1964

1964 is the publication year of W. D. Hamilton’s two papers that the book presents as a major advance in explaining kin altruism. The year anchors an argument that runs from the probability relatives share genes to the behavior that can follow from that shared inheritance, while also opening questions about how kinship is recognized and represented in human societies.

Hamilton’s Papers and Kin Selection

The book describes Hamilton’s two papers of 1964 as among the most important contributions to social ethology. Their central problem is how an apparently altruistic act can succeed through the copies of a gene carried by relatives. An individual that dies saving close kin may lose one copy of a gene, yet preserve more copies in those relatives than the sacrifice removes. Hamilton’s work gives this reasoning a more precise basis than the loose terms “close relatives” and “a larger number.”

The explanation depends on relatedness: the chance that two individuals share a particular gene. A sibling pair has an average relatedness of one-half, as does a parent and child; the parent-child value is always exactly one-half, while the sibling value can vary with the genetic inheritance of the particular pair. From this logic, the book gives thresholds for costly aid: a gene favoring the rescue of more than two siblings, children, or parents may spread; the corresponding thresholds are more than four half-siblings and more than eight first cousins. The decisive measure is not the emotional appearance of an act but whether enough copies of the gene persist among those helped.

A Practical Measure of Kinship

The discussion turns the abstract idea into a rough method for calculating relatedness. First identify the most recent common ancestors of two individuals, then count the steps from one individual up to an ancestor and back down to the other. For first cousins, the shared grandparents are the relevant common ancestors; for an uncle and nephew, the generation distance is three. The method is offered as a practical rule, even for interpreting family resemblances or making a will, though the book cautions that it fails in cases of incestuous mating and in certain insects.

The arithmetic also depends on uncertainty. If two individuals might be either half-brothers or full brothers, their relatedness cannot be treated as a known fixed value: the usable estimate is an average weighted by the likelihood of each relationship. The book’s example supposes the same mother and only a one-in-ten chance of the same father, producing an effective relatedness of 0.275. Such calculations distinguish an observer’s estimate from what an animal may be able to identify in practice.

Care, Kinship, and Social Meaning

Kin selection makes parental care a special case rather than a genetically unique form of altruism. An adult is related to its own child and its orphaned younger sibling by the same degree, one-half; in the book’s gene-centered account, a gene promoting care by an older sibling could therefore spread as successfully as one promoting parental care. The text stresses that this is an oversimplification and that sibling care is much less common in nature, but the genetic comparison challenges the assumption that parent-child care has a special status in itself.

Human kinship, however, is not simply a reading of genetic relatedness. People learn who their relations are through names, formal marriages, written records, and memory. Kinship customs, ancestor worship, family loyalties, blood-feuds, and inter-clan warfare are discussed as social practices that can be interpreted alongside Hamilton’s theory, while incest taboos are distinguished from altruistic kin selection: the book suggests their genetic advantage concerns the harms associated with inbreeding. Its account therefore separates biological relatedness from the cultural ideas and obligations through which people understand kin.

Recognition Rules and Their Limits

Animals cannot be assumed to possess exact knowledge of genealogies. Instead, the book asks what simple inherited rules might produce behavior that benefits relatives on average. A tendency to favor individuals who physically resemble oneself could sometimes direct aid toward kin, but it would only make statistically reliable choices under particular conditions. If a species began living in much larger groups, the same rule could misfire; the text also proposes racial prejudice as a possible irrational extension of a tendency to identify with similar-looking individuals and reject those who look different.

Adoption offers another example of how such rules can fail. An unrelated female that cares for an orphan may spend time and energy that could otherwise benefit her own kin, so the book treats many adoptions as possible misfires of a built-in rule rather than straightforward evidence against gene-centered explanation. It notes that orphans are often left to die, and that a bereaved monkey mother has sometimes been observed to take and care for another female’s infant. That more extreme case raises unresolved questions about the adopter’s relatedness to the infant, the real mother’s response, and whether adoption is frequent enough to matter evolutionarily.

The year’s importance in the book is thus both historical and conceptual: Hamilton’s papers provide a quantitative framework for explaining kin-directed altruism, while the surrounding discussion shows that relatedness must be estimated, translated into workable behavior, and distinguished from the social meanings humans attach to family.