In biology, one of the two most common and best understood forms of altruism. The other is reciprocal altruism. Kin altruism occurs because close relatives (kin) have a higher probability of sharing genes than random members of the population.

Obviously, parents take care of or invest in their children. Even parents who are no longer there when their eggs hatch, have provided their offspring with nutrition in the form of the egg. This is so obviously necessary for the perpetuation of the lineage that this parental altruism is often not regarded as altruism; and the term kin altruism is sometimes restricted to other forms of investment: in younger siblings, nieces and nephews, grandchildren, and so on.

But, although there are particular reasons why the parent-child relationship is slightly different from other forms of kinship, the genetic basis is essentially the same in all cases, and the same effect is occurring.

In normal sexually reproducing organisms, parents and children share half their genes; as do full sisters and brothers. More distant relatives share proportionately less. Your genes are likely to benefit if you help your sister; less so if you help your cousin. "Your" genes are not just those copies in your body, inherited from one or other of your parents, but the same gene in anyone else who has inherited it from the same source.

In all this, though I'm using human kin terms, it does not really apply to humans, because we have too many cultural reasons to behave altruistically or otherwise, and culture develops faster than biological evolution. However, the effects of kin selection are still sometimes detectable: it is reported that stepchildren are killed more often than blood children.

Natural selection that occurs as a result of the good done by kin altruism is called kin selection. This is occasionally erroneously (or at least very unhelpfully) classed as a form of group selection, that is where changes occur for the benefit of a group (a family, a species, whatever). It is generally agreed that group selection in this proper sense does not and cannot occur.

Most sexual organisms are diploid, having chromosomes in pairs, one member of each pair coming from each parent. This gives the coefficient of relationship of 1/2 between parent and child, or between full siblings, 1/4 between half siblings, and so on: the probability that any given gene in one body will also be in the other body, because of their common descent.

A haplodiploid creature has different sexual arrangements, and this typically leads them to being eusocial, like bees and ants, with a single fertile female and a sterile caste. The sterile creatures don't want children of their own because they are more closely related to their siblings than they would be to their own children. So it's to the advantage of their genes to work for or guard the hive or nest, and not breed. It is still kin altruism at work, but with different probabilities. (See Sister power! Close relationships and hymenopterans for a more detailed discussion.)

Organisms do not practise kin altruism because it is good for the other individual they are benefiting. The other individual benefits, but that is not the reason for it. Nor does the organism do it for selfish reasons, i.e. to benefit itself. By definition, altruism is a direct loss of benefit to itself - though it may gain by later return of the favour: but if it does, this is the unrelated phenomenon of reciprocal altruism, which has a different genetic basis.

There is no intrinsic reason to favour your kin as such. The reason they're valuable to your genes is that they are likely to contain your genes. One gene prospers when it helps many copies of itself arise and survive. One way of doing this is by causing your body to have many offspring, each of which is likely (probability = 0.5) to contain you (the gene).

Another way is to be able to recognize other bodies that are likely to contain you. And one good bet on that count is if you can recognize kin in general. One good way of doing that is if you behave altruistically to individuals brought up with you. These are all probabilistic devices for taking a good guess at who shares your genes. Cuckoos exploit this by the fact that usually the thing in a mummy bird's nest is mummy's baby bird, so mummy is usually on the right track genetically by feeding any bird in its nest.

Another way of recognizing kin is by clusters of features: how much they resemble you in looks, smells, voice, or whatever. If you're a gene in a body that closely resembles another body, you have no particular interest in fostering the other genes that cause those, but they mark the likelihood that the body is of recent common descent with you and therefore could contain you.

Having (say) red hair, and being altruistic, are two entirely different effects of genes. It is possible that one gene could have two separate effects, (i) giving you red hair, and (ii) making you behave altruistically to people who giggle a lot. One gene having two unrelated effects is called pleiotropy. But it is vanishingly unlikely that a single gene could have the two pleiotropic effects of having red hair, and behaving altruistically to people with red hair. The chemical pathways for these effects are totally different. Richard Dawkins called this hypothetical coincidence the green-beard effect. Years later an example was reported in nature. [Don't click on the link: I haven't noded it yet, and it's pretty much explained here.]

But normally it's impossible to recognize that another individual actually bears the same gene for altruism as you do: so if you behave altruistically to those most likely to bear it (your kin), on average the gene for altruism will be helped to survive. So all the other genes in the helped individual's body will also benefit.

Further reading: The Extended Phenotype (1981) by Richard Dawkins contains vastly more detailed discussion. His much more accessible The Selfish Gene (1976, revised 1989) helps me understand the other book.

Kin Altruism and Homosexuality

Caveat: I believe that human sexuality is a rich spectrum, not binarily hetero or homo. When I use the term homosexual in this writeup, I mean individuals exhibiting strictly same-sex sexual behavior. I know it's shorthand, but it makes things smoother in the text. Also, I presume the studies showing a moderate heritability of homosexuality are correct, i.e. that there is some genetic basis for its presence in humans.

Homosexuality presents quite a problem to evolutionary thinking. How could homosexuality have ever survived evolutionary forces to be as common as it today across the world? After all, time was it took a man and a woman to spread genes by making babies. Since same-sex pairings can't make babies, aren't "the gays" a genetic dead end?

(This line of thinking is what led cackling plastic sculpture Dr. Laura Schlessinger to announce that homosexuality is a "biological error", with all of its terrifying Third Reich inferences. But even without the vitriol, the question seems fairly tough at first.)

The short answer is no, because of course it's not that simple.

As Gritchka notes in the above writeup of the general theory of kin altruism, evolution supports preferential behavior towards individuals who share genetic makeup. In his 1978 Pulitzer Prize-winning book On Human Nature, Edward Wilson extended this idea (it was his in the first place, Dawkins just made it more accessible) to posit that while exclusive homosexuality does not directly contribute to the gene pool, the kin altruism of homosexuals has a greater effect than that of heterosexual kin because the homosexuals are unburdened with children of their own, and this compensates. Put another way, your kids are more likely to get more presents on their birthday if you have a gay sibling than a straight one. (That is, of course, if you're on good terms with them and don't call them biological errors.)

Other theorists have elaborated indirect evidence to support the theory:

  • Male homosexuals are poor heterosexual mating prospects, and so the fitness costs for not having their own progeny is low.
  • They are on average more empathic and nurturing, and therefore more likely to act on their kin altruism.
  • Homosexuals tend to be born later rather than earlier in birth order, increasing the likelihood that there would be nieces and nephews around to spoil.

David M. Buss in his 2004 book The Evolution of Desire, cites three problems with the theory.

  1. It doesn't account for exclusive homosexual behavior in the first place: why wouldn't it encourage asexuality instead?
  2. To be valid, there must be some evidence to indicate that greater empathy translates to specific kin benefit.
  3. The kin benefits would have to be quite high to compensate for the loss of direct reproduction.

I'm not an evolutionary scientist, and this last one seems like it would take more statistics that I can muster, but the first two seem easy to address from a layman's point of view.

Why homosexuality rather than asexuality? Because the sexual drive is evolutionarily much older and much more deeply embedded than mammalian communal drives. For this reason it seems that it would be much easier for evolution to tweak sexual drive than to eradicate it. Furthermore, the theory doesn't seek to explain the origin of homosexuality (after all, it appears in thousands of species much older that that arrogant upstart Homo Sapiens Sapiens), just the reasons why it became optimized at a consistent 1%-4% rate across the species.

Why must greater empathy translate to specific benefit? I think this is a straw man argument, because increased empathy is not the lynchpin of the theory. It would still hold true if homosexual kin had the exact same levels of empathy as their heterosexual counterparts, because they are unburdened with children of their own. As long as kin altruism exists, this translates into some gain for the children, and evolution would seek to optimize it.

For his death knell to the theory, Buss cites a cross-sectional study by David Bobrow and Michael Bailey that indicates that modern gay and lesbian kin investment is equal to heterosexual kin investment. But does saying it doesn't work that way now mean it couldn't have been that way in the past? The evolutionary forces that shaped homosexuality in our species may simply no longer pertain. (Just as the evolutionary forces that optimized our spinal column for brachiation don't apply now either, but that's the spine we got. See Gould's spandrel theory.) Modern mobility and the stigmatization of homosexuality have clearly lessened the likelihood, opportunity, and amount of investment in the grand scheme of things.

Seen this way, the recent (circa 1600 CE) Western stigmatization of homosexuality is strangely self-denying. A society truly interested in family values should embrace their gay uncles and lesbian aunts. It should encourage their kin altruism as much as possible. And yet. And yet. And yet.

Childless Relatives

As Tato pointed out above, relatives without children of their own can contribute to the future success of their other relatives' children.  Single-parents today often experience the most stress when raising their children.  Even couples often find the need to ask grandparents for help in taking care of their children.  If the general environment changes in such a way such that parents find they have to spend more and more time just to (for example) be able to pay the bills, then they may find that they need to recruit more and more people to help in the raising of their children.  If a couple is better than a single-parent in a situation like this, one might imagine that a couple plus a gay / childless sibling is better than a couple alone.

Matryoshka Dolls

I believe an important question to ask is: Who are our kin anyway?  If you follow the idea that we all descended from Adam and Eve, then we are in fact all kin (some more distant than others, of course).  Similarly, in the evolutionary sense, if our species came from common ancestors, then we are still all kin.  So when discussing "kin altruism" and preserving the survival of your "selfish genes", one might say that by helping any human being, we are in fact helping our own genes survive, as opposed to, for example, helping the genes of bacteria survive.

When considering the concept of competing genes, it is important to look at the scale of competition. If it's individual competition, then you'd screw anyone in your family, even your children, to favor your clones.  If it's family competition, then you'd screw anyone you don't consider to be part of your family, or too distant of a relative.  If it's ethnic group competition, then you'd screw anyone you don't consider to be in your ethnic group. If it's species competition, then you'd screw any organism that your species can't mate with (unless that species were useful in other ways, such as domesticated chickens or cabbage).  Beyond species, if your "selfish genes" were just trying to propagate themselves, then they may "predispose" you to favor primates, mammals, chordates, animals, organisms, organic matter, or whatever.

Beyond Genes

Imagine a science fiction future in which humans came in contact with an alien civilization (that we can't mate with) - perhaps they aren't even composed of biological matter, but merely mechanical beings.  Would they have a history of social organization similar to ours?  Perhaps and perhaps not.  We may introduce to them our civilization's own past experience with monarchy, dictatorship, democracy, capitalism, communism, anarchism, social democracy, etc.  Perhaps even then we still haven't worked out a very good solution.  They too may introduce to us their own history of various different kinds of social organization.

What happens after this kind of memetic cross-pollination?  Some of us may start to take sides with some of the aliens in fighting for one ideology, while others of us take sides with other aliens to fight for other ideologies. When this occurs, "kin altruism" no longer happens in terms of helping your genetic relatives, but instead, it happens in terms of helping your "memetic relatives" - those who hold ideas similar to your own.  Some may be focused on actually attacking their memetic competition, others may be focused on merely ensuring the survival or security of their own "memetic kin", while yet more may be focused on trying to convert your "non-kin" into your "kin".

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