In biology, altruism has a purely descriptive economic meaning: the active donation of resources to one or more individuals at cost to the donor. Moral values or conscious motivations are not implied, and the ideas are as applicable to plants as to animals. Four evolutionary causes of altruism will be considered here: kin selection, reciprocation, manipulation, and group selection. Each implies demonstrably different patterns for what individuals donate what resources to whom and under what circumstances and may suggest different motivational and emotional experiences by both donor and recipient.

It may seem that Darwinian EVOLUTION, directed by natural selection, could never favor altruism. Any avoidable activity that imposes a cost, always measured as reduced reproductive fitness, would be eliminated in evolution. This view is too simple. Natural selection should minimize costs whenever possible, but successful reproduction always requires donation of resources to offspring, at least by females putting nutrients into eggs. A closer look shows that offspring are important to natural selection only because they bear their parents' genes, but this is true of all relatives. From the perspective of genetics and natural selection, the survival and reproduction of any relative are partly equivalent to one's own survival and reproduction. So there must be an evolutionary force of kin selection that favors altruism between associated relatives.

Kin selection, first clearly formulated by Hamilton (1964), can be defined as selection among individuals for the adaptive use of cues indicative of kinship. The products of mitotic cell division are exactly similar genetically, and their special physical contact is reliable evidence of full kinship. This accounts for the subservience of somatic cells in a multicellular organism to the reproductive interests of the germ cells. Kin selection also accounts for the generally benign relations among young animals in the same nest. Such early proximity is often a cue indicative of close kinship. Nestmates are often full sibs, with a genetic relationship of 0.50. They could also be half sibs if the mother mated with more than one male. They may not be related at all if one or more eggs were deposited by females other than the apparent mother. Such nest parasites are often of the same species in birds, but some species, such as the European cuckoo and the American cowbird, reproduce exclusively by parasitizing other species. Their young's competition with nest mates has not been tempered by kin selection, and this accounts for their lethal eviction of the offspring of the parasitized pair. Many sorts of cues other than early proximity can be used to assess kinship, such as the odors used by mammals and insects to recognize relatives and make genetically appropriate adjustments in altruism. The classic work on mechanisms of kin recognition is Fletcher and Michener (1987); see Slater (1994) for a critical updating.

In the insect order Hymenoptera (ants, bees, and wasps), a male has only one chromosome set, from an unfertilized egg of his mother, and his sperm are all exactly the same genetically. So his offspring by a given female have a relationship of 0.75 to one another. This factor has been used to explain the multiple independent instances, in this insect order, of the evolution of sterile worker castes that are entirely female. These workers derive greater genetic success by helping their mothers produce sisters than they would by producing their own offspring, which would have only a 0.50 genetic similarity. These special relationships are not found in termites (order Isoptera) and, as expected, both males and females form the termite worker castes.

Reciprocity is another evolutionary factor that can favor altruism. The basic theory was introduced by Trivers (1971) and refined by Axelrod and Hamilton (1980). One organism has a net gain by helping another if the other reciprocates with benefits (simultaneous or delayed) that balance the donor's cost. Cleaning symbiosis between a large fish and a small one of a different species may provide simultaneous reciprocal benefits: the large fish gets rid of parasites; the small one gets food. This reciprocation implies that the small fish is more valuable as a cleaner to the large fish than it would be as food. Reciprocity is a pervasive factor in the socioeconomic lives of many species, especially our own. It requires safeguards, often in the form of evolved adaptations for the detection of cheating (Wright 1994).

Manipulation is another source of altruism. The donation results from actual or implied threat or deception by the recipient. In any social hierarchy, individuals of lower rank will often yield to the higher by abandoning a food item or possible mate, thereby donating the coveted resource to the dominant individual. Deception often works between species: a snapper may donate its body to an anglerfish that tempts it with its lure; some orchids have flowers that resemble females of an insect species, so that deceived males donate time and energy transporting pollen with no payoff to themselves. The nest parasitism discussed above is another example. Our own donations of money or labor or blood to public appeals can be considered manipulation of donors by those who make the appeals.

Group selection is another possibility. Individuals may donate resources as a group-level adaptation, which evolution can favor by operating at the level of competing groups rather than their competing members. A group of individuals that aid each other may prevail over a more individually selfish group. A difficulty here is that if selfishness is advantageous within a group, that group is expected to evolve a higher level of individual selfishness, no matter what the effect on group survival. The original concept of group selection focused on separate populations within a species (Wynne-Edwards 1962; Wade 1996). This idea has few adherents, because of the paucity of apparent population-level adaptations (Williams 1996: 51-53), because altruistic populations are readily subverted by the immigration of selfish individuals, and because the low rate of proliferation and extinction of populations, compared to the reproduction and death of individuals, would make selection among populations a relatively weak force.

More recently attention has been given to selection among temporary social groupings or trait groups (Wilson 1980), such as fish schools or flocks of birds. Trait groups with more benign and cooperative members may feed more efficiently and avoid predators more effectively. The more selfish individuals still thrive best within each group, and the evolutionary result reflects the relative strengths of selection within and between groups. In human history, groups with more cooperative relations among members must often have prevailed in conflicts with groups of more consistently self-seeking individuals (Wilson and Sober 1994). The resulting greater prevalence of human altruism would be more likely to result from culturally transmitted than genetic differences. It should be noted that any form of group selection can only produce modifications that benefit the sorts of groups among which selection takes place. It need not produce benefits for whole species or more inclusive groups.

A given instance of altruistic behavior may, of course, result from more than one of these four evolutionary causes. Genealogical relatives are especially likely to indulge in both reciprocation and manipulation. If reproductive processes result in stable associations of relatives, these kin-groups are inevitably subject to natural selection. The most extreme examples of altruism, those of social insects, probably resulted from the operation of all the factors discussed here, and social insect colonies may aptly be termed superorganisms (Seeley 1989). Excellent detailed discussions of altruism in the animal kingdom and in human evolution, and of the history of thought on these topics, are available (Ridley 1996; Wright 1994).

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-- George C. Williams


Axelrod, R., and W. D. Hamilton. (1980). The evolution of cooperation. Science 211:1390-1396.

Fletcher, D. J. C., and C. D. Michener. (1987). Kin Recognition in Animals. New York: Wiley-Interscience.

Hamilton, W. D. (1964). The genetical theory of social behaviour, parts 1 and 2. Journal of Theoretical Biology 7:1-52.

Ridley, M. (1996). The Origins of Virtue. New York: Viking Press.

Seeley, T. D. (1989). The honey bee as a superorganism. American Scientist 77:546-553.

Slater, P. J. B. (1994). Kinship and altruism. In P. J. B. Slater and T. R. Halliday, Eds., Behavior and Evolution. Cambridge University Press.

Trivers, R. L. (1971). The evolution of reciprocal altruism. Quarterly Review of Biology 46:35-57.

Wade, M. J. (1996). Adaptation in subdivided populations: kin selection and interdemic selection. In M. R. Rose and G. V. Lauder, Eds., Adaptation. San Diego: Academic Press.

Williams, G. C. (1996). Plan and Purpose in Nature. London: Weidenfeld and Nicholson.

Wilson, D. S. (1980). Natural Selection of Populations and Communities. Boston: Benjamin/Cummings.

Wilson, D. S., and E. Sober. (1994). Re-introducing group selection to the human behavioral sciences. Behavioral and Brain Sciences 17:585-654.

Wright, R. (1994). The Moral Animal: Why We Are the Way We Are. Vintage Books.

Wynne-Edwards, V. C. (1961). Animal Dispersion in Relation to Social Behavior. London: Oliver and Boyd.