Mechanisms for recognizing kin are found throughout the plant and animal kingdoms, regardless of an organism's social or mental complexity. Improvements in the general understanding of these mechanisms have turned some biologists' attention to the question of why kin recognition occurs at all. One response to this question is offered by the inclusive fitness theory, which was developed in the 1960s. The theory is based on the realization that an organism transmits its genetic attributes to succeeding generations not solely through its offspring, but more generally through all of its close relatives. Whereas the traditional view of evolution held that natural selection favors the continued genetic representation of individuals within a species that produce the greatest number of offspring, the inclusive fitness theory posits that natural selection similarly favors organisms that help their relatives, because doing so also increases their own total genetic representation. The theory has helped to explain previously mysterious phenomena, including the evolution of social insect species like the honeybee, most of whose members do not produce offspring and exist only to nurture relatives.

Inclusive fitness theory has also been applied usefully to new findings concerning cannibalism within animal species. Based on the theory, cannibals should have evolved to avoid eating their own kin because of the obvious genetic costs of such a practice. Spadefoot toad tadpoles provide an illustration. Biologists have found that all tadpoles of that species begin life as omnivores, feeding mainly on organic debris in their soon-to-be-dry pool in the desert, but that occasionally one tadpole eats another or eats a freshwater shrimp. This event can trigger changes in the tadpole's physiology and dietary preference, causing the tadpole to become larger and exclusively carnivorous, feasting on other animals including members of its own species. Yet the cannibals have a procedure of discrimination whereby they nip at other tadpoles, eating nonsiblings but releasing siblings unharmed. This suggests that the inclusive fitness theory offers at least a partial answer to why kin recognition develops. Interestingly, a cannibal tadpole is less likely to avoid eating kin when it becomes very hungry, apparently putting its own unique genetic makeup ahead of its siblings'.

But there may be other reasons why organisms recognize kin. For example, it has recently been found that tiger salamander larvae, also either omnivorous or cannibalistic, are plagued in nature by a deadly bacterium. Furthermore, it was determined that cannibal larvae are especially likely to be infected by eating diseased species members. The fact that this bacterium is more deadly when it comes from a close relative with a similar immune system suggests that natural selection may favor cannibals that avoid such pathogens by not eating kin. For tiger salamanders then, kin recognition can be explained simply as a means by which an organism preserves its own life, not as a means to aid in relatives' survival.