Page Actions

Cousin statistics

From ISOGG Wiki

It is helpful to have an understanding of the probabilities of matching cousins at different degrees of relationship when contemplating autosomal DNA testing or when interpreting the results of an existing test. This article reports some relevant cousin statistics.

Company statistics

The following table shows the information provided by the three main testing companies on the probability that two cousins will share enough DNA for the relationship to be detected.[1] AncestryDNA claim to be able to detect more relatives in the third to sixth cousin range which possibly relates to their use of phasing. Note that each company sets its own autosomal DNA match thresholds and some more distant matches might not appear because of these restrictions.

Relationship 23andMe AncestryDNA Family Tree DNA Family Finder
First cousins 100% 100% 100%
Second cousins 100% 100% >99%
Third cousins 89.7% 98% >90%
Fourth cousins 45.9% 71% >50%
Fifth cousins 14.9% 32% >10%
Sixth cousins 4.1% 11% Remote (typically less than 2%)[2]
Seventh cousins 1.1 3.2%
Eighth cousins 0.24 0.91%
Ninth cousins 0.06%
Tenth cousins 0.002%

Theoretical probabilities

The content of the following two tables is derived from Table 1 in the paper The probability that related individuals share some section of genome identical by descent by Kevin P Donnelly, Statistical Laboratory, Cambridge University, Cambridge, England. (Source: Theoretical Population Biology 1983: 23, 34-63) A copy of the paper is available here.

Cousin relationships.jpg

Ancestor relationships.jpg

How many cousins do we have?

Although there is only a low chance of sharing enough DNA with a specific distant cousin for the relationship to be detected, we have a large number of distant cousins and so many of these more distant cousins will appear in our match lists. The following table from the paper Cryptic distant relatives are common in both isolated and cosmopolitan genetic samples by Henn et al (2012) shows the expected number of cousins at different degrees of relationship and the expected number of detectable cousins along with the expected amount of Identical by descent (IBD) sharing if the relationship is detected.

How many cousins.jpg

Mathematician and genetic genealogist Paul Rakow has done his own computer simulations on family sizes and has published the results in an essay on Counting cousins (published online 31 March 2016).

A study by AncestryDNA, based on British birth rates, census data, parliamentary research briefings[3] and other sources for the last 200 years, produced the following statistics on the number of cousins that the average British person would be expected to have.[4]

Relationship Number of cousins
First cousins 5
Second cousins 28
Third cousins 175
Fourth cousins 1,570
Fifth cousins 17,300
Sixth cousins 174,000

It is not clear if these statistics relate to the whole of the United Kingdom or just England and Wales.

Further reading

Tools

See also

Acknowledgements

Thanks to Jon Hamm for compiling the spreadsheets based on data from Donnelly (1983) that have been used in the section on theoretical probabilities.

Footnotes

  1. The 23andMe data was extracted from the paper by Henn et al Cryptic distant relatives are common in both isolated and cosmopolitan genetic samples published online in PLOS One on 3 April 2012. 23andMe has a simplified version of this data in their customer FAQ The probability of detecting different types of cousins. The AncestryDNA data is from Table 1 in the AncestryDNA help topic menu “Should other family members get tested?” (available only to AncestryDNA test takers). The Family Finder percentages are from an article in the Family Tree DNA Learning Center What is the probability that my relative and I share enough DNA to be detected by Family Finder?.
  2. The figure quoted by Family Tree DNA refers to "6th cousins and more distant".
  3. A Century of Change: Trends in UK statistics since 1900 House of Commons Research Paper 99/111. Published 21 December 1999.
  4. Average British person has 193,000 living cousins says new research . Daily Mirror. 17 June 2015.