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.
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).
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.
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.
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.
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.
I think there’s at least decent truth to it. One study:
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.
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).
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.
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:
And this, where the highlights are:
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.
From Gemini Pro 2.0: