allowed to marry and have children, particularly prior to 1800.

So how many out of the total population of Iceland at any time saw to the propagation of the human stock? In 1703 the number of married women between the ages of 20 and 44 was 3284. The number of husbands and wives therefore totalled 6568, amounting to 13% of the total population at the time. The census figures for 1801 reveal a similar proportion for that year, i.e. 13%. On this basis we can estimate that, out of a population of 50,000, there would have been at most around 6500 individuals involved in producing the generation that followed them; if the population had been 60,000 this figure would be 7,800. And Jón Arason, famously, was not a man to stint himself when it came to the duty of producing offspring!

Translated by Nicholas Jones.