# Brothers and sisters **URL:** \ **Category:** Cecil's Columns/Staff Reports\ **Created:** [September 2, 2018, 8:13am UTC](https://boards.straightdope.com/t/brothers-and-sisters/820637 "2018-09-02T08:13:55Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![tavaritz](https://avatars.discourse-cdn.com/v4/letter/t/71e660/32.png) [@tavaritz](https://boards.straightdope.com/u/tavaritz)\ **Post date:** [September 2, 2018, 8:13am UTC](https://boards.straightdope.com/t/brothers-and-sisters/820637/1 "2018-09-02T08:13:55Z")
In the article: > **[If twins marry twins, are the children from the two families cousins–or...](https://www.straightdope.com/columns/read/1602/if-twins-marry-twins-are-the-children-from-the-two-families-cousins-or-brothers-and-sisters/)** > > A STAFF REPORT FROM THE STRAIGHT DOPE SCIENCE ADVISORY BOARD Comments that children of two families where fathers and mothers are both identical twins the siblings are not brothers and sisters but cousins. I agree but want to state that this is trivially true if you think about adoption. Adopted child is a brother or sister to biological siblings or several adopted childs are brothers and sisters. So being a brother or sister is not about genetics but about living and groving in same household. That’s why humans and chimpansees are cousins not brothers and sisters. ---
**Author:** ![Broomstick](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/broomstick/32/246_2.png) [@Broomstick](https://boards.straightdope.com/u/Broomstick)\ **Post date:** [September 2, 2018, 7:36pm UTC](https://boards.straightdope.com/t/brothers-and-sisters/820637/2 "2018-09-02T19:36:00Z")
If a pair of identical twin men marry a pair of identical twin women then all of their children will be _genetically_ siblings but _socially_ the children of one married set will be siblings and _socially_ cousins to the siblings of the other married pair. I really don’t get why this is so confusing to people. On the flip side, if a married pair adopts unrelated children then _genetically_ their adopted children aren’t related to the parents, but _socially_ they are their parents’ children. In sum: genetic and social relatives are not always the same thing, although they often overlap. ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [September 2, 2018, 7:54pm UTC](https://boards.straightdope.com/t/brothers-and-sisters/820637/3 "2018-09-02T19:54:28Z")
> [@Broomstick](#): > > If a pair of identical twin men marry a pair of identical twin women then all of their children will be _genetically_ siblings but _socially_ the children of one married set will be siblings and _socially_ cousins to the siblings of the other married pair. I think you are adding to the confusion here. You want to introduce the terminology “social siblings” vs “genetic siblings”, but that’s not used in genetics. In my experience, the terms “sibling” and “cousin” refer solely to familial relationships, not genetics. The genetic concept is the coefficient of relationship, r. > **[Coefficient of relationship](https://en.wikipedia.org/wiki/Coefficient_of_relationship)** > > The coefficient of relationship is a measure of the degree of consanguinity (or biological relationship) between two individuals. The term coefficient of relationship was defined by Sewall Wright in 1922, and was derived from his definition of the coefficient of inbreeding of 1921. The measure is most commonly used in genetics and genealogy. A coefficient of inbreeding can be calculated for an individual, and is typically one-half the coefficient of relationship between the parents. > In general,... So in the case of the children of two families where the fathers and mothers who are two pairs of identical twins: we would still call them cousins, but note they have the same genetic coefficient of relationship (r=0.5) that siblings usually do. ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [September 2, 2018, 8:14pm UTC](https://boards.straightdope.com/t/brothers-and-sisters/820637/4 "2018-09-02T20:14:52Z")
ETA: to Broomstick, I see that googling “genetic siblings” does get some hits, so I apologize - it does appear the term is sometimes colloquially used. But I’ll stick to my claim that it’s not the usual terminology in genetics (I’m an evolutionary biologist). ---
**Author:** ![Broomstick](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/broomstick/32/246_2.png) [@Broomstick](https://boards.straightdope.com/u/Broomstick)\ **Post date:** [September 2, 2018, 10:03pm UTC](https://boards.straightdope.com/t/brothers-and-sisters/820637/5 "2018-09-02T22:03:15Z")
I’m totally OK with geneticists using different terminology than the rest of us. I suspect that geneticists are primarily concerned with (surprise!) the genetics. Some of the rest of us put the social/cultural relationships first. Perhaps it is akin to the way that to a geneticist “clone” refers to any number of individuals sharing a genotype and the rest of us might refer to “identical twins” or “identical triplet” or “clone” (if we’re not specifying them being of near identical age or specific number) or even “monozygotic twins” if we have just enough knowledge to be dangerous. 😉 So… there’s the specific jargon of genetic science, and then there’s the rest of us using language in a slightly different manner. ---
**Author:** ![Beckdawrek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beckdawrek/32/3647_2.png) [@Beckdawrek](https://boards.straightdope.com/u/Beckdawrek)\ **Post date:** [September 3, 2018, 12:21am UTC](https://boards.straightdope.com/t/brothers-and-sisters/820637/6 "2018-09-03T00:21:08Z")
Double first cousins. My Daddy had some. His Dad and Uncle married a pair of twin sisters. It’s never been confusing at our reunions. ---
**Author:** ![Lemur866](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lemur866/32/434_2.png) [@Lemur866](https://boards.straightdope.com/u/Lemur866)\ **Post date:** [September 6, 2018, 7:15pm UTC](https://boards.straightdope.com/t/brothers-and-sisters/820637/7 "2018-09-06T19:15:47Z")
You’re correct, but double first cousins would also be if any pair of siblings married another pair of siblings, they don’t have to be twins. ---
**Author:** ![elfkin477](https://avatars.discourse-cdn.com/v4/letter/e/a9a28c/32.png) [@elfkin477](https://boards.straightdope.com/u/elfkin477)\ **Post date:** [September 9, 2018, 3:31am UTC](https://boards.straightdope.com/t/brothers-and-sisters/820637/8 "2018-09-09T03:31:41Z")
> [@Broomstick](#): > > I really don’t get why this is so confusing to people. Have you met people? There are grown adults who don’t know the difference between step siblings and half siblings, and some who don’t realize boy/girl twins can’t be identical. This is a more difficult concept of either of those things.