W. D. Hamilton (1964), Journal of Theoretical Biology 7(1): 1–16 (received 13 May 1963, revised 24 February 1964) — doi:10.1016/0022-5193(64)90038-4
Key takeaways
- If an individual’s genes affect the fitness of relatives, selection acts as though each organism maximized its inclusive fitness: its own fitness stripped of its social environment’s effects, plus the effects it causes on others weighted by its relationship to them (§2).
- An action that lowers the actor’s fitness spreads only if the ratio of benefit to cost exceeds 1/r: twice the cost for a full sib, four times for a half-sib, eight times for a first cousin (§5). A selfish action is held back only by the same weighting.
- Placing a benefit on a relative of degree r instead of on oneself slows selection by the factor r (§3a), and “viscous” populations should show the most giving traits (§3b).
Standing in the record
Filed on 2026-10-02 at the owner’s request, as one of the evolutionary sources of morality-as-cooperation. NOTE-034, the record’s reading of Curry, Mullins & Whitehouse’s sixty-society test (LIT-047), names Hamilton’s kin selection first in the theory’s lineage, and Curry’s 2016 statement of the theory (LIT-503) cites “Hamilton, 1964” for its kin domain. Its sequel, with the applications, is LIT-506.
NOTE-401 is the close reading of 2026-10-02, and it placed the work: Active. It is the place the quantity “inclusive fitness” and the 1/r threshold are first derived. What it grounds for morality-as-cooperation is narrow and real: a selective reason to favour kin over non-kin in proportion to relatedness, which is the kin domain (LIT-047’s “family”). It grounds no other domain, and it does not mention morality. The rb > c form that later work calls Hamilton’s rule is a restatement: Hamilton writes −k > 1/r with k the ratio of the effect on neighbours to the effect on oneself.
Two things a reader citing it should know. First, the theorem that mean inclusive fitness increases is proved only when no genotype’s total effect on others is negative; for harming traits Hamilton says only that the change is “aimed somewhere in the direction of a local maximum” (§2). Second, the coefficient r is exact only for loci not under selection and for ancestors, and Hamilton says he “has so far failed to reach any definite general conclusions” on the error this introduces (§4). The later derivations of the rule from the Price equation are not in the record; Luque’s analysis of the Price equation (LIT-205) defends the equation against critics including Nowak and Highfield, but its reading (NOTE-101) found that it has no section on Hamilton’s rule.
Topic. natural-sciences first: it is population genetics.
mathematics holds the model. It is not tagged moral-psychology: it
says nothing about morality, moral judgement or norms, and ADR-018 keeps
the cooperation sources out of that word unless they are read as accounts
of morality.
Anthology. No instruction for machine-learning practice; the anthology does not hold it.