W. D. Hamilton

W. D. Hamilton is portrayed as one of the central architects of the gene-centered explanation of social behavior. His work showed how altruism can evolve when an act benefits copies of the actor’s genes in relatives, and how degrees of relatedness generate testable predictions about behavior and reproductive investment. The book follows that argument from family relationships to insect societies, population clustering, parasite–host conflicts, and the analysis of cooperation between strategies.

Kinship and the mathematics of altruism

Hamilton’s key insight, as the book presents it, was that the genetic logic already used to explain parental care also applies to siblings and other close relatives. A gene that leads an individual to help a relative may persist because the beneficiary has an above-average chance of carrying that same gene. In the stark case of an individual dying to save ten close relatives, one copy of an altruistic gene may be lost with the actor while multiple copies are preserved among those saved. The argument shifts the question from whether an act looks self-sacrificing to how many copies of the relevant gene survive through its consequences.

The author singles out Hamilton’s two papers of 1964 as among the most important contributions to social ethology, while noting that they had been neglected: Hamilton’s name was absent from the indexes of two major ethology textbooks published in 1970. Their mathematical reasoning can, the book argues, be grasped intuitively, although that comes at the cost of simplification. Relatedness is the probability that two individuals share a particular gene by descent. A pair of brothers has an average relatedness of one-half, as does a parent and child; a pair of first cousins has a lower value because the shared genes must pass through more generations.

Hamilton’s approach turns that probability into a practical rule for evaluating altruism. For each common ancestor, the contribution to relatedness is one-half multiplied by itself once for every generational step along the path connecting the two individuals. Contributions from multiple recent common ancestors are added. The book illustrates the method with an uncle and nephew, whose generation distance is three, and with first cousins, who share two recent common ancestors and have a generation distance of four through each. The resulting estimate is an average, not a guarantee about any particular pair: the genes shared by two brothers can be more or fewer than the expected half.

From these relationships, the author derives thresholds for when lethal help could favor an altruistic gene. Saving five cousins would not, on average, compensate for the actor’s death; saving five brothers or ten first cousins could. The corresponding thresholds are more than two siblings, children, or parents; more than four half-siblings, uncles, aunts, nephews, nieces, grandparents, or grandchildren; and more than eight first cousins. The point is not that animals calculate such totals, but that selection can favor behavioral rules whose average effects preserve enough copies of a gene.

What counts as kin

The account distinguishes genetic relatedness from the categories people use to recognize and organize kin. Human beings identify relatives through names, formal marriages, records, and memory, while social customs and rituals give kinship strong cultural force. Ancestor worship, family obligations, and loyalties are cited as examples; blood-feuds and inter-clan warfare are described as interpretable through Hamilton’s theory. The author also separates the likely evolutionary rationale for incest taboos from altruism, proposing that their genetic advantage concerns harmful recessive traits appearing through inbreeding.

For nonhuman animals, Hamilton’s framework does not require a conscious ability to identify a family tree. The relevant question is what simple behavioral rule could, under ordinary conditions, lead to help reaching relatives more often than unrelated individuals. Resemblance might serve as an imperfect cue: animals disposed to favor those that look like themselves could, statistically, direct some aid toward kin. The rule would not always work, especially if circumstances changed, and the author suggests racial prejudice as a possible irrational extension of a tendency to associate with physically similar individuals. The example underscores the difference between a behavior’s possible evolutionary effect and an animal’s awareness of that effect.

The author uses the adoption of orphaned young to show how kin-directed tendencies can misfire. Monkey-watchers may call an adopting female an aunt, although in many cases there is no evidence that she is related to the orphan. Adoption may therefore be a mistake of a built-in rule rather than an act that benefits the adopter’s genes: the female spends time and energy caring for the orphan instead of investing them in her own kin, including possible future offspring. The book suggests that selection may not have made maternal behavior more discriminating because such cases occur too rarely. In other cases, an orphan is left to die.

A more difficult example is a bereaved monkey mother who takes another female’s baby and cares for it. The author calls this a double mistake: the adopter incurs the cost of care, while the child’s mother is released from that burden and may reproduce again sooner. Rather than treating the case as a settled refutation of gene-centered theory, the book identifies questions that research would need to answer: how often it occurs, how closely related adopter and child tend to be, and whether the child’s mother might benefit by inducing a young female to adopt. The example keeps the theory open to investigation rather than allowing a striking anecdote to decide the issue.

Kin selection and the limits of group selection

The author accepts kin selection as a useful name for selection favoring altruism within families, with stronger effects expected as relatedness rises, but disputes the way the term can blur distinct levels of explanation. He objects to E. O. Wilson’s definition of kin selection as a special case of group selection. A family can be called a kind of group, but Hamilton’s theory does not divide animals into a family inside which altruism is expected and outsiders toward whom it is not. Relatedness is a matter of degree: second cousins, for instance, are expected to receive altruism less often than offspring or siblings, not to fall on the other side of a fixed family boundary.

On this account, kin selection is a consequence of gene selection, not an intermediate category between individual and group selection. The author also challenges Wilson’s exclusion of offspring from kin selection, despite their genetic relationship to their parents. Parental care and care between siblings evolve for the same basic reason: the beneficiary has a substantial chance of carrying copies of the altruistic gene. The distinction matters because familiar parental behavior can otherwise seem to require an explanation separate from the one Hamilton offered for helping other relatives.

Social insects and conflicting interests

Hamilton’s reasoning reaches a particularly striking result in the discussion of social insects. In ants, bees, and wasps, a female worker may be more closely related to her full sisters than to her own offspring. This creates a possible evolutionary advantage to remaining sterile and directing effort toward producing sisters through the queen. The book presents this as a dramatic application of kin selection, developed by Hamilton and later taken further by Robert Trivers and Hare.

That arrangement does not mean queen and workers have identical interests. Workers would benefit, in the model, from a reproductive investment ratio of three females to one male, whereas a mother’s stable ratio is one to one. The same gene can therefore favor different outcomes depending on whether it is acting through a queen’s body or a worker’s body: each body provides different means of influencing the next generation. The author describes the resulting conflict as a contest over who controls the colony’s reproductive output.

Trivers and Hare tested the prediction by examining the investment in male and female reproductives among 20 ant species, weighing the reproductive forms because males and females can differ considerably in size. Their results closely fitted the workers’ predicted three-to-one female-to-male investment ratio. In the species studied, the workers appeared to prevail, consistent with the practical power workers exercise as guardians of the nurseries. Hamilton’s framework thus connected degrees of relatedness to an observable difference in how a colony allocates reproductive resources.

Slave-making ants offered a contrasting test. Their specialized workers conduct raids on another species’ nest and carry away its unhatched young; the captives grow up performing ordinary nest duties, while the raiders continue their expeditions. Because the enslaved workers’ genes are not expected to enter the captors’ reproductives, they cannot effectively influence the next generation in the captors’ colony. The theory therefore predicts that slave-making species should tend toward equal investment in the two reproductive sexes rather than the workers’ three-to-one preference. Trivers and Hare found a ratio approaching one to one in the two slave-making species they examined, a result presented as a case in which the queen could prevail.

The account also marks complications rather than presenting the model as a perfect fit. Honeybees appear to invest more in males than in queens, but Hamilton proposed that the workers accompanying a departing queen swarm should count as part of the investment in reproductive females: producing each departing queen requires many additional workers, who are lost to the parent hive. A different complication arises when queens mate with several males. Their daughters then share fewer genes on average, weakening the workers’ special relatedness to sisters. The author resists turning this into a simple story of workers acting as a united class: each worker’s evolutionary interests are her own, not those of a permanent workers’ trade union.

From populations to wider evolutionary arguments

Hamilton’s work also provides the framework for the book’s account of the selfish herd. In a herd threatened by a predator that tends to attack the nearest prey, each individual is at risk according to its position, particularly when it is on the edge. Hamilton’s geometrical model describes the area in which a given prey animal is the nearest target as its domain of danger. The author stresses that a selfish herd means a herd of selfish individuals, not a group acting for a collective good. Hamilton’s idea links the local arrangement of animals to each individual’s attempt to reduce its own exposure.

The book also invokes population viscosity to explain how cooperative strategies might become locally concentrated. Individuals who remain near their birthplace are more likely to live among relatives than among randomly selected members of the wider population. The author recalls visiting a remote island off Ireland’s west coast, where many inhabitants had conspicuously large ears; he attributes the shared trait not to adaptation to the windy setting but to the inhabitants’ close kinship. Relatedness can make behavioral tendencies cluster as well as physical traits, allowing a strategy such as Tit for Tat to meet copies of itself locally even when it is rare in the population as a whole.

Hamilton’s influence reaches the book’s discussion of parasites too. When a parasite’s genes are transmitted by a route different from the host’s, the two sets of genes can have conflicting reproductive interests. A fluke that makes a snail produce a thicker shell may prolong the host’s life, but the shell costs resources that the snail could otherwise devote to reproduction. Since the fluke does not depend on the snail’s offspring as its own route to the future, it may benefit from survival even at a cost to the snail’s reproductive success. The book presents this as a conjecture about how parasite genes could alter a host’s body for their own advantage.

The corresponding principle is about transmission. If parasite and host genes reach future generations through the same vehicles, their interests are expected to converge: the parasite can benefit from helping the host survive and reproduce, and over evolutionary time the association may become cooperative or merge into host tissues. Hamilton’s gene-centered reasoning therefore supports a broader question about whether two sets of genes share a route onward, rather than relying only on whether one organism appears to help another.

Finally, Hamilton’s evolutionary perspective intersects with the book’s account of repeated cooperation. After Axelrod’s second Prisoner’s Dilemma tournament, the author urged him to contact Hamilton, who was working in another department at the University of Michigan. Axelrod and Hamilton subsequently collaborated on a paper published in Science in 1981, which received the Newcomb Cleveland Prize of the American Association for the Advancement of Science. Their work gave explicit recognition to the evolutionarily stable strategy approach, which the book contrasts with tournament rankings dependent on the particular strategies submitted. Across these examples, Hamilton’s significance lies in a consistent explanatory move: tracing social outcomes back to the conditions under which genes persist and are transmitted.