What does this equation mean?

Here r is the coefficient of relatedness between the actor and the recipient, b is the fitness benefit the recipient gains, and c is the fitness cost the actor pays. A gene that disposes its carrier to help a relative will spread whenever the benefit to the recipient, discounted by how likely the recipient is to carry a copy of the same gene, exceeds the cost to the helper. As Robert Trivers summarized it a few years later, opening his own paper on the harder problem of altruism between non-relatives, “Hamilton (1964) has demonstrated that degree of relationship is an important parameter in predicting how selection will operate” on behavior that looks altruistic [ 2 ] . Under Hamilton’s rule, a parent who works to feed a child, or a sibling who takes a real risk for a full sibling, is not a puzzle. The genes involved are, in the relevant sense, helping copies of themselves.