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Rafael Harth2mo-44

I would have been shocked if twin sisters cared equally about nieces and kids. Genetic similarity is one factor, not the entire story.

Ustice2mo32

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3 How identical twin sisters feel about nieces vs their own daughters

by Dave Lindbergh

9th Feb 2025

1 min read

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(cross posted from https://mugwumpery.com/how-identical-twin-sisters-feel-about-nieces-vs-their-own-daughters/)

It seems to be generally assumed that twin sisters feel the same way as other sisters – closer to their own children.

But per Hamilton/Trivers, they shouldn’t. They should feel equally related and care equally about daughters and nieces.

Identical twins share 100% of their genes, and their nieces are just as closely related as their own daughters (r=0.5 for both, vs. r=0.25 for nieces of non-identical sisters).

I’m told that in fact twin sisters usually care more about their own children than about their nieces.

I suspect that this is because identical twins are rare and our genes just program us to assume nieces are less closely related than our children, even tho with identical twins it’s not true.

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[-]Kaj_Sotala2mo197

How true is the thing about caring for your relatives in proportion to their genetic similarity in general? I get why it makes evolutionary sense but I think that in general, people's degree of caring doesn't actually follow that rule very much and it would have been hard for evolution to program in very consistently. E.g. I care more about my close friends with no relation to me, than I care about some of my cousins who I've rarely if ever met. (Sorry cousins! But also c'mon, you'd choose your friends over me too.)

[-]Jiao Bu2mo30

Relevant information might include whether mothers sometimes have differing degrees of care for each of their own daughters. I am guessing without looking that there are studies indicating favorites can exist among equally-genetically-related-to-oneself children, also breaking the strong link to "caring in proportion to their genetic similarity to you."

[-]localdeity2mo20

I think there's at least decent truth to it. One study:

This study examines gift giving at Israeli weddings. In accordance with kin selection theory, we hypothesized that wedding guests possessing greater genetic relatedness to the newlyweds would offer greater sums of money as wedding gifts. We also hypothesized that family members stemming from the maternal side (where the genetic lineage has higher kinship certainty) would offer the newlyweds more money than those stemming from the paternal side. Data on the monetary gift sums of the wedding guests from 30 weddings were collapsed according to two criteria: (a) genetic relatedness (0%, 6.25%, 12.5%, 25%, and 50%) and (b) kinship certainty (maternal or paternal lineage). Both hypotheses were supported.

I think I had also heard of studies that looked into either "how devastated would you feel", or "how devastated did you feel", regarding the death of a family member, and that these also fit the "genetic closeness" predictions. I don't know exactly how they were done—obviously genetic closeness will correlate highly with family structure that gives you actual closeness, and one must control for that. But my impression is that the effect is real and significant, though of course not all-consuming.

There's also an interview with someone who studied identical twins a lot, with various interesting things to say.

[-]Kaj_Sotala2mo64

30 weddings in one particular culture doesn't sound like a particularly representative sample. I would expect that in formal situations like weddings, social norms and expectations would determine gift-giving as strongly if not more strongly than genuine liking. And "closer relatives should give more generous gifts than more distant ones" sounds like a pretty natural social norm.

With regard to those other studies, I don't think you can conclude anything from just a relatedness-grief correlation. As you note yourself, there's also a relatedness-closeness correlation, so we should expect a relatedness-grief correlation even in worlds with no genetic effect. There's also a cultural mechanism where you are expected to feel grief when people related to you die.

And none of these studies establish a mechanism for how the effect is supposed to work. There are some simple and straightforward mechanisms for establishing closeness with close relatives - e.g. "you grow to care about your parents who you have known for as long you can remember", "you grow to care about children that you personally gave birth to", "you grow to care about people you spend a lot of time with", etc..

But in the case of a cousin who you might never have met, by what mechanism is evolution going to get you to care about them? Before the invention of DNA testing, the only evidence for them being related to you was someone claiming that they are your cousin. And if that was enough to win over someone's trust, we'd expect there to be a lot more con schemes that tried to establish that the con artist was the mark'a long-lost cousin (or even better, sibling).

[-]ChristianKl2mo40

Taking one study about how much wedding gifts come from each side in one specific culture of Israeli weddings, seems very bad reasoning. Depending of the economics of marriage, wedding gifts differ from culture to culture.

In Judaism, religion passes primarily through the maternal lineage by cultural custom, so there are a lot of other reasons besides kinship certainty.

[-]localdeity2mo20

In Judaism, you're not supposed to marry a non-Jew unless they convert to Judaism (a lengthy process from what I've heard), so I suspect the families on both sides of the deal are usually equally religious.

In any case, googling for "grief and genetic closeness study" yields this:

A Twin Loss Survey was completed by MZ and same-sex DZ twins following loss of a cotwin and nontwin relatives. Twin survivors (N = 612; MZ = 506; DZ, n = 106) included twins whose age at loss was 15 years or older. Participation age was M = 47.66 years (SD = 15.31). Hamilton's inclusive fitness theory generated two hypotheses: (1) MZ twins will recall greater grief intensity at loss than DZ twins; (2) loss of a twin will receive greater grief intensity ratings than loss of nontwin relatives. [...] Part I: Hypotheses regarding grief intensity were supported.

And this, where the highlights are:

Surviving MZ twins grieve more intensely for deceased co-twins than surviving DZ twins.
Female twins grieve more intensely for deceased co-twins than male twins.
Twins grieve more intensely for deceased co-twins than for other deceased relatives.
Survivors' grief intensity varies with genetic relatedness to the deceased.

[-]Kaj_Sotala2mo20

Twins grieving more strongly for deceased co-twins seems to me explained by twins having a more closely coupled history than non-twin relatives. MZ twins grieving more strongly than DZ twins seems to me explained by MZ having larger similarity in personality so bonding more strongly due to that.

[-]ChristianKl2mo20

From Gemini Pro 2.0:

Traditionally, the Man's Family Provides More (Bride Price/Bridewealth):
Many African Cultures: Bride price (also called bridewealth) is a common tradition across many African societies. It involves the groom's family giving gifts of money, livestock, goods, or other valuables to the bride's family. It's seen as compensation for the loss of the bride's labor and a way to strengthen ties between the families. The specific form and amount vary greatly. Examples include, but are not by any means limited to: many communities in Nigeria, Kenya, South Africa, Ghana, and Uganda.
Many Asian Cultures:
China: Traditionally, the groom's family provided a "bride price" to the bride's family. The gifts symbolised wealth and prosperity.
India: While dowry (from the bride's family) is more commonly discussed (and now illegal), there have also been traditions in some communities of bride price, though it's less prevalent.
Thailand: "Sin Sod" is a payment made by the groom to the bride's family. It's seen as a way to show respect and gratitude.
Indonesia: Diverse archipelago with varying customs. Bride price exists in some communities, often involving goods, livestock, or money.
Papua New Guinea: Bride price is a very significant part of marriage customs in many communities, often involving pigs, shells, and other valuables.
Some Middle Eastern Cultures: Historically and, in some areas, continuing today, mahr (dower) in Islamic traditions has sometimes functioned similarly to a bride price. It is a payment or gift from the groom (or his family) to the bride, and it becomes her property. It is important that it's her property. However, in practice, social pressures sometimes meant families influenced how it was used.
Some Indigenous Cultures of the Americas: While practices varied greatly, some indigenous communities had traditions where the groom or his family provided gifts to the bride's family.
Some Pacific Island Cultures: Bride price is common in parts of Melanesia (e.g., Vanuatu, Solomon Islands) and Polynesia.
Traditionally, the Woman's Family Provides More (Dowry):
India: Dowry is the most well-known example. Despite being illegal, the practice of the bride's family giving substantial gifts of cash, jewelry, land, and household goods to the groom's family persists in many areas. It's a deeply ingrained social custom, though it's increasingly challenged.
Historically in Europe: Dowry was common in many European societies throughout history, especially among the upper classes. It was a way to provide for the daughter's future and enhance her marriage prospects. This was the case in Ancient Greece, Rome, and through the medieval and early modern periods.
Bangladesh: Dowry, though illegal, is still practiced in some areas, similar to India.
Nepal: Similar to India and Bangladesh, dowry, though illegal, persists in some communities.
Some parts of the Balkans: Historically, dowries were prevalent, and vestiges of the tradition might still be found in some rural areas.
Sri Lanka: Dowry is a factor.

[-]notfnofn2mo149

This whole thing about "I would give my life for two brothers or eight cousins" is just nonsense formed by taking a single concept way too far. Blood relation matters but it isn't everything. People care about their adopted children and close unrelated friends.

[-]eukaryote2mo71

Advice: The AI-generated diagram here doesn't add anything and in fact indicates strongly that I wouldn't want to read the post. One of the things about diagrams being so important and eye-catching associated with writing is that they communicate information, so if a diagram is clearly half-assed and wrong, it makes one assume that the text is too. (Half-assed is maybe not the word - minimally-assed? MS Paint stick figures would be fine here, for instance.)

There's extraneous detail. The text is garbled and irrelevant.

I think if you use image-generating AI to make diagrams you should then edit it afterwards to make sure it's actually, like, good and represents what you wanted, and add your own captions.

[-]Dave Lindbergh2mo30

Good point. I'll try to remove it.

[-]dr_s2mo54

This feels like a classic case of overthinking. Suggestion: maybe twin sisters care more about their own children than their nieces because they are the ones whom they carried in their womb and then nurtured and actually raised as their own children. Genetics inform our behaviour but ultimately what they do align us to is something like "you shall be attached to cute little baby like things you spend a lot of time raising". That holds for our babies, it holds for babies born with other people's sperm/eggs, it holds for adopted babies, heck it even transfers to dogs and cats and other cute animals.

The genetically determined mechanism is not particularly clever or discerning. It just points us in a vague direction. There was no big evolutionary pressure in the ancestral environment to worry much about genetic markers specifically. Just "the baby that you hold in your arms" was a good enough proxy for that.

[-]ChristianKl2mo34

The process of birth is a strong bonding process between the mother and the child. If evolution chose to use that as the way to create the bonding that makes mothers care a lot about their child describing that as "genes just program us to assume nieces are less closely related than our children" feels really strange.

[-]localdeity2mo20

Everything you say seems straightforwardly correct or a logical guess. I'd add:

I expect identical twin sisters do feel closer to their nieces than non-identical sisters feel to their nieces. There would probably be a higher degree of discovering that this person just happens to have traits like your own.
Even from a fully logical "optimize my inclusive genetic fitness" perspective, there is value, all else being equal, in most parent-child relationships that isn't in most aunt-niece relationships. Because you raised this child, she probably trusts you, you know her strengths and weaknesses and where she needs help, etc.; you have a comparative advantage at helping your daughter vs helping your niece. (I speak of good parental relationships; of course, there are some where the parent has a comparative disadvantage. Also, if you know your child is a no-good criminal or something, and you know nothing about your identical sister's child, then the latter may be a better bet for investing your resources; hence my saying "all else being equal".)

[-]PaulBecon2mo90

Unless the identical twin sisters marry identical brother twins, the preferential love for their own husband would distinguish their feelings for their own children

[-]spencerb2mo10

Isn't this obvious from the inheritance of non-genetic factors? I would assume that twins would care more about their adopted children then their niblings. Probably correctly; unless you think that your values are overwhelmingly heritable, children you raise personnally are going to better represent your values than genetically close children raised by someone else

[-]robo2mo10

Eliezer Yudkowsky wrote a story Kindness to Kin about aliens who love(?) their family members proportionally to the Hamilton's "I'd lay down my life for two brothers or eight cousins" rule. It gives an idea to how alien it is.

Then again, Proto-Indo-European had detailed family words that correspond rather well to confidence of genetic kinship, so maybe it's a cultural thing.

[-]Rafael Harth2mo-44

I would have been shocked if twin sisters cared equally about nieces and kids. Genetic similarity is one factor, not the entire story.

[-]Ustice2mo32

I agree.

I’m not a twin, but I am a parent, and I have a a nephew, and my son has a stepsister who has called me Uncle Jason since she could talk.

I don’t feel closer to my nephew than I am with my “niece.” I normally wouldn’t make a distinction based on genetics, except that it is relevant here. I’m not closer with my sister’s kids than I am with the other two.

Also, I’m not sure closeness is really even a good distinction. I’m not generally responsible for my niece or nephew, but if they or my son needed me to travel across the country to rescue them from some bad situation, I’d do it. I love those kids.

Being responsible for a child may present as being closer to them, So does spending a lot of time with a child. One could argue that these are two aspects of closeness. Neither of those things have anything to do with genetics.

Personality can be a huge factor in closeness too, and there is a huge variation in personality, even amongst identical twins.

Genetics seems only tangentially related to closeness, and mostly because the vast majority of children are genetically related to their parents. Family is complex, and often has more to do with shared history than anything else.

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