Epistemic status: Wild guesswork based on half-understood studies from way outside my field. More food for thought than trustworthy information.
tl/dr: Estimates of familial relatedness between people should help promote empathy, so here's how to make them - and might this be useful for Effective Altruism?
I don't know how it is for you, but for me, knowing I'm related to someone makes a specific emotional difference. Scenario: I'm at a big family-and-friends get-together, I meet a guy, we get along. (For clarity, let's assume no sexual tension.) And then we're told we're third cousins via some weird aunt. From the moment I'm told, I feel different towards him. Firm, forthcoming, obliging. Some kind of basic kinship emotion, I guess, noticeable when it shifts on these rare occasions but basically going on, deep down in System 1, every time that emailing a remote uncle feels different from emailing a similarly remote associate.
Meanwhile, my System 2 has heard that all humans are at least 50th degree cousins and likes to point out everyone I've ever had sex with was a cousin of some degree. That similarly remote associate where I don't have that kinship feeling - he's a relative too, just a more distant one. And when I notice that, I get a bit of that kinship feeling too...
With me so far? Here's my thesis: the two human feelings of kinship and empathy are closely connected, and to make one of them more salient is to increase the salience of the other.
I don't think this has been tested properly. A. J. Jacobs, who is running a huge family reunion event in New York this summer, said "some ambitious psychology professor needs to conduct a study about whether we deliver lower electrical shocks to people if we know we’re related" and I think he's exactly right.
Has anybody here not heard of circles of empathy? They're a concept invented by the very cool 19th century rationalist William Edward Hartpole Lecky in his "History of European Morals From Augustus to Charlemagne". Peter Singer summarizes it as follows:
Lecky wrote of human concern as an expanding circle which begins with the individual, then embraces the family and ‘soon the circle... includes first a class, then a nation, then a coalition of nations, then all humanity, and finally, its influence is felt in the dealings of man [sic] with the animal world’.
There's more to read about this in Peter Singer's "The Expanding Circle" or Steven Pinker's "The Better Angels of Our Nature", but what strikes me about it is contained in that single sentence: The expansion that is described tracks actual genetic relatedness, or Consanguinity. The list goes down a gradient of (expected) genetic relatedness. This makes the size of the circle of empathy seem to depend on a threshold of how related you need to be to someone in order to care about them.
(Note that Becky published his "History of European Morals" - with this inclusion of concern about animals - in 1869, i.e. only ten years after the publication of "On the Origin of Species". There was some animal rights legislation before Darwin, but animal rights as a movement only arose after we knew animals to be our relatives.)
On the other hand, those who would promote empathy have always relied on familial vocabulary, chiefly "brother" and "sister", to refer to people who evidently weren't actual brothers or sisters. Martin Luther King, Jesus, the Buddha, Mandela, Gandhi, they all do this. So maybe it works a bit. Maybe it helps trigger that emotional kinship response and that somehow helps people get along.
Now to see how these emotional responses would arise, we could discuss reciprocal altruism and gene-centered Darwinism and whatnot, but "The Selfish Gene" is required reading anyway and I assume you've done your homework. I'd like to instead go to the second part of my thesis, the one about increasing salience.
Recognizing you're related to somebody does something. (Especially if you have an incest fetish, of course.) I propose that whatever it does increases empathy. And empathy might not be a categorically good thing, but it comes pretty close, at least until you extend it to all food groups. So maybe we could increase empathy among people by pointing out their relatedness. And maybe we can do this more vividly, more strikingly than by simply saying "we're all descended from apes, so we're all related, duh" or by boring the non-nerd majority to death with talk of human genetic clustering and fixation indexes.
So I'd like to revisit that "brothers and sisters" thing from MLK and those other guys. Maybe they shouldn't have used figurative language. Maybe a more lasting feeling of kinship can be created by literal language: By telling people how related they are. Detailed ancestry information is being collected at various Wiki-like sites, but even assuming they'll grow and become less US-centric, they don't go back very far (except around very famous people) and what came before remains guesswork. So let's do some Fermi-ish estimates.
The drop dead amazing Nature Article Modelling the recent common ancestry of all living humans is way too careful and scientific to put an exact number on how long ago the last common ancestor lived, unfortunately. But the mean date their simulations come up with is 1415 BC, which will be approximately 120 generations ago, so let's say really remote people like the Karitiana tribe are, at most, something like 125th degree cousins of all of us. So that's a useful upper bound for the degree of cousinhood between any two arbitrary humans, such as you and me.
The lower bound could be something like 3 - if you and I were that closely related, we'd share a great-great-grandparent and could probably ascertain rather than guess that. With fairly extensive genealogy, the lower bound might go up to around 5 - which is the level where you need to look at 64 ancestors for each of us who lived in the middle of the 19th century and failed to use Facebook. We'd find it hard to ascertain whether your great-great-great-great-grandmother Mary was identical to mine.
There are a lot of special cases where the lower bound can be higher. If both people involved know their family more than 3 generations were deep-rooted peasant folks from two distinct populations, the history books might tell them how many centuries further back are very unlikely to contain a common ancestor. (This will of course be much rarer among descendants of immigrants, like Americans, than it is for citizens of older or more rural countries.) If they're of different ethnicities, castes or classes that wouldn't normally date each other 80 years ago, the lower bound should probably go up a few more generations. If both people involved are Icelanders, they can just look up their last common ancestor in the comprehensive Icelandic family tree. But let's assume you and I don't have any of these special cases, and we're stuck with a lower bound of 3. Now between that and 125, how do we narrow it down?
Turns out the authors of that gorgeous Nature paper don't hand out access to their simulations to random dudes who just email them. So lets see how far we get on the hard way.
In a completely random mating model (where people do not tend to mate with people who happen to live near them, i.e. happen to be descendants of the same people), your number of ancestors doubles with every generation you go back, in a sort of ancestor tree that grows backwards. We're looking for the point where the two ancestor trees first meet. If we assume generations have homogenous lengths (which implies further simplifying assumptions like moms and dads are the same age) and further assume only people from within the same generation have kids with each other, cousins of the Nth degree have a common ancestor N+1 generations ago, and each has 2N+1 ancestors belonging to that generation.
This means that for you and me to be, say, 15th degree cousins, our two sets of 215+1=65536 ancestors have to have one person in common, some 480 years ago, assuming 30 years as mean parenthood age. Of course we each probably have less than 65536 unique ancestors due to... um... "reticulations".
But empirically, it seems that "a pair of modern Europeans living in neighboring populations share around 2–12 genetic common ancestors from the last 1,500 years" and even individuals from opposite ends of Europe will normally have common ancestors if you search back 3000 years (source). That isn't what you get from the simplistic model above - the numbers of ancestors it calculates exceed the world population less than 32 generations (about 800 years) ago. The empirical genetic data from this paper would indicate that it is likely the median first common ancestor between me and anybody in central Europe is somewhere like 1200 years (or 40 generations) ago and any two people anywhere in Europe would probably be at most 100th degree cousins.
Around 600 years ago is a good time to look at, because that's shortly before intercontinental travel started to intricately connect all regions of the world, including genetically. If most of your 600-years-ago ancestors lived outside Europe, you and I might still be <25 degrees cousins - maybe you have some ancestor who left for Europe 300 years ago, leaving siblings behind (your ancestors) and having kids in Europe (mine). Or vice versa. But that kind of thing is unlikely and since we're doing rough estimates I suggest we round that probability down to zero.
In genetic studies, no other continent is anywhere near as well-studied as Europe, so I guess we'll just have to roll with it and assume that other places are about the same as this paper found and the nice exponential drop-off with geographic distance that's the case in Europe is also the case elsewhere. America and Australia as continents of immigrants continue to be a special cases. But for two people with families from, say, West Africa, I'd be comfortable assuming that if they're from roughly the same large region (say around the Bight of Benin) they're probably something like 40th degree cousins and if not, they're still something like 100 degree cousins at least.
It gets only slightly more complicated if the set of ancestors you know - say your four grandparents - are a mix of descendants from different regions or continents. Just add the number of generations between you and them to your expected degree of cousinhood to everybody from that region or continent.
Needless to say these are all wild guesses. I'm basically hoping someone more qualified than me will see this and be horrified enough to go do the job properly.
Now I'm not an American but statistically you probably are, and you might be more interested in know how closely you're related to other Americans - your boss, your sexual partners, or Mel Gibson. The bad news is that as a member of a nation of relatively recent immigrants, and particularly if your ancestors didn't all come from different continents, you have a harder time estimating most recent common ancestors with people than most other people on Earth. The good news, however, is that the data collected at the large ancestry sites ancestry.com, FamilySearch.org, Geni.com and WikiTree.com are all growing fastest in the US-centric part of their "world trees".
For cousinhood between people whose ancestors seem to have lived on entirely seperate continents as far as anyone knows, I think we can only fall back on our upper bound of 125 degrees of cousinhood. Things get fuzzy so far back, the world population was much smaller, and the population of those who have descendants living today is smaller still. Shared ancestry within any particular generation remains unlikely, but over the centuries and millenia, between trade (particularly in slaves), the various empires and the mass rapes of warfare, genes did get mixed around. Again, see that spectacular Nature paper if you still haven't.
Side note: The most recent common ancestor of two arbitrarily chosen people on different continents is likely to be someone who had kids on different continents. So it is probably a very rich person, a sailor or a soldier, i.e. a male. In general, the number of unique males in anybody's ancestor tree will likely be much smaller than the number of unique females. I expect the difference will be sharper in most recent common ancestors of humans from different continents, because women have shorter fertility windows inside which to travel intercontinentally and don't seem to have moved nearly as much as men except as slaves.
The point of all this is simple. Now you can look at somebody and figure she's not only your cousin, you even have a guess as to the degree of cousin she is. I like to do that when I'm angry with people, because for me, it makes a distinct emotional difference. Maybe try if that works for you too.
Relation to the care allocation problem
I suspect this cousinhood thing could be a fairly principled solution to the problem of how to allocate caring between humans and animals, which Yvain/Scott laid out in a recent SSC post. Why not go by actual (known or estimated) blood relations, and privilege closer relatives over more distant ones?
Our last common ancestor with chimps was something like 5 to 6 million years ago, so our ancestor trees merge about 250000 (human) generations ago, making chimps something like quarter-million-degrees-cousins of all of us. Generations get a lot shorter further back, so our last common ancestor with cattle and dogs, about 92 million years ago, may be 30 million generations ago. Birds would be much more distant, our last common ancestor with them was around 310 million years ago, and so forth. (Richard Dawkins The Ancestor's Tale has much more on this.) For me, this maps rather nicely onto my intuitive prejudices as to how much I should care about which creatures. It fails to map my caring for plants far more than I care for bacteria, but EA has nothing to improve on in that department.
If EA has to have impartiality in the sense that your neighbor can't be more important to you than a tribesman in Mongolia, this isn't EA. Quoth Yvain:
allowing starving Third World people into the circle of concern totally pushes out most First World charities like art museums and school music programs and holiday food drives. This is a scary discovery and most people shy away from it. Effective altruists are the people who are selected for not having shied away from it.
So anybody trying to grow EA might want to make that step easier. Maybe a "closeness multiplier" on units of caring works better than a series of unprincipled exceptions, and still gets across the idea that units of caring are to be distributed between everybody (or everybody's QALYs), if unevenly. And then to become more impartial would be to have that multiplier approach 1.
And if that were the case, my personal preference for how to design that multiplier would be that it shouldn't rely on arbitrary constructs like citizenships. Maybe if EAs want to find a principled solution to the care allocation problem, consanguinity should be one of the options.