Inclusive fitness theory provides explanations for many cooperative behaviors – particularly among kin – that enhance one’s likelihood of reproductive fitness. In short, any allele responsible for cooperative social behaviors may be naturally selected if its possessor were able to reproduce more frequently than other members of the same species who do not possess that allele (i.e., those who possess a competing allele). Since the inception of inclusive fitness theory, evolutionary scientists have hypothesized various mechanisms and behaviors that could be the product of specific naturally selected genes. These mechanisms include kin recognition, kin selection, parental investment, parent–offspring conflict, sexual and emotional jealousy, and aggression. Inclusive fitness theory is also one of the most widely misunderstood theories in evolutionary psychology. In this chapter, we describe inclusive fitness theory and expand upon these mechanisms by reviewing various studies within the evolutionary psychological literature, while also addressing the key misunderstandings of inclusive fitness theory.