Sociobiology, a developing scientific field that investigates the biological bases of the social behavior of animals, such as aggression, territoriality, social systems, and mate selection. Sociobiology seeks to extend the concept of natural selection to social systems and social behavior of animals, including humans. Sociobiologists believe that behavior patterns come into being, are modified, and even disappear through the process of natural selection.

Because of the efforts of some prominent sociobiologists to extend such analyses to the complex behavior of humans, sociobiology initially aroused a great deal of controversy. Those opposed to this perspective on the origins of human behavior consider sociobiology a purveyor of biological determinism and, in effect, a supporter of existing social systems; sociobiologists dispute such charges, and subsequent research has suggested a high degree of innate control in certain aspects of human behavior, such as language acquisition, mate choice, gestural communication, incest avoidance, and some personality and cognitive traits. The work of sociobiologists has been most successful in helping to understand the evolution of the behavior patterns of social insects, patterns that are hard to account for in terms of the survival of individual animals alone.

Although the term sociobiology is of recent coinage, the problems the discipline seeks to resolve have been recognized for many years. Indeed, in the 19th century the main founder of evolutionary theory, Charles Darwin, had already attempted to deal with the question of altruism: the willingness of one individual to do favors for another, even if the donor thereby reduces or even forecloses its own opportunity to have offspring. Examples of altruism include the caring for young by sterile worker ants and bees and the sacrificial defense of young by older animals. Even grooming a fellow creature takes time away from selfish activities that, in the long run, might increase an animal’s reproductive potential.

In attempting to reconcile altruism with natural selection, Darwin foreshadowed the thesis later developed by sociobiologists: that the performer of an altruistic act, though forfeiting some part of its own contribution to the gene pool of the next generation, nevertheless contributes to the survival of others of the species. But how selection can reward such sacrifice was unclear to Darwin; in time, the genes predisposing altruism should become uncommon and then go extinct, since their possessors are reproducing less often than animals lacking them.

The first major advance in understanding altruism came around 1960 when British biologist W. D. Hamilton developed the concept of kin selection. His theory shows with mathematical precision that individuals within a species can best enhance their own reproductive success by aiding their close relatives so long as the gain conferred on the recipient is much greater than the cost to the donor. Many aspects of the evolution of not only social insects, but also such close-knit mammalian societies as lions, can be explained in this way.

On the other hand, altruism often occurs in the absence of close genetic relatedness; that is, it can involve other than direct kin. R. L. Trivers, an American sociobiologist, proposed that reciprocal altruism can account for such cases. According to this concept, individuals in need receive aid on the “understanding” that they will reciprocate such aid. For reciprocal altruism to work, however, the members of the group need to be able to identify and exclude any who do not play their part—a state of affairs most likely in small groups, such as those of primates, including primitive human societies.

Many other such theories dealing with the pressures of natural selection and the environment on animal groups went into the making of sociobiology. Even the mathematics of game theory has played a role: The English biologist J. Maynard Smith has shown how reciprocal altruism can evolve in a species that is completely selfish at the outset, and that such altruism can lead to higher reproductive success. These various theories and supporting data were brought together by the American biologist E. O. Wilson in Sociobiology: The New Synthesis (1975), a book that has become the cornerstone of sociobiology as a distinct field of study. In its last, speculative chapter on the evolution of human behavior, the book also helped to arouse the controversy that has captured the interest of many nonscientists.