What criteria for the 'Families with twins' rule?

Development
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I was wondering what criteria is used for the Families with twins rule?

Does it look for duplicate dates of birth? Or does it check Offspring having multiple ‘Primary’ roles for Birth events? Does it find step/foster families?

I was writing a custom filter to find Persons with multiple Roles for Birth events and work backwards. But then realized there are a lot of factors that I couldn’t check. Maybe there aren’t multiple Primary Roles for the Birth. (Like someone recorded a ‘midwife’ or ‘attending’ Role in addition to the Primary for the Birth. Or a sperm/egg donor Role. Or felt the need to explicitly define the Role normally handled implicitly via the Relationship structure.)

From what I read, natural hyperovulation is genetically tied to mothers with about 1:60 chance of fraternal multiple births. (Doubling to 1:30 with fertility treatments. And jumping to 1:5… or even 1:4… for IVF.) But the recessive gene in males can be expressed in granddaughters having multiple births. Identicals are more random.

I wanted to see whether the plethora of multiple births in my lines follow the statistical norm. Now I’m wondering if I can start with the built-in Rule?

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Code is in “…/gramps/gen/filters/rules/family/_hasfamily.py”. It looks for two or more children with BIRTH role of same mother and same birthdate.

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Thank you. Added the info to the wiki

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