in 1964, william d. hamilton published a couple of papers outlining his theory of inclusive fitness in which he suggested that individual organisms may increase their total fitness (i.e. the propogation of their genes) not only by reproducing themselves, but also by helping out other individuals with whom they share genes to reproduce successfully. if, in addition to having two children myself (or none at all for that matter), i also help my two siblings — with whom i share genes — to raise a couple of kids each, then i have helped to propogate even more copies of my own genes than those that my children carry. i have increased my fitness.

hamilton gave some examples in one of these papers of how this might work in various animal societies. he talked a lot about ants, for one, and how inclusive fitness explains the quirky social structure of the ant colony — i.e. that (in most ant societies) only the queen reproduces and all the rest of the ants just work to help raise her kids. why would any ant in their right mind do such a thing? well, the key is that, because of the way ants reproduce (don’t ask — it’s too complicated!), the worker ants actually share three-quarters (0.75) of their dna with their sisters rather than, say, one-half (like in humans), so it makes more sense for them fitness-wise to help raise their sisters than their own offspring. hamilton had cracked the mystery of the seemingly incomprehensible altruistic behaviors of ants.

much hilarity research into inclusive fitness and altruism has ensued since the publication of hamilton’s articles.

and while that’s all really interesting (really, really interesting!), hamilton thought that inclusive fitness could explain not just altruistic behaviors, but all sorts of social behaviors in organisms (including humans, of course), as is evidenced by the title of another one of his papers: “Innate Social Aptitudes of Man: an Approach from Evolutionary Genetics.” not the “innate altruistic aptitudes of man” but the “innate social aptitudes of man.” many different types of human behaviors hinge on inclusive fitness from the altruistic to the (depending on your p.o.v.) very un-altruistic.

one set of behaviors that seems to be influenced by inclusive fitness considerations is the control of reproduction in others. this has been fairly well established in certain social animals (mongooses, meerkats, naked mole-rats); but it also — not surprisingly — seems to be the case in humans. i haven’t discussed it much on the ol’ blog here, but i have brought it up once or twice. well, once anyways. humans — like mongooses, meerkats and naked mole-rats — take a keen interest in who their relatives choose to mate with — and they’re (we’re) more interested in the mating choices of closer relatives than more distant ones. this makes sense because we share more genes with more closely related relatives.

so, all sorts of social behaviors beyond altruistic ones are probably affected by inclusive fitness.

the other thing to keep in mind (which i’ve been babbling about at length here on the hbd chick blog) is that different populations of people have different degrees of relatedness to their fellows due to different mating patterns. this makes the inclusive fitness thing all the more interesting because, for instance, in many societies around the world, peoples’ children are also their cousins and so they (probably) share more genes with their children than we do with ours. therefore, their inclusive fitness interests in their children will be somewhat — or, perhaps, very — different from ours.

different degrees of relatedness (looking away from any inbreeding for a sec) have been shown to affect the behaviors of family members toward one another. the different types of grandmothers (paternal versus maternal), for instance, behave differently towards their male and female grandchildren because of inclusive fitness-related interests. (grandmothers are related to their granchildren to various degrees due to the differential inheritance of the sex chromosomes.) imagine what happens when inbreeding occurs and these different degrees of relatedness are, therefore, amplified.

mating patterns affect social behaviors right across entire societies because the relatedness between individuals in different societies differs. thus you get the rampant nepotism and clannishness in places like iraq and afghanistan that makes, as steve sailer, parapundit, stanley kurtz and robin fox have all pointed out, a political system based on democracy a non-starter in those places. the people in those populations are too genetically invested in their fellow family members to ever want to cooperate in a civil society with unrelated individuals. there is too much genetically at stake for them to do so.

the flip-side of inbreeding (too much?) is outbreeding (too much?). outbreeding results in a different set of inclusive fitness-related drives and issues as compared to inbreeding. in a population that is not inbred you get, as steve sailer put it, “broad but shallow regional blood ties.” inclusive fitness interests in an outbred population operate in such a way as to make, not the members of a clan (a very extended family) driven to cooperate against all outsiders like in an inbred society, but rather a much larger number of more distantly related individuals — a nation you might call it — really eager to work together. this is because the members’ genes are spread out over a wider population.

and, in a nice recursive twist, inclusive fitness and genetic relatedness affect the structures of societies — which, in turn, become new environments with new factors for natural selection to “use” upon individuals. a shift in mating patterns can mean a shift in the social environment, as we’ve seen in medieval europe (if you’ve been following along), which can mean that different behavioral traits will be selected for in a population. (yes, i know — i should read “A Farewell to Alms.”) what goes around comes around.

so, inclusive fitness underlies not only altruism and social control of reproduction, but nepotism and tribalism — and even individualism and democracy and universalism. genetic relatedness and inclusive fitness are the keystones of human social behaviors from the small (whether your favor your son or your daughter) to the great (western civilization as we know it). am i over-estimating its importance? maybe. but i don’t think so. it’s all biology, after all. (or maybe chemistry … or physics ….)

edit: boilerplate and boilerplate 2.0

see also: cousin marriage conundrum addendum and everything in the “Inbreeding in Europe Series” listed below in the left-hand column.

update 11/23/11: see also four things.

(note: comments do not require an email. altruism?)


  1. How does inclusive fitness in ants come about? Proteins don’t know what they’re doing so how did they develop a system where proteins in one organism help proteins in another organism?


  2. @big nose kate – “How does inclusive fitness in ants come about? Proteins don’t know what they’re doing so how did they develop a system where proteins in one organism help proteins in another organism?”

    natural selection. what works, works.


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s