--- title: "Kin Selection, Relatedness and When Helping Pays" description: "Helping a relative passes on shared alleles indirectly, so a costly act can spread when the benefit to the recipient, weighted by relatedness, exceeds the cost to the helper. Working out relatedness t" canonical: https://lightmysky.com/learn/science/kin-selection-relatedness-and-when-helping-pays-mt_scVf8G34WI source: https://lightmysky.com/learn/science/kin-selection-relatedness-and-when-helping-pays-mt_scVf8G34WI.md retrieved: 2026-09-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Kin Selection, Relatedness and When Helping Pays Helping a relative passes on shared alleles indirectly, so a costly act can spread when the benefit to the recipient, weighted by relatedness, exceeds the cost to the helper. Working out relatedness turns an argument about altruism into a calculation. Subject: Science · Area: Ecology & Conservation · Ages 20 to 22 Page: https://lightmysky.com/learn/science/kin-selection-relatedness-and-when-helping-pays-mt_scVf8G34WI ## Ready when they can - Calculates relatedness between stated relatives and uses it in the inequality. - Explains why helping a full sibling can pay when helping an unrelated individual does not. - Separates kin selection from cooperation between unrelated individuals. ## Lesson: When helping family pays Animals often help relatives at a cost to themselves, and kin selection explains why evolution favors it. Relatedness is the chance that two relatives share a gene copy inherited from the same ancestor. Full siblings and parents with children share about one half, half siblings about one quarter, and cousins about one eighth. **Example.** Try the accounting on a full sibling. Aid costing you 2 that gives your sibling 6 pays, because 0.5 times 6 is 3, which beats the cost of 2. Saving two siblings can pass on as many of your genes as raising one child of your own. Sterile worker ants and bees live this logic: they raise the queen's young because those young carry many of their genes. Helping pays whenever the benefit to the relative, scaled by relatedness, exceeds your own cost. That is why aid flows freely toward siblings and cousins. Help toward strangers needs some other return path, such as returned favors, reputation, or shared interests, because no shared genes pay the bill. Aggression toward kin is muted for the same reason: injuring a relative harms your own genetic interests. **Tip.** Test any helping case by plugging its three numbers into the inequality: relatedness times benefit against cost. Shared genes alone can justify help to kin. For strangers, always name the return path, or the behavior makes no evolutionary sense yet. **Recap.** Helping a relative pays when the benefit scaled by relatedness beats your cost, which is why aid follows family lines. ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Sexual Selection, Mate Choice and Parental Investment](https://lightmysky.com/learn/science/sexual-selection-mate-choice-and-parental-investment-mt_6oTPGFw_p5) - [Meiosis and Why Gametes Are Haploid](https://lightmysky.com/learn/science/meiosis-and-why-gametes-are-haploid-mt_hvu0bczcfX)