Hamilton’s Rule [...]

Rule to predict kin selection effects in evolution.

From Wikipedia:

Formally, genes should increase in frequency when

rB > C


r = the genetic relatedness of the recipient to the actor, often defined as the probability that a gene picked randomly from each at the same locus is identical by descent.

B = the additional reproductive benefit gained by the recipient of the altruistic act,

C = the reproductive cost to the individual performing the act.

This inequality is known as Hamilton’s rule after W. D. Hamilton who in 1964 published the first formal quantitative treatment of kin selection.