A dominance hierarchy (in humans: social hierarchy) is the organization of individuals in a group that occurs when competition of resources lead to aggression. Schjelderup-Ebbe, who studied the often-cited example of the pecking order in chickens, found that such social structures lead to more stable flocks in which aggression was reduced among individuals.

Dominance hierarchies can be despotic or linear. In a despotic hierarchy, only one individual is dominant, while the others are all submissive. In a linear hierarchy, for example, in the above cited pecking order of chickens, each individual dominates all individuals below him and not those above him.

Dominance hierarchies occur in most social animal species, including primates who normally live in groups. Dominance hierarchies have been extensively studied in fish, birds, and mammals. Dominance hierarchies can be simple linear structures, which often arise from the physical differences among individuals in a group in relation to their access to resources. They are also influenced by the complex social interactions among individuals in the group.

In animal groups, the dominant individual is the leader. Dominance is often won through battles or displays of dominance. The dominant position and gaining dominance is linked to mating in many animals.

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The most basic interaction that can establish a Dominance Hierarchy is the dyad, or paired interaction among individuals. To study the formation of hierarchies, scientists have often used the dyadic method, in which two individuals are forced to interact isolated from others. All individuals in the group are paired with each other (i.e. a round-robin), in isolation, until a hierarchy can be deduced. The process of deducing the hierarchy involves the construction of a dominance matrix, in which wins/ties are expressed in relation to each fish in the group.

Recently, it has been postulated that paired interactions alone can not account for the emergence of dominance hierarchies. This is because in nature, such paired interactions rarely occur in isolation. Thus, a relatively new concept has now emerged in animal behavior: the study of socially-embedded dyads. Such phenomena as the audience effect, the context-dependent audience effect in Betta fish (Betta splendens), the observer effect, and the winner-loser effect[How to reference and link to summary or text], may play important roles in the formation of dominance hierarchies in social groups. Furthermore, it has been argued that the social group forms a complex signaling network: interaction that occur among just two individuals of the group are in turn affected by other signals transmitted by individuals in direct communication with them. In many animals, these putative signals can include postural changes, as well as changes in “state” (such as color changes).

Dominance hierarchies, though often more subtle, can be observed in human societies and are important for understanding the organization of family, tribe or clan, work organizations, politics, etc. in normal and abnormal social situations. It is not clear how much of dominance hierarchy in humans is due to the intrinsic biology of our brains, derived from evolution, and how much is due to cultural factors.

Dominance relationships require the cooperation of both parties. The dominant party tacitly agrees not to kill or injure the submissive party provided the submitter concedes and does not interfere with the dominant party's access to resources or compete for mates. The ability to identify and remember members of the group along with their dominance status is also necessary. These hierarchies may have developed, in evolutionary terms, for the sake of efficiency and in order to reduce the likelihood of injury among group members who may share genes. As with the case of altruism in animals, the voluntary abbreviation of hostilities into symbolic form remains something of a Darwinian Puzzle.

Individuals with greater hierarchical status tend to displace those ranked lower from access to space, to food and to mating opportunities. Thus, individuals with higher social status tend to have greater reproductive success by mating more often and having more resources to invest in the survival of offspring.
Hence it serves as an intrinsic factor for population control, insuring adequate resources for the dominant individuals and thus preventing widespread starvation. Territorialbehavior enhances this effect.[1][2]

These hierarchies are not fixed and depend on any number of changing factors, among them are age, gender, body size, intelligence, and aggressiveness. Status may also be affected by the ability to marshal the support of others. Indeed, the need to maintain social position and social knowledge may be an impetus for the evolution of larger brains in humans and other animals.[3]