Double Cousin Relationship Predictions with Ped-sim

Double cousin predictions are now updated with many more relationship types and a peer-reviewed data source

Brit Nicholson
4 min readMar 9, 2022


Groundbreaking developments are happening in genetic genealogy at this very moment. As of today, we can now say:

Previously, the only double cousin relationship predictor included only two types of 3/4 siblings, which were then averaged together, and double 1st cousins. That relationship predictor was also developed and hosted by

The following double cousin relationship types are now included in predictions:

  • 3/4 Siblings: Child of your father and maternal aunt
  • 3/4 Siblings: Child of your mother and paternal uncle
  • 3/4 Siblings: Child of your father and maternal half-sister
  • 3/4 Siblings: Child of your father and maternal grandmother (opposite perspective of the immediately above type)
  • 3/4 Siblings: Child of your mother and paternal half-brother
  • 3/4 Siblings: Child of your mother and paternal grandfather (opposite perspective of the immediately above type)
  • Double First Cousins (2x 1C): “Two brothers married two sisters” or “A brother and a sister married a sister and a brother” is how it’s often said
  • 1C + Half- Aunt/Uncle/Niece/Nephew: A cousin has both of the listed relationship types to you
  • 1C + First cousin once removed (1C1R)
  • 1C + Half-1C
  • Double 1C1R: with or without FIR
  • Double Second Cousin (2x 2C): with or without FIR
  • Double 2C1R: with or without FIR
  • Double Third cousins: with or without FIR

For many of the double cousin relationships, two types are included here. One type cannot share fully-identical regions (FIR) and the other type usually will. Including both was necessary because there are differences between the two in the half-identical region (HIR) amount, which is the only way cM values are reported at Ancestry. For HIR comparisons of relationships, the type that cannot include FIR will actually have a higher cM value. This is because a double 2nd cousin (2C) pair, for example, who cannot share FIR, will have an average shared amount of 6.25%, all of which comes from HIR. Conversely, a double 2C pair who do share FIR will usually only have about 6.05% HIR, the other 0.2% coming from FIR. Also, double cousin types with possible FIR will also have slightly lower amounts of full IBD sharing at genotyping sites. That’s because some segments that are FIR will be below the low-cM threshold, causing twice the amount of shared cM to be discarded for those segments when the cutoff is applied.

How do you know if your double relationship could include FIR? Here’s the test: If both of your parents are related to the match and you’re related to both of your match’s parents, then you could share FIR with your match. Please note that as double relationships get more distant, it becomes less likely that they’re aligned in exactly the right way to produce FIR.

The tool includes options for three different measuring metrics, in order of the most useful predictions: total identical-by-descent sharing (IBD, like what’s reported at 23andMe), FIR (which can be seen at 23andMe or GEDmatch), or HIR (the default for all sites other than 23andMe).

Here are what the FIR probability curves look like for the double cousin relationships:

Do you have a suggestion for a type of double or multiple cousin relationship to add to this tool? If so, please leave a comment here.

Probabilities are included for other (non-double) relationships as far back as 8C1R. This tool does not yet include population weights, which are easier to implement with traditional relationships. Double cousin relationships are less common and there may not be a perfect way to add population weights to a relationship predictor that includes them, although I have an idea.

For more information about the methodology and discoveries associated with this tool, click here.

The data used for these predictions came from Ped-sim. In this case, the refined genetic map of Bhérer et al. (2017) was used as well as the crossover interference parameters of Campbell et al. (2015).

Feel free to ask a question or leave a comment. And make sure to check out these ranges of shared DNA percentages or shared centiMorgans, which are the only published values that match peer-reviewed standard deviations. Or, try a tool that lets you find the amount of an ancestor’s DNA you cover when combining multiple kits.