In evolutionary biology inclusive fitness theory is a model for the evolution of social behaviors (traits), first set forward by W. D. Hamilton in 1963 and 1964. Instead of a trait's frequency increase being thought of only via its average effects on an organism's direct reproduction, Hamilton argued that its average effects on indirect reproduction, via identical copies of the trait in other individuals, also need to be taken into account. Hamilton's theory, alongside reciprocal altruism, is considered one of the two primary mechanisms for the evolution of social behaviors in natural species.