Altruism spreads when relatedness times benefit to the recipient exceeds cost to the actor.
Hamilton's 1964 rule, rB > C. It explains why sterile worker castes exist in bees and why people take larger risks for siblings than for strangers, without requiring anyone to compute anything.
In practice
- An employee covers a sibling's shift at real cost and would not cover a stranger's for the same money.
- A family firm tolerates a level of underperformance from a cousin that no hiring process would have accepted.