Summary
Altruism evolves when relatedness times benefit exceeds the cost, rb greater than c.
In depth
rB > C. Relatedness r is 0.5 for siblings and 0.125 for cousins. It explains worker sterility and cooperative breeding, but debates continue about inclusive fitness versus other models.
Costs and benefits are measured in lifetime reproductive success. Relatedness r is the probability that an allele is shared by common descent. The rule also underlies parent-offspring conflict and nepotism.
Remember: inclusive fitness sums direct and indirect effects on genes.
W. D. Hamilton showed in 1964 that a gene for altruism can spread if rB > C, where r is relatedness between actor and recipient, B is the fitness benefit to the recipient and C is the cost to the actor. Relatedness is the chance of sharing a gene by common descent: 0.5 for full siblings and 0.125 for first cousins.
The rule underlies inclusive fitness, which includes effects on relatives. It helps explain sterile workers in social insects, whose haplodiploid genetics make sisters unusually related (0.75), and helping behaviour in birds and mammals.
Example: a worker bee helps her mother produce sisters.
Remember: it predicts help is directed toward close kin.
Frequently asked questions
What is Hamilton's Rule?
Altruism evolves when relatedness times benefit exceeds the cost, rb greater than c.
Which area of biology covers Hamilton's Rule, and how advanced is it?
Hamilton's Rule is one of the 53 topics in Animal Behaviour. It is rated Advanced, so it suits learners who want specialist depth. Related topics in this area include Altruism, Cooperative Breeding, Game Theory in Behaviour.
Can you give an example of Hamilton's Rule?
Helpers at the nest in Florida scrub jays are mostly relatives.
What should I learn before Hamilton's Rule?
Work through the Beginner topics in Animal Behaviour first, such as Animal Behaviour, Innate Behaviour, Learned Behaviour, then return to Hamilton's Rule. Advanced topics assume the Intermediate vocabulary.
What topics are related to Hamilton's Rule?
The closest topics to Hamilton's Rule are Altruism, Cooperative Breeding, Game Theory in Behaviour, Animal Personality. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Hamilton's Rule?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Hamilton's Rule. To practise the whole of Animal Behaviour, use the Animal Behaviour quiz.
How is Hamilton's Rule different from Altruism?
Hamilton's Rule: Altruism evolves when relatedness times benefit exceeds the cost, rb greater than c. Altruism: Behaviour that benefits others at a cost to the actor, explained by kin selection and reciprocity. Read both side by side and note what each one explains.
How should I study Hamilton's Rule step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Altruism and explain the difference aloud. Finally, take the Animal Behaviour quiz and come back to this page for any question you miss.