‘Hamilton’s Rule’: a gene for altruism will spread if the cost to the altruist, C, is less than the value, B, to the beneficiary, devalued by the coefficient of relatedness, r, between them. r is a proportion between 0 and 1. It has the value 1 for identical twins; 0.5 for offspring and full siblings; 0.25 for grandchildren, half-siblings, and nieces; 0.125 for first cousins.
