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).
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).